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javascript iterate all items with different data types [duplicate]

I have a JavaScript array like:
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
How would I go about merging the separate inner arrays into one like:
["$6", "$12", "$25", ...]

ES2019
ES2019 introduced the Array.prototype.flat() method which you could use to flatten the arrays. It is compatible with most environments, although it is only available in Node.js starting with version 11, and not at all in Internet Explorer.
const arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
const merge3 = arrays.flat(1); //The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
console.log(merge3);
Older browsers
For older browsers, you can use Array.prototype.concat to merge arrays:
var arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
var merged = [].concat.apply([], arrays);
console.log(merged);
Using the apply method of concat will just take the second parameter as an array, so the last line is identical to this:
var merged = [].concat(["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]);

Here's a short function that uses some of the newer JavaScript array methods to flatten an n-dimensional array.
function flatten(arr) {
return arr.reduce(function (flat, toFlatten) {
return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
}, []);
}
Usage:
flatten([[1, 2, 3], [4, 5]]); // [1, 2, 3, 4, 5]
flatten([[[1, [1.1]], 2, 3], [4, 5]]); // [1, 1.1, 2, 3, 4, 5]

There is a confusingly hidden method, which constructs a new array without mutating the original one:
var oldArray = [[1],[2,3],[4]];
var newArray = Array.prototype.concat.apply([], oldArray);
console.log(newArray); // [ 1, 2, 3, 4 ]

It can be best done by javascript reduce function.
var arrays = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"], ["$0"], ["$15"],["$3"], ["$75"], ["$5"], ["$100"], ["$7"], ["$3"], ["$75"], ["$5"]];
arrays = arrays.reduce(function(a, b){
return a.concat(b);
}, []);
Or, with ES2015:
arrays = arrays.reduce((a, b) => a.concat(b), []);
js-fiddle
Mozilla docs

There's a new native method called flat to do this exactly.
(As of late 2019, flat is now published in the ECMA 2019 standard, and core-js#3 (babel's library) includes it in their polyfill library)
const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
// Flatten 2 levels deep
const arr3 = [2, 2, 5, [5, [5, [6]], 7]];
arr3.flat(2);
// [2, 2, 5, 5, 5, [6], 7];
// Flatten all levels
const arr4 = [2, 2, 5, [5, [5, [6]], 7]];
arr4.flat(Infinity);
// [2, 2, 5, 5, 5, 6, 7];

Most of the answers here don't work on huge (e.g. 200 000 elements) arrays, and even if they do, they're slow.
Here is the fastest solution, which works also on arrays with multiple levels of nesting:
const flatten = function(arr, result = []) {
for (let i = 0, length = arr.length; i < length; i++) {
const value = arr[i];
if (Array.isArray(value)) {
flatten(value, result);
} else {
result.push(value);
}
}
return result;
};
Examples
Huge arrays
flatten(Array(200000).fill([1]));
It handles huge arrays just fine. On my machine this code takes about 14 ms to execute.
Nested arrays
flatten(Array(2).fill(Array(2).fill(Array(2).fill([1]))));
It works with nested arrays. This code produces [1, 1, 1, 1, 1, 1, 1, 1].
Arrays with different levels of nesting
flatten([1, [1], [[1]]]);
It doesn't have any problems with flattening arrays like this one.

Update: it turned out that this solution doesn't work with large arrays. It you're looking for a better, faster solution, check out this answer.
function flatten(arr) {
return [].concat(...arr)
}
Is simply expands arr and passes it as arguments to concat(), which merges all the arrays into one. It's equivalent to [].concat.apply([], arr).
You can also try this for deep flattening:
function deepFlatten(arr) {
return flatten( // return shalowly flattened array
arr.map(x=> // with each x in array
Array.isArray(x) // is x an array?
? deepFlatten(x) // if yes, return deeply flattened x
: x // if no, return just x
)
)
}
See demo on JSBin.
References for ECMAScript 6 elements used in this answer:
Spread operator
Arrow functions
Side note: methods like find() and arrow functions are not supported by all browsers, but it doesn't mean that you can't use these features right now. Just use Babel — it transforms ES6 code into ES5.

You can use Underscore:
var x = [[1], [2], [3, 4]];
_.flatten(x); // => [1, 2, 3, 4]

Generic procedures mean we don't have to rewrite complexity each time we need to utilize a specific behaviour.
concatMap (or flatMap) is exactly what we need in this situation.
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// your sample data
const data =
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
console.log (flatten (data))
foresight
And yes, you guessed it correctly, it only flattens one level, which is exactly how it should work
Imagine some data set like this
// Player :: (String, Number) -> Player
const Player = (name,number) =>
[ name, number ]
// team :: ( . Player) -> Team
const Team = (...players) =>
players
// Game :: (Team, Team) -> Game
const Game = (teamA, teamB) =>
[ teamA, teamB ]
// sample data
const teamA =
Team (Player ('bob', 5), Player ('alice', 6))
const teamB =
Team (Player ('ricky', 4), Player ('julian', 2))
const game =
Game (teamA, teamB)
console.log (game)
// [ [ [ 'bob', 5 ], [ 'alice', 6 ] ],
// [ [ 'ricky', 4 ], [ 'julian', 2 ] ] ]
Ok, now say we want to print a roster that shows all the players that will be participating in game …
const gamePlayers = game =>
flatten (game)
gamePlayers (game)
// => [ [ 'bob', 5 ], [ 'alice', 6 ], [ 'ricky', 4 ], [ 'julian', 2 ] ]
If our flatten procedure flattened nested arrays too, we'd end up with this garbage result …
const gamePlayers = game =>
badGenericFlatten(game)
gamePlayers (game)
// => [ 'bob', 5, 'alice', 6, 'ricky', 4, 'julian', 2 ]
rollin' deep, baby
That's not to say sometimes you don't want to flatten nested arrays, too – only that shouldn't be the default behaviour.
We can make a deepFlatten procedure with ease …
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[0, [1, [2, [3, [4, 5], 6]]], [7, [8]], 9]
console.log (flatten (data))
// [ 0, 1, [ 2, [ 3, [ 4, 5 ], 6 ] ], 7, [ 8 ], 9 ]
console.log (deepFlatten (data))
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
There. Now you have a tool for each job – one for squashing one level of nesting, flatten, and one for obliterating all nesting deepFlatten.
Maybe you can call it obliterate or nuke if you don't like the name deepFlatten.
Don't iterate twice !
Of course the above implementations are clever and concise, but using a .map followed by a call to .reduce means we're actually doing more iterations than necessary
Using a trusty combinator I'm calling mapReduce helps keep the iterations to a minium; it takes a mapping function m :: a -> b, a reducing function r :: (b,a) ->b and returns a new reducing function - this combinator is at the heart of transducers; if you're interested, I've written other answers about them
// mapReduce = (a -> b, (b,a) -> b, (b,a) -> b)
const mapReduce = (m,r) =>
(acc,x) => r (acc, m (x))
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.reduce (mapReduce (f, concat), [])
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[ [ [ 1, 2 ],
[ 3, 4 ] ],
[ [ 5, 6 ],
[ 7, 8 ] ] ]
console.log (flatten (data))
// [ [ 1. 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]
console.log (deepFlatten (data))
// [ 1, 2, 3, 4, 5, 6, 7, 8 ]

To flatten an array of single element arrays, you don't need to import a library, a simple loop is both the simplest and most efficient solution :
for (var i = 0; i < a.length; i++) {
a[i] = a[i][0];
}
To downvoters: please read the question, don't downvote because it doesn't suit your very different problem. This solution is both the fastest and simplest for the asked question.

Another ECMAScript 6 solution in functional style:
Declare a function:
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
and use it:
flatten( [1, [2,3], [4,[5,[6]]]] ) // -> [1,2,3,4,5,6]
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
console.log( flatten([1, [2,3], [4,[5],[6,[7,8,9],10],11],[12],13]) )
Consider also a native function Array.prototype.flat() (proposal for ES6) available in last releases of modern browsers. Thanks to #(Константин Ван) and #(Mark Amery) mentioned it in the comments.
The flat function has one parameter, specifying the expected depth of array nesting, which equals 1 by default.
[1, 2, [3, 4]].flat(); // -> [1, 2, 3, 4]
[1, 2, [3, 4, [5, 6]]].flat(); // -> [1, 2, 3, 4, [5, 6]]
[1, 2, [3, 4, [5, 6]]].flat(2); // -> [1, 2, 3, 4, 5, 6]
[1, 2, [3, 4, [5, 6]]].flat(Infinity); // -> [1, 2, 3, 4, 5, 6]
let arr = [1, 2, [3, 4]];
console.log( arr.flat() );
arr = [1, 2, [3, 4, [5, 6]]];
console.log( arr.flat() );
console.log( arr.flat(1) );
console.log( arr.flat(2) );
console.log( arr.flat(Infinity) );

You can also try the new Array.flat() method. It works in the following manner:
let arr = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]].flat()
console.log(arr);
The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the 1 layer of depth (i.e. arrays inside arrays)
If you want to also flatten out 3 dimensional or even higher dimensional arrays you simply call the flat method multiple times. For example (3 dimensions):
let arr = [1,2,[3,4,[5,6]]].flat().flat().flat();
console.log(arr);
Be careful!
Array.flat() method is relatively new. Older browsers like ie might not have implemented the method. If you want you code to work on all browsers you might have to transpile your JS to an older version. Check for MDN web docs for current browser compatibility.

A solution for the more general case, when you may have some non-array elements in your array.
function flattenArrayOfArrays(a, r){
if(!r){ r = []}
for(var i=0; i<a.length; i++){
if(a[i].constructor == Array){
flattenArrayOfArrays(a[i], r);
}else{
r.push(a[i]);
}
}
return r;
}

What about using reduce(callback[, initialValue]) method of JavaScript 1.8
list.reduce((p,n) => p.concat(n),[]);
Would do the job.

const common = arr.reduce((a, b) => [...a, ...b], [])

You can use Array.flat() with Infinity for any depth of nested array.
var arr = [ [1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]], [[1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]]] ];
let flatten = arr.flat(Infinity)
console.log(flatten)
check here for browser compatibility

Please note: When Function.prototype.apply ([].concat.apply([], arrays)) or the spread operator ([].concat(...arrays)) is used in order to flatten an array, both can cause stack overflows for large arrays, because every argument of a function is stored on the stack.
Here is a stack-safe implementation in functional style that weighs up the most important requirements against one another:
reusability
readability
conciseness
performance
// small, reusable auxiliary functions:
const foldl = f => acc => xs => xs.reduce(uncurry(f), acc); // aka reduce
const uncurry = f => (a, b) => f(a) (b);
const concat = xs => y => xs.concat(y);
// the actual function to flatten an array - a self-explanatory one-line:
const flatten = xs => foldl(concat) ([]) (xs);
// arbitrary array sizes (until the heap blows up :D)
const xs = [[1,2,3],[4,5,6],[7,8,9]];
console.log(flatten(xs));
// Deriving a recursive solution for deeply nested arrays is trivially now
// yet more small, reusable auxiliary functions:
const map = f => xs => xs.map(apply(f));
const apply = f => a => f(a);
const isArray = Array.isArray;
// the derived recursive function:
const flattenr = xs => flatten(map(x => isArray(x) ? flattenr(x) : x) (xs));
const ys = [1,[2,[3,[4,[5],6,],7],8],9];
console.log(flattenr(ys));
As soon as you get used to small arrow functions in curried form, function composition and higher order functions, this code reads like prose. Programming then merely consists of putting together small building blocks that always work as expected, because they don't contain any side effects.

ES6 One Line Flatten
See lodash flatten, underscore flatten (shallow true)
function flatten(arr) {
return arr.reduce((acc, e) => acc.concat(e), []);
}
or
function flatten(arr) {
return [].concat.apply([], arr);
}
Tested with
test('already flatted', () => {
expect(flatten([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats first level', () => {
expect(flatten([1, [2, [3, [4]], 5]])).toEqual([1, 2, [3, [4]], 5]);
});
ES6 One Line Deep Flatten
See lodash flattenDeep, underscore flatten
function flattenDeep(arr) {
return arr.reduce((acc, e) => Array.isArray(e) ? acc.concat(flattenDeep(e)) : acc.concat(e), []);
}
Tested with
test('already flatted', () => {
expect(flattenDeep([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats', () => {
expect(flattenDeep([1, [2, [3, [4]], 5]])).toEqual([1, 2, 3, 4, 5]);
});

Using the spread operator:
const input = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]];
const output = [].concat(...input);
console.log(output); // --> ["$6", "$12", "$25", "$25", "$18", "$22", "$10"]

I recommend a space-efficient generator function:
function* flatten(arr) {
if (!Array.isArray(arr)) yield arr;
else for (let el of arr) yield* flatten(el);
}
// Example:
console.log(...flatten([1,[2,[3,[4]]]])); // 1 2 3 4
If desired, create an array of flattened values as follows:
let flattened = [...flatten([1,[2,[3,[4]]]])]; // [1, 2, 3, 4]

If you only have arrays with 1 string element:
[["$6"], ["$12"], ["$25"], ["$25"]].join(',').split(',');
will do the job. Bt that specifically matches your code example.

I have done it using recursion and closures
function flatten(arr) {
var temp = [];
function recursiveFlatten(arr) {
for(var i = 0; i < arr.length; i++) {
if(Array.isArray(arr[i])) {
recursiveFlatten(arr[i]);
} else {
temp.push(arr[i]);
}
}
}
recursiveFlatten(arr);
return temp;
}

A Haskellesque approach
function flatArray([x,...xs]){
return x ? [...Array.isArray(x) ? flatArray(x) : [x], ...flatArray(xs)] : [];
}
var na = [[1,2],[3,[4,5]],[6,7,[[[8],9]]],10];
fa = flatArray(na);
console.log(fa);

ES6 way:
const flatten = arr => arr.reduce((acc, next) => acc.concat(Array.isArray(next) ? flatten(next) : next), [])
const a = [1, [2, [3, [4, [5]]]]]
console.log(flatten(a))
ES5 way for flatten function with ES3 fallback for N-times nested arrays:
var flatten = (function() {
if (!!Array.prototype.reduce && !!Array.isArray) {
return function(array) {
return array.reduce(function(prev, next) {
return prev.concat(Array.isArray(next) ? flatten(next) : next);
}, []);
};
} else {
return function(array) {
var arr = [];
var i = 0;
var len = array.length;
var target;
for (; i < len; i++) {
target = array[i];
arr = arr.concat(
(Object.prototype.toString.call(target) === '[object Array]') ? flatten(target) : target
);
}
return arr;
};
}
}());
var a = [1, [2, [3, [4, [5]]]]];
console.log(flatten(a));

if you use lodash, you can just use its flatten method: https://lodash.com/docs/4.17.14#flatten
The nice thing about lodash is that it also has methods to flatten the arrays:
i) recursively: https://lodash.com/docs/4.17.14#flattenDeep
ii) upto n levels of nesting: https://lodash.com/docs/4.17.14#flattenDepth
For example
const _ = require("lodash");
const pancake = _.flatten(array)

I was goofing with ES6 Generators the other day and wrote this gist. Which contains...
function flatten(arrayOfArrays=[]){
function* flatgen() {
for( let item of arrayOfArrays ) {
if ( Array.isArray( item )) {
yield* flatten(item)
} else {
yield item
}
}
}
return [...flatgen()];
}
var flatArray = flatten([[1, [4]],[2],[3]]);
console.log(flatArray);
Basically I'm creating a generator that loops over the original input array, if it finds an array it uses the yield* operator in combination with recursion to continually flatten the internal arrays. If the item is not an array it just yields the single item. Then using the ES6 Spread operator (aka splat operator) I flatten out the generator into a new array instance.
I haven't tested the performance of this, but I figure it is a nice simple example of using generators and the yield* operator.
But again, I was just goofing so I'm sure there are more performant ways to do this.

just the best solution without lodash
let flatten = arr => [].concat.apply([], arr.map(item => Array.isArray(item) ? flatten(item) : item))

I would rather transform the whole array, as-is, to a string, but unlike other answers, would do that using JSON.stringify and not use the toString() method, which produce an unwanted result.
With that JSON.stringify output, all that's left is to remove all brackets, wrap the result with start & ending brackets yet again, and serve the result with JSON.parse which brings the string back to "life".
Can handle infinite nested arrays without any speed costs.
Can rightly handle Array items which are strings containing commas.
var arr = ["abc",[[[6]]],["3,4"],"2"];
var s = "[" + JSON.stringify(arr).replace(/\[|]/g,'') +"]";
var flattened = JSON.parse(s);
console.log(flattened)
Only for multidimensional Array of Strings/Numbers (not Objects)

Ways for making flatten array
using Es6 flat()
using Es6 reduce()
using recursion
using string manipulation
[1,[2,[3,[4,[5,[6,7],8],9],10]]] - [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
// using Es6 flat()
let arr = [1,[2,[3,[4,[5,[6,7],8],9],10]]]
console.log(arr.flat(Infinity))
// using Es6 reduce()
let flatIt = (array) => array.reduce(
(x, y) => x.concat(Array.isArray(y) ? flatIt(y) : y), []
)
console.log(flatIt(arr))
// using recursion
function myFlat(array) {
let flat = [].concat(...array);
return flat.some(Array.isArray) ? myFlat(flat) : flat;
}
console.log(myFlat(arr));
// using string manipulation
let strArr = arr.toString().split(',');
for(let i=0;i<strArr.length;i++)
strArr[i]=parseInt(strArr[i]);
console.log(strArr)

I think array.flat(Infinity) is a perfect solution. But flat function is a relatively new function and may not run in older versions of browsers. We can use recursive function for solving this.
const arr = ["A", ["B", [["B11", "B12", ["B131", "B132"]], "B2"]], "C", ["D", "E", "F", ["G", "H", "I"]]]
const flatArray = (arr) => {
const res = []
for (const item of arr) {
if (Array.isArray(item)) {
const subRes = flatArray(item)
res.push(...subRes)
} else {
res.push(item)
}
}
return res
}
console.log(flatArray(arr))

How to remove an array over the object in javascript [duplicate]

I have a JavaScript array like:
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
How would I go about merging the separate inner arrays into one like:
["$6", "$12", "$25", ...]

ES2019
ES2019 introduced the Array.prototype.flat() method which you could use to flatten the arrays. It is compatible with most environments, although it is only available in Node.js starting with version 11, and not at all in Internet Explorer.
const arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
const merge3 = arrays.flat(1); //The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
console.log(merge3);
Older browsers
For older browsers, you can use Array.prototype.concat to merge arrays:
var arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
var merged = [].concat.apply([], arrays);
console.log(merged);
Using the apply method of concat will just take the second parameter as an array, so the last line is identical to this:
var merged = [].concat(["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]);

Here's a short function that uses some of the newer JavaScript array methods to flatten an n-dimensional array.
function flatten(arr) {
return arr.reduce(function (flat, toFlatten) {
return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
}, []);
}
Usage:
flatten([[1, 2, 3], [4, 5]]); // [1, 2, 3, 4, 5]
flatten([[[1, [1.1]], 2, 3], [4, 5]]); // [1, 1.1, 2, 3, 4, 5]

There is a confusingly hidden method, which constructs a new array without mutating the original one:
var oldArray = [[1],[2,3],[4]];
var newArray = Array.prototype.concat.apply([], oldArray);
console.log(newArray); // [ 1, 2, 3, 4 ]

It can be best done by javascript reduce function.
var arrays = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"], ["$0"], ["$15"],["$3"], ["$75"], ["$5"], ["$100"], ["$7"], ["$3"], ["$75"], ["$5"]];
arrays = arrays.reduce(function(a, b){
return a.concat(b);
}, []);
Or, with ES2015:
arrays = arrays.reduce((a, b) => a.concat(b), []);
js-fiddle
Mozilla docs

There's a new native method called flat to do this exactly.
(As of late 2019, flat is now published in the ECMA 2019 standard, and core-js#3 (babel's library) includes it in their polyfill library)
const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
// Flatten 2 levels deep
const arr3 = [2, 2, 5, [5, [5, [6]], 7]];
arr3.flat(2);
// [2, 2, 5, 5, 5, [6], 7];
// Flatten all levels
const arr4 = [2, 2, 5, [5, [5, [6]], 7]];
arr4.flat(Infinity);
// [2, 2, 5, 5, 5, 6, 7];

Most of the answers here don't work on huge (e.g. 200 000 elements) arrays, and even if they do, they're slow.
Here is the fastest solution, which works also on arrays with multiple levels of nesting:
const flatten = function(arr, result = []) {
for (let i = 0, length = arr.length; i < length; i++) {
const value = arr[i];
if (Array.isArray(value)) {
flatten(value, result);
} else {
result.push(value);
}
}
return result;
};
Examples
Huge arrays
flatten(Array(200000).fill([1]));
It handles huge arrays just fine. On my machine this code takes about 14 ms to execute.
Nested arrays
flatten(Array(2).fill(Array(2).fill(Array(2).fill([1]))));
It works with nested arrays. This code produces [1, 1, 1, 1, 1, 1, 1, 1].
Arrays with different levels of nesting
flatten([1, [1], [[1]]]);
It doesn't have any problems with flattening arrays like this one.

Update: it turned out that this solution doesn't work with large arrays. It you're looking for a better, faster solution, check out this answer.
function flatten(arr) {
return [].concat(...arr)
}
Is simply expands arr and passes it as arguments to concat(), which merges all the arrays into one. It's equivalent to [].concat.apply([], arr).
You can also try this for deep flattening:
function deepFlatten(arr) {
return flatten( // return shalowly flattened array
arr.map(x=> // with each x in array
Array.isArray(x) // is x an array?
? deepFlatten(x) // if yes, return deeply flattened x
: x // if no, return just x
)
)
}
See demo on JSBin.
References for ECMAScript 6 elements used in this answer:
Spread operator
Arrow functions
Side note: methods like find() and arrow functions are not supported by all browsers, but it doesn't mean that you can't use these features right now. Just use Babel — it transforms ES6 code into ES5.

You can use Underscore:
var x = [[1], [2], [3, 4]];
_.flatten(x); // => [1, 2, 3, 4]

Generic procedures mean we don't have to rewrite complexity each time we need to utilize a specific behaviour.
concatMap (or flatMap) is exactly what we need in this situation.
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// your sample data
const data =
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
console.log (flatten (data))
foresight
And yes, you guessed it correctly, it only flattens one level, which is exactly how it should work
Imagine some data set like this
// Player :: (String, Number) -> Player
const Player = (name,number) =>
[ name, number ]
// team :: ( . Player) -> Team
const Team = (...players) =>
players
// Game :: (Team, Team) -> Game
const Game = (teamA, teamB) =>
[ teamA, teamB ]
// sample data
const teamA =
Team (Player ('bob', 5), Player ('alice', 6))
const teamB =
Team (Player ('ricky', 4), Player ('julian', 2))
const game =
Game (teamA, teamB)
console.log (game)
// [ [ [ 'bob', 5 ], [ 'alice', 6 ] ],
// [ [ 'ricky', 4 ], [ 'julian', 2 ] ] ]
Ok, now say we want to print a roster that shows all the players that will be participating in game …
const gamePlayers = game =>
flatten (game)
gamePlayers (game)
// => [ [ 'bob', 5 ], [ 'alice', 6 ], [ 'ricky', 4 ], [ 'julian', 2 ] ]
If our flatten procedure flattened nested arrays too, we'd end up with this garbage result …
const gamePlayers = game =>
badGenericFlatten(game)
gamePlayers (game)
// => [ 'bob', 5, 'alice', 6, 'ricky', 4, 'julian', 2 ]
rollin' deep, baby
That's not to say sometimes you don't want to flatten nested arrays, too – only that shouldn't be the default behaviour.
We can make a deepFlatten procedure with ease …
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[0, [1, [2, [3, [4, 5], 6]]], [7, [8]], 9]
console.log (flatten (data))
// [ 0, 1, [ 2, [ 3, [ 4, 5 ], 6 ] ], 7, [ 8 ], 9 ]
console.log (deepFlatten (data))
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
There. Now you have a tool for each job – one for squashing one level of nesting, flatten, and one for obliterating all nesting deepFlatten.
Maybe you can call it obliterate or nuke if you don't like the name deepFlatten.
Don't iterate twice !
Of course the above implementations are clever and concise, but using a .map followed by a call to .reduce means we're actually doing more iterations than necessary
Using a trusty combinator I'm calling mapReduce helps keep the iterations to a minium; it takes a mapping function m :: a -> b, a reducing function r :: (b,a) ->b and returns a new reducing function - this combinator is at the heart of transducers; if you're interested, I've written other answers about them
// mapReduce = (a -> b, (b,a) -> b, (b,a) -> b)
const mapReduce = (m,r) =>
(acc,x) => r (acc, m (x))
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.reduce (mapReduce (f, concat), [])
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[ [ [ 1, 2 ],
[ 3, 4 ] ],
[ [ 5, 6 ],
[ 7, 8 ] ] ]
console.log (flatten (data))
// [ [ 1. 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]
console.log (deepFlatten (data))
// [ 1, 2, 3, 4, 5, 6, 7, 8 ]

To flatten an array of single element arrays, you don't need to import a library, a simple loop is both the simplest and most efficient solution :
for (var i = 0; i < a.length; i++) {
a[i] = a[i][0];
}
To downvoters: please read the question, don't downvote because it doesn't suit your very different problem. This solution is both the fastest and simplest for the asked question.

Another ECMAScript 6 solution in functional style:
Declare a function:
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
and use it:
flatten( [1, [2,3], [4,[5,[6]]]] ) // -> [1,2,3,4,5,6]
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
console.log( flatten([1, [2,3], [4,[5],[6,[7,8,9],10],11],[12],13]) )
Consider also a native function Array.prototype.flat() (proposal for ES6) available in last releases of modern browsers. Thanks to #(Константин Ван) and #(Mark Amery) mentioned it in the comments.
The flat function has one parameter, specifying the expected depth of array nesting, which equals 1 by default.
[1, 2, [3, 4]].flat(); // -> [1, 2, 3, 4]
[1, 2, [3, 4, [5, 6]]].flat(); // -> [1, 2, 3, 4, [5, 6]]
[1, 2, [3, 4, [5, 6]]].flat(2); // -> [1, 2, 3, 4, 5, 6]
[1, 2, [3, 4, [5, 6]]].flat(Infinity); // -> [1, 2, 3, 4, 5, 6]
let arr = [1, 2, [3, 4]];
console.log( arr.flat() );
arr = [1, 2, [3, 4, [5, 6]]];
console.log( arr.flat() );
console.log( arr.flat(1) );
console.log( arr.flat(2) );
console.log( arr.flat(Infinity) );

You can also try the new Array.flat() method. It works in the following manner:
let arr = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]].flat()
console.log(arr);
The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the 1 layer of depth (i.e. arrays inside arrays)
If you want to also flatten out 3 dimensional or even higher dimensional arrays you simply call the flat method multiple times. For example (3 dimensions):
let arr = [1,2,[3,4,[5,6]]].flat().flat().flat();
console.log(arr);
Be careful!
Array.flat() method is relatively new. Older browsers like ie might not have implemented the method. If you want you code to work on all browsers you might have to transpile your JS to an older version. Check for MDN web docs for current browser compatibility.

A solution for the more general case, when you may have some non-array elements in your array.
function flattenArrayOfArrays(a, r){
if(!r){ r = []}
for(var i=0; i<a.length; i++){
if(a[i].constructor == Array){
flattenArrayOfArrays(a[i], r);
}else{
r.push(a[i]);
}
}
return r;
}

What about using reduce(callback[, initialValue]) method of JavaScript 1.8
list.reduce((p,n) => p.concat(n),[]);
Would do the job.

const common = arr.reduce((a, b) => [...a, ...b], [])

You can use Array.flat() with Infinity for any depth of nested array.
var arr = [ [1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]], [[1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]]] ];
let flatten = arr.flat(Infinity)
console.log(flatten)
check here for browser compatibility

Please note: When Function.prototype.apply ([].concat.apply([], arrays)) or the spread operator ([].concat(...arrays)) is used in order to flatten an array, both can cause stack overflows for large arrays, because every argument of a function is stored on the stack.
Here is a stack-safe implementation in functional style that weighs up the most important requirements against one another:
reusability
readability
conciseness
performance
// small, reusable auxiliary functions:
const foldl = f => acc => xs => xs.reduce(uncurry(f), acc); // aka reduce
const uncurry = f => (a, b) => f(a) (b);
const concat = xs => y => xs.concat(y);
// the actual function to flatten an array - a self-explanatory one-line:
const flatten = xs => foldl(concat) ([]) (xs);
// arbitrary array sizes (until the heap blows up :D)
const xs = [[1,2,3],[4,5,6],[7,8,9]];
console.log(flatten(xs));
// Deriving a recursive solution for deeply nested arrays is trivially now
// yet more small, reusable auxiliary functions:
const map = f => xs => xs.map(apply(f));
const apply = f => a => f(a);
const isArray = Array.isArray;
// the derived recursive function:
const flattenr = xs => flatten(map(x => isArray(x) ? flattenr(x) : x) (xs));
const ys = [1,[2,[3,[4,[5],6,],7],8],9];
console.log(flattenr(ys));
As soon as you get used to small arrow functions in curried form, function composition and higher order functions, this code reads like prose. Programming then merely consists of putting together small building blocks that always work as expected, because they don't contain any side effects.

ES6 One Line Flatten
See lodash flatten, underscore flatten (shallow true)
function flatten(arr) {
return arr.reduce((acc, e) => acc.concat(e), []);
}
or
function flatten(arr) {
return [].concat.apply([], arr);
}
Tested with
test('already flatted', () => {
expect(flatten([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats first level', () => {
expect(flatten([1, [2, [3, [4]], 5]])).toEqual([1, 2, [3, [4]], 5]);
});
ES6 One Line Deep Flatten
See lodash flattenDeep, underscore flatten
function flattenDeep(arr) {
return arr.reduce((acc, e) => Array.isArray(e) ? acc.concat(flattenDeep(e)) : acc.concat(e), []);
}
Tested with
test('already flatted', () => {
expect(flattenDeep([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats', () => {
expect(flattenDeep([1, [2, [3, [4]], 5]])).toEqual([1, 2, 3, 4, 5]);
});

Using the spread operator:
const input = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]];
const output = [].concat(...input);
console.log(output); // --> ["$6", "$12", "$25", "$25", "$18", "$22", "$10"]

I recommend a space-efficient generator function:
function* flatten(arr) {
if (!Array.isArray(arr)) yield arr;
else for (let el of arr) yield* flatten(el);
}
// Example:
console.log(...flatten([1,[2,[3,[4]]]])); // 1 2 3 4
If desired, create an array of flattened values as follows:
let flattened = [...flatten([1,[2,[3,[4]]]])]; // [1, 2, 3, 4]

If you only have arrays with 1 string element:
[["$6"], ["$12"], ["$25"], ["$25"]].join(',').split(',');
will do the job. Bt that specifically matches your code example.

I have done it using recursion and closures
function flatten(arr) {
var temp = [];
function recursiveFlatten(arr) {
for(var i = 0; i < arr.length; i++) {
if(Array.isArray(arr[i])) {
recursiveFlatten(arr[i]);
} else {
temp.push(arr[i]);
}
}
}
recursiveFlatten(arr);
return temp;
}

A Haskellesque approach
function flatArray([x,...xs]){
return x ? [...Array.isArray(x) ? flatArray(x) : [x], ...flatArray(xs)] : [];
}
var na = [[1,2],[3,[4,5]],[6,7,[[[8],9]]],10];
fa = flatArray(na);
console.log(fa);

ES6 way:
const flatten = arr => arr.reduce((acc, next) => acc.concat(Array.isArray(next) ? flatten(next) : next), [])
const a = [1, [2, [3, [4, [5]]]]]
console.log(flatten(a))
ES5 way for flatten function with ES3 fallback for N-times nested arrays:
var flatten = (function() {
if (!!Array.prototype.reduce && !!Array.isArray) {
return function(array) {
return array.reduce(function(prev, next) {
return prev.concat(Array.isArray(next) ? flatten(next) : next);
}, []);
};
} else {
return function(array) {
var arr = [];
var i = 0;
var len = array.length;
var target;
for (; i < len; i++) {
target = array[i];
arr = arr.concat(
(Object.prototype.toString.call(target) === '[object Array]') ? flatten(target) : target
);
}
return arr;
};
}
}());
var a = [1, [2, [3, [4, [5]]]]];
console.log(flatten(a));

if you use lodash, you can just use its flatten method: https://lodash.com/docs/4.17.14#flatten
The nice thing about lodash is that it also has methods to flatten the arrays:
i) recursively: https://lodash.com/docs/4.17.14#flattenDeep
ii) upto n levels of nesting: https://lodash.com/docs/4.17.14#flattenDepth
For example
const _ = require("lodash");
const pancake = _.flatten(array)

I was goofing with ES6 Generators the other day and wrote this gist. Which contains...
function flatten(arrayOfArrays=[]){
function* flatgen() {
for( let item of arrayOfArrays ) {
if ( Array.isArray( item )) {
yield* flatten(item)
} else {
yield item
}
}
}
return [...flatgen()];
}
var flatArray = flatten([[1, [4]],[2],[3]]);
console.log(flatArray);
Basically I'm creating a generator that loops over the original input array, if it finds an array it uses the yield* operator in combination with recursion to continually flatten the internal arrays. If the item is not an array it just yields the single item. Then using the ES6 Spread operator (aka splat operator) I flatten out the generator into a new array instance.
I haven't tested the performance of this, but I figure it is a nice simple example of using generators and the yield* operator.
But again, I was just goofing so I'm sure there are more performant ways to do this.

just the best solution without lodash
let flatten = arr => [].concat.apply([], arr.map(item => Array.isArray(item) ? flatten(item) : item))

I would rather transform the whole array, as-is, to a string, but unlike other answers, would do that using JSON.stringify and not use the toString() method, which produce an unwanted result.
With that JSON.stringify output, all that's left is to remove all brackets, wrap the result with start & ending brackets yet again, and serve the result with JSON.parse which brings the string back to "life".
Can handle infinite nested arrays without any speed costs.
Can rightly handle Array items which are strings containing commas.
var arr = ["abc",[[[6]]],["3,4"],"2"];
var s = "[" + JSON.stringify(arr).replace(/\[|]/g,'') +"]";
var flattened = JSON.parse(s);
console.log(flattened)
Only for multidimensional Array of Strings/Numbers (not Objects)

Ways for making flatten array
using Es6 flat()
using Es6 reduce()
using recursion
using string manipulation
[1,[2,[3,[4,[5,[6,7],8],9],10]]] - [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
// using Es6 flat()
let arr = [1,[2,[3,[4,[5,[6,7],8],9],10]]]
console.log(arr.flat(Infinity))
// using Es6 reduce()
let flatIt = (array) => array.reduce(
(x, y) => x.concat(Array.isArray(y) ? flatIt(y) : y), []
)
console.log(flatIt(arr))
// using recursion
function myFlat(array) {
let flat = [].concat(...array);
return flat.some(Array.isArray) ? myFlat(flat) : flat;
}
console.log(myFlat(arr));
// using string manipulation
let strArr = arr.toString().split(',');
for(let i=0;i<strArr.length;i++)
strArr[i]=parseInt(strArr[i]);
console.log(strArr)

I think array.flat(Infinity) is a perfect solution. But flat function is a relatively new function and may not run in older versions of browsers. We can use recursive function for solving this.
const arr = ["A", ["B", [["B11", "B12", ["B131", "B132"]], "B2"]], "C", ["D", "E", "F", ["G", "H", "I"]]]
const flatArray = (arr) => {
const res = []
for (const item of arr) {
if (Array.isArray(item)) {
const subRes = flatArray(item)
res.push(...subRes)
} else {
res.push(item)
}
}
return res
}
console.log(flatArray(arr))

Merge multiple arrays from one array in Javascript [duplicate]

I have a JavaScript array like:
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
How would I go about merging the separate inner arrays into one like:
["$6", "$12", "$25", ...]

ES2019
ES2019 introduced the Array.prototype.flat() method which you could use to flatten the arrays. It is compatible with most environments, although it is only available in Node.js starting with version 11, and not at all in Internet Explorer.
const arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
const merge3 = arrays.flat(1); //The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
console.log(merge3);
Older browsers
For older browsers, you can use Array.prototype.concat to merge arrays:
var arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
var merged = [].concat.apply([], arrays);
console.log(merged);
Using the apply method of concat will just take the second parameter as an array, so the last line is identical to this:
var merged = [].concat(["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]);

Here's a short function that uses some of the newer JavaScript array methods to flatten an n-dimensional array.
function flatten(arr) {
return arr.reduce(function (flat, toFlatten) {
return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
}, []);
}
Usage:
flatten([[1, 2, 3], [4, 5]]); // [1, 2, 3, 4, 5]
flatten([[[1, [1.1]], 2, 3], [4, 5]]); // [1, 1.1, 2, 3, 4, 5]

There is a confusingly hidden method, which constructs a new array without mutating the original one:
var oldArray = [[1],[2,3],[4]];
var newArray = Array.prototype.concat.apply([], oldArray);
console.log(newArray); // [ 1, 2, 3, 4 ]

It can be best done by javascript reduce function.
var arrays = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"], ["$0"], ["$15"],["$3"], ["$75"], ["$5"], ["$100"], ["$7"], ["$3"], ["$75"], ["$5"]];
arrays = arrays.reduce(function(a, b){
return a.concat(b);
}, []);
Or, with ES2015:
arrays = arrays.reduce((a, b) => a.concat(b), []);
js-fiddle
Mozilla docs

There's a new native method called flat to do this exactly.
(As of late 2019, flat is now published in the ECMA 2019 standard, and core-js#3 (babel's library) includes it in their polyfill library)
const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
// Flatten 2 levels deep
const arr3 = [2, 2, 5, [5, [5, [6]], 7]];
arr3.flat(2);
// [2, 2, 5, 5, 5, [6], 7];
// Flatten all levels
const arr4 = [2, 2, 5, [5, [5, [6]], 7]];
arr4.flat(Infinity);
// [2, 2, 5, 5, 5, 6, 7];

Most of the answers here don't work on huge (e.g. 200 000 elements) arrays, and even if they do, they're slow.
Here is the fastest solution, which works also on arrays with multiple levels of nesting:
const flatten = function(arr, result = []) {
for (let i = 0, length = arr.length; i < length; i++) {
const value = arr[i];
if (Array.isArray(value)) {
flatten(value, result);
} else {
result.push(value);
}
}
return result;
};
Examples
Huge arrays
flatten(Array(200000).fill([1]));
It handles huge arrays just fine. On my machine this code takes about 14 ms to execute.
Nested arrays
flatten(Array(2).fill(Array(2).fill(Array(2).fill([1]))));
It works with nested arrays. This code produces [1, 1, 1, 1, 1, 1, 1, 1].
Arrays with different levels of nesting
flatten([1, [1], [[1]]]);
It doesn't have any problems with flattening arrays like this one.

Update: it turned out that this solution doesn't work with large arrays. It you're looking for a better, faster solution, check out this answer.
function flatten(arr) {
return [].concat(...arr)
}
Is simply expands arr and passes it as arguments to concat(), which merges all the arrays into one. It's equivalent to [].concat.apply([], arr).
You can also try this for deep flattening:
function deepFlatten(arr) {
return flatten( // return shalowly flattened array
arr.map(x=> // with each x in array
Array.isArray(x) // is x an array?
? deepFlatten(x) // if yes, return deeply flattened x
: x // if no, return just x
)
)
}
See demo on JSBin.
References for ECMAScript 6 elements used in this answer:
Spread operator
Arrow functions
Side note: methods like find() and arrow functions are not supported by all browsers, but it doesn't mean that you can't use these features right now. Just use Babel — it transforms ES6 code into ES5.

You can use Underscore:
var x = [[1], [2], [3, 4]];
_.flatten(x); // => [1, 2, 3, 4]

Generic procedures mean we don't have to rewrite complexity each time we need to utilize a specific behaviour.
concatMap (or flatMap) is exactly what we need in this situation.
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// your sample data
const data =
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
console.log (flatten (data))
foresight
And yes, you guessed it correctly, it only flattens one level, which is exactly how it should work
Imagine some data set like this
// Player :: (String, Number) -> Player
const Player = (name,number) =>
[ name, number ]
// team :: ( . Player) -> Team
const Team = (...players) =>
players
// Game :: (Team, Team) -> Game
const Game = (teamA, teamB) =>
[ teamA, teamB ]
// sample data
const teamA =
Team (Player ('bob', 5), Player ('alice', 6))
const teamB =
Team (Player ('ricky', 4), Player ('julian', 2))
const game =
Game (teamA, teamB)
console.log (game)
// [ [ [ 'bob', 5 ], [ 'alice', 6 ] ],
// [ [ 'ricky', 4 ], [ 'julian', 2 ] ] ]
Ok, now say we want to print a roster that shows all the players that will be participating in game …
const gamePlayers = game =>
flatten (game)
gamePlayers (game)
// => [ [ 'bob', 5 ], [ 'alice', 6 ], [ 'ricky', 4 ], [ 'julian', 2 ] ]
If our flatten procedure flattened nested arrays too, we'd end up with this garbage result …
const gamePlayers = game =>
badGenericFlatten(game)
gamePlayers (game)
// => [ 'bob', 5, 'alice', 6, 'ricky', 4, 'julian', 2 ]
rollin' deep, baby
That's not to say sometimes you don't want to flatten nested arrays, too – only that shouldn't be the default behaviour.
We can make a deepFlatten procedure with ease …
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[0, [1, [2, [3, [4, 5], 6]]], [7, [8]], 9]
console.log (flatten (data))
// [ 0, 1, [ 2, [ 3, [ 4, 5 ], 6 ] ], 7, [ 8 ], 9 ]
console.log (deepFlatten (data))
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
There. Now you have a tool for each job – one for squashing one level of nesting, flatten, and one for obliterating all nesting deepFlatten.
Maybe you can call it obliterate or nuke if you don't like the name deepFlatten.
Don't iterate twice !
Of course the above implementations are clever and concise, but using a .map followed by a call to .reduce means we're actually doing more iterations than necessary
Using a trusty combinator I'm calling mapReduce helps keep the iterations to a minium; it takes a mapping function m :: a -> b, a reducing function r :: (b,a) ->b and returns a new reducing function - this combinator is at the heart of transducers; if you're interested, I've written other answers about them
// mapReduce = (a -> b, (b,a) -> b, (b,a) -> b)
const mapReduce = (m,r) =>
(acc,x) => r (acc, m (x))
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.reduce (mapReduce (f, concat), [])
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[ [ [ 1, 2 ],
[ 3, 4 ] ],
[ [ 5, 6 ],
[ 7, 8 ] ] ]
console.log (flatten (data))
// [ [ 1. 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]
console.log (deepFlatten (data))
// [ 1, 2, 3, 4, 5, 6, 7, 8 ]

To flatten an array of single element arrays, you don't need to import a library, a simple loop is both the simplest and most efficient solution :
for (var i = 0; i < a.length; i++) {
a[i] = a[i][0];
}
To downvoters: please read the question, don't downvote because it doesn't suit your very different problem. This solution is both the fastest and simplest for the asked question.

Another ECMAScript 6 solution in functional style:
Declare a function:
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
and use it:
flatten( [1, [2,3], [4,[5,[6]]]] ) // -> [1,2,3,4,5,6]
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
console.log( flatten([1, [2,3], [4,[5],[6,[7,8,9],10],11],[12],13]) )
Consider also a native function Array.prototype.flat() (proposal for ES6) available in last releases of modern browsers. Thanks to #(Константин Ван) and #(Mark Amery) mentioned it in the comments.
The flat function has one parameter, specifying the expected depth of array nesting, which equals 1 by default.
[1, 2, [3, 4]].flat(); // -> [1, 2, 3, 4]
[1, 2, [3, 4, [5, 6]]].flat(); // -> [1, 2, 3, 4, [5, 6]]
[1, 2, [3, 4, [5, 6]]].flat(2); // -> [1, 2, 3, 4, 5, 6]
[1, 2, [3, 4, [5, 6]]].flat(Infinity); // -> [1, 2, 3, 4, 5, 6]
let arr = [1, 2, [3, 4]];
console.log( arr.flat() );
arr = [1, 2, [3, 4, [5, 6]]];
console.log( arr.flat() );
console.log( arr.flat(1) );
console.log( arr.flat(2) );
console.log( arr.flat(Infinity) );

You can also try the new Array.flat() method. It works in the following manner:
let arr = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]].flat()
console.log(arr);
The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the 1 layer of depth (i.e. arrays inside arrays)
If you want to also flatten out 3 dimensional or even higher dimensional arrays you simply call the flat method multiple times. For example (3 dimensions):
let arr = [1,2,[3,4,[5,6]]].flat().flat().flat();
console.log(arr);
Be careful!
Array.flat() method is relatively new. Older browsers like ie might not have implemented the method. If you want you code to work on all browsers you might have to transpile your JS to an older version. Check for MDN web docs for current browser compatibility.

A solution for the more general case, when you may have some non-array elements in your array.
function flattenArrayOfArrays(a, r){
if(!r){ r = []}
for(var i=0; i<a.length; i++){
if(a[i].constructor == Array){
flattenArrayOfArrays(a[i], r);
}else{
r.push(a[i]);
}
}
return r;
}

What about using reduce(callback[, initialValue]) method of JavaScript 1.8
list.reduce((p,n) => p.concat(n),[]);
Would do the job.

const common = arr.reduce((a, b) => [...a, ...b], [])

You can use Array.flat() with Infinity for any depth of nested array.
var arr = [ [1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]], [[1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]]] ];
let flatten = arr.flat(Infinity)
console.log(flatten)
check here for browser compatibility

Please note: When Function.prototype.apply ([].concat.apply([], arrays)) or the spread operator ([].concat(...arrays)) is used in order to flatten an array, both can cause stack overflows for large arrays, because every argument of a function is stored on the stack.
Here is a stack-safe implementation in functional style that weighs up the most important requirements against one another:
reusability
readability
conciseness
performance
// small, reusable auxiliary functions:
const foldl = f => acc => xs => xs.reduce(uncurry(f), acc); // aka reduce
const uncurry = f => (a, b) => f(a) (b);
const concat = xs => y => xs.concat(y);
// the actual function to flatten an array - a self-explanatory one-line:
const flatten = xs => foldl(concat) ([]) (xs);
// arbitrary array sizes (until the heap blows up :D)
const xs = [[1,2,3],[4,5,6],[7,8,9]];
console.log(flatten(xs));
// Deriving a recursive solution for deeply nested arrays is trivially now
// yet more small, reusable auxiliary functions:
const map = f => xs => xs.map(apply(f));
const apply = f => a => f(a);
const isArray = Array.isArray;
// the derived recursive function:
const flattenr = xs => flatten(map(x => isArray(x) ? flattenr(x) : x) (xs));
const ys = [1,[2,[3,[4,[5],6,],7],8],9];
console.log(flattenr(ys));
As soon as you get used to small arrow functions in curried form, function composition and higher order functions, this code reads like prose. Programming then merely consists of putting together small building blocks that always work as expected, because they don't contain any side effects.

ES6 One Line Flatten
See lodash flatten, underscore flatten (shallow true)
function flatten(arr) {
return arr.reduce((acc, e) => acc.concat(e), []);
}
or
function flatten(arr) {
return [].concat.apply([], arr);
}
Tested with
test('already flatted', () => {
expect(flatten([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats first level', () => {
expect(flatten([1, [2, [3, [4]], 5]])).toEqual([1, 2, [3, [4]], 5]);
});
ES6 One Line Deep Flatten
See lodash flattenDeep, underscore flatten
function flattenDeep(arr) {
return arr.reduce((acc, e) => Array.isArray(e) ? acc.concat(flattenDeep(e)) : acc.concat(e), []);
}
Tested with
test('already flatted', () => {
expect(flattenDeep([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats', () => {
expect(flattenDeep([1, [2, [3, [4]], 5]])).toEqual([1, 2, 3, 4, 5]);
});

Using the spread operator:
const input = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]];
const output = [].concat(...input);
console.log(output); // --> ["$6", "$12", "$25", "$25", "$18", "$22", "$10"]

I recommend a space-efficient generator function:
function* flatten(arr) {
if (!Array.isArray(arr)) yield arr;
else for (let el of arr) yield* flatten(el);
}
// Example:
console.log(...flatten([1,[2,[3,[4]]]])); // 1 2 3 4
If desired, create an array of flattened values as follows:
let flattened = [...flatten([1,[2,[3,[4]]]])]; // [1, 2, 3, 4]

If you only have arrays with 1 string element:
[["$6"], ["$12"], ["$25"], ["$25"]].join(',').split(',');
will do the job. Bt that specifically matches your code example.

I have done it using recursion and closures
function flatten(arr) {
var temp = [];
function recursiveFlatten(arr) {
for(var i = 0; i < arr.length; i++) {
if(Array.isArray(arr[i])) {
recursiveFlatten(arr[i]);
} else {
temp.push(arr[i]);
}
}
}
recursiveFlatten(arr);
return temp;
}

A Haskellesque approach
function flatArray([x,...xs]){
return x ? [...Array.isArray(x) ? flatArray(x) : [x], ...flatArray(xs)] : [];
}
var na = [[1,2],[3,[4,5]],[6,7,[[[8],9]]],10];
fa = flatArray(na);
console.log(fa);

ES6 way:
const flatten = arr => arr.reduce((acc, next) => acc.concat(Array.isArray(next) ? flatten(next) : next), [])
const a = [1, [2, [3, [4, [5]]]]]
console.log(flatten(a))
ES5 way for flatten function with ES3 fallback for N-times nested arrays:
var flatten = (function() {
if (!!Array.prototype.reduce && !!Array.isArray) {
return function(array) {
return array.reduce(function(prev, next) {
return prev.concat(Array.isArray(next) ? flatten(next) : next);
}, []);
};
} else {
return function(array) {
var arr = [];
var i = 0;
var len = array.length;
var target;
for (; i < len; i++) {
target = array[i];
arr = arr.concat(
(Object.prototype.toString.call(target) === '[object Array]') ? flatten(target) : target
);
}
return arr;
};
}
}());
var a = [1, [2, [3, [4, [5]]]]];
console.log(flatten(a));

if you use lodash, you can just use its flatten method: https://lodash.com/docs/4.17.14#flatten
The nice thing about lodash is that it also has methods to flatten the arrays:
i) recursively: https://lodash.com/docs/4.17.14#flattenDeep
ii) upto n levels of nesting: https://lodash.com/docs/4.17.14#flattenDepth
For example
const _ = require("lodash");
const pancake = _.flatten(array)

I was goofing with ES6 Generators the other day and wrote this gist. Which contains...
function flatten(arrayOfArrays=[]){
function* flatgen() {
for( let item of arrayOfArrays ) {
if ( Array.isArray( item )) {
yield* flatten(item)
} else {
yield item
}
}
}
return [...flatgen()];
}
var flatArray = flatten([[1, [4]],[2],[3]]);
console.log(flatArray);
Basically I'm creating a generator that loops over the original input array, if it finds an array it uses the yield* operator in combination with recursion to continually flatten the internal arrays. If the item is not an array it just yields the single item. Then using the ES6 Spread operator (aka splat operator) I flatten out the generator into a new array instance.
I haven't tested the performance of this, but I figure it is a nice simple example of using generators and the yield* operator.
But again, I was just goofing so I'm sure there are more performant ways to do this.

just the best solution without lodash
let flatten = arr => [].concat.apply([], arr.map(item => Array.isArray(item) ? flatten(item) : item))

I would rather transform the whole array, as-is, to a string, but unlike other answers, would do that using JSON.stringify and not use the toString() method, which produce an unwanted result.
With that JSON.stringify output, all that's left is to remove all brackets, wrap the result with start & ending brackets yet again, and serve the result with JSON.parse which brings the string back to "life".
Can handle infinite nested arrays without any speed costs.
Can rightly handle Array items which are strings containing commas.
var arr = ["abc",[[[6]]],["3,4"],"2"];
var s = "[" + JSON.stringify(arr).replace(/\[|]/g,'') +"]";
var flattened = JSON.parse(s);
console.log(flattened)
Only for multidimensional Array of Strings/Numbers (not Objects)

Ways for making flatten array
using Es6 flat()
using Es6 reduce()
using recursion
using string manipulation
[1,[2,[3,[4,[5,[6,7],8],9],10]]] - [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
// using Es6 flat()
let arr = [1,[2,[3,[4,[5,[6,7],8],9],10]]]
console.log(arr.flat(Infinity))
// using Es6 reduce()
let flatIt = (array) => array.reduce(
(x, y) => x.concat(Array.isArray(y) ? flatIt(y) : y), []
)
console.log(flatIt(arr))
// using recursion
function myFlat(array) {
let flat = [].concat(...array);
return flat.some(Array.isArray) ? myFlat(flat) : flat;
}
console.log(myFlat(arr));
// using string manipulation
let strArr = arr.toString().split(',');
for(let i=0;i<strArr.length;i++)
strArr[i]=parseInt(strArr[i]);
console.log(strArr)

I think array.flat(Infinity) is a perfect solution. But flat function is a relatively new function and may not run in older versions of browsers. We can use recursive function for solving this.
const arr = ["A", ["B", [["B11", "B12", ["B131", "B132"]], "B2"]], "C", ["D", "E", "F", ["G", "H", "I"]]]
const flatArray = (arr) => {
const res = []
for (const item of arr) {
if (Array.isArray(item)) {
const subRes = flatArray(item)
res.push(...subRes)
} else {
res.push(item)
}
}
return res
}
console.log(flatArray(arr))

Terse way to intersperse element between all elements in JavaScript array?

Say I have an array var arr = [1, 2, 3], and I want to separate each element by an element eg. var sep = "&", so the output is [1, "&", 2, "&", 3].
Another way to think about it is I want to do Array.prototype.join (arr.join(sep)) without the result being a string (because the elements and separator I am trying to use are Objects, not strings).
Is there a functional/nice/elegant way to do this in either es6/7 or lodash without something that feels clunky like:
_.flatten(arr.map((el, i) => [el, i < arr.length-1 ? sep : null])) // too complex
or
_.flatten(arr.map(el => [el, sep]).slice(0,-1) // extra sep added, memory wasted
or even
arr.reduce((prev,curr) => { prev.push(curr, sep); return prev; }, []).slice(0,-1)
// probably the best out of the three, but I have to do a map already
// and I still have the same problem as the previous two - either
// inline ternary or slice
Edit: Haskell has this function, called intersperse

Using a generator:
function *intersperse(a, delim) {
let first = true;
for (const x of a) {
if (!first) yield delim;
first = false;
yield x;
}
}
console.log([...intersperse(array, '&')]);
Thanks to #Bergi for pointing out the useful generalization that the input could be any iterable.
If you don't like using generators, then
[].concat(...a.map(e => ['&', e])).slice(1)

A spread and explicit return in reducing function will make it more terse:
const intersperse = (arr, sep) => arr.reduce((a,v)=>[...a,v,sep],[]).slice(0,-1)
// intersperse([1,2,3], 'z')
// [1, "z", 2, "z", 3]

In ES6, you'd write a generator function that can produce an iterator which yields the input with the interspersed elements:
function* intersperse(iterable, separator) {
const iterator = iterable[Symbol.iterator]();
const first = iterator.next();
if (first.done) return;
else yield first.value;
for (const value of iterator) {
yield separator;
yield value;
}
}
console.log(Array.from(intersperse([1, 2, 3], "&")));

One straightforward approach could be like feeding the reduce function with an initial array in size one less than the double of our original array, filled with the character to be used for interspersing. Then mapping the elements of the original array at index i to 2*i in the initially fed target array would do the job perfectly..
In this approach i don't see (m)any redundant operations. Also since we are not modifying any of the array sizes after they are set, i wouldn't expect any background tasks to run for memory reallocation, optimization etc.
One other good part is using the standard array methods since they check all kinds of mismatch and whatnot.
This function returns a new array, in which the called upon array's items are interspersed with the provided argument.
var arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
Array.prototype.intersperse = function(s){
return this.reduce((p,c,i) => (p[2*i]=c,p), new Array(2*this.length-1).fill(s));
}
document.write("<pre>" + JSON.stringify(arr.intersperse("&")) + "</pre>");

Using reduce but without slice
var arr = ['a','b','c','d'];
var lastIndex = arr.length-1;
arr.reduce((res,x,index)=>{
res.push(x);
if(lastIndex !== index)
res.push('&');
return res;
},[]);

If you have Ramda in your dependencies or if willing to add it, there is intersperse method there.
From the docs:
Creates a new list with the separator interposed between elements.
Dispatches to the intersperse method of the second argument, if present.
R.intersperse('n', ['ba', 'a', 'a']); //=> ['ba', 'n', 'a', 'n', 'a']
Or you can check out the source for one of the ways to do it in your codebase. https://github.com/ramda/ramda/blob/v0.24.1/src/intersperse.js

You could use Array.from to create an array with the final size, and then use the callback argument to actually populate it:
const intersperse = (arr, sep) => Array.from(
{ length: Math.max(0, arr.length * 2 - 1) },
(_, i) => i % 2 ? sep : arr[i >> 1]
);
// Demo:
let res = intersperse([1, 2, 3], "&");
console.log(res);

ONE-LINER and FAST
const intersperse = (ar,s)=>[...Array(2*ar.length-1)].map((_,i)=>i%2?s:ar[i/2]);
console.log(intersperse([1, 2, 3], '&'));

javascript has a method join() and split()
var arr = ['a','b','c','d'];
arr = arr.join('&');
document.writeln(arr);
Output should be: a&b&c&d
now split again:
arr = arr.split("");
arr is now:
arr = ['a','&','b','&','c','&','d'];

if (!Array.prototype.intersperse) {
Object.defineProperty(Array.prototype, 'intersperse', {
value: function(something) {
if (this === null) {
throw new TypeError( 'Array.prototype.intersperse ' +
'called on null or undefined' );
}
var isFunc = (typeof something == 'function')
return this.concat.apply([],
this.map(function(e,i) {
return i ? [isFunc ? something(this[i-1]) : something, e] : [e] }.bind(this)))
}
});
}

you can also use the following:
var arr =['a', 'b', 'c', 'd'];
arr.forEach(function(element, index, array){
array.splice(2*index+1, 0, '&');
});
arr.pop();

My take:
const _ = require('lodash');
_.mixin({
intersperse(array, sep) {
return _(array)
.flatMap(x => [x, sep])
.take(2 * array.length - 1)
.value();
},
});
// _.intersperse(["a", "b", "c"], "-")
// > ["a", "-", "b", "-", "c"]

const arr = [1, 2, 3];
function intersperse(items, separator) {
const result = items.reduce(
(res, el) => [...res, el, separator], []);
result.pop();
return result;
}
console.log(intersperse(arr, '&'));

A few years later, here's a recursive generator solution. Enjoy!
const intersperse = function *([first, ...rest], delim){
yield first;
if(!rest.length){
return;
}
yield delim;
yield * intersperse(rest, delim);
};
console.log([...intersperse(array, '&')]);

export const intersperse = (array, insertSeparator) => {
if (!isArray(array)) {
throw new Error(`Wrong argument in intersperse function, expected array, got ${typeof array}`);
}
if (!isFunction(insertSeparator)) {
throw new Error(`Wrong argument in intersperse function, expected function, got ${typeof insertSeparator}`);
}
return flatMap(
array,
(item, index) => index > 0 ? [insertSeparator(item, index), item] : [item] // eslint-disable-line no-confusing-arrow
);
};

Here is also a version with reduce only:
const intersperse = (xs, s) => xs.reduce((acc, x) => acc ? [...acc, s, x] : [x], null)
const a = [1, 2, 3, 4, 5]
console.log(intersperse(a, 0))
// [1, 0, 2, 0, 3, 0, 4, 0, 5]

Updated for objects not using join method:
for (var i=0;i<arr.length;i++;) {
newarr.push(arr[i]);
if(i>0) {
newarr.push('&');
}
}
newarr should be:
newarr = ['a','&','b','&','c','&','d'];

Function to bring multidimensional array to single dimension? [duplicate]

I have a JavaScript array like:
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
How would I go about merging the separate inner arrays into one like:
["$6", "$12", "$25", ...]

ES2019
ES2019 introduced the Array.prototype.flat() method which you could use to flatten the arrays. It is compatible with most environments, although it is only available in Node.js starting with version 11, and not at all in Internet Explorer.
const arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
const merge3 = arrays.flat(1); //The depth level specifying how deep a nested array structure should be flattened. Defaults to 1.
console.log(merge3);
Older browsers
For older browsers, you can use Array.prototype.concat to merge arrays:
var arrays = [
["$6"],
["$12"],
["$25"],
["$25"],
["$18"],
["$22"],
["$10"]
];
var merged = [].concat.apply([], arrays);
console.log(merged);
Using the apply method of concat will just take the second parameter as an array, so the last line is identical to this:
var merged = [].concat(["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]);

Here's a short function that uses some of the newer JavaScript array methods to flatten an n-dimensional array.
function flatten(arr) {
return arr.reduce(function (flat, toFlatten) {
return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
}, []);
}
Usage:
flatten([[1, 2, 3], [4, 5]]); // [1, 2, 3, 4, 5]
flatten([[[1, [1.1]], 2, 3], [4, 5]]); // [1, 1.1, 2, 3, 4, 5]

There is a confusingly hidden method, which constructs a new array without mutating the original one:
var oldArray = [[1],[2,3],[4]];
var newArray = Array.prototype.concat.apply([], oldArray);
console.log(newArray); // [ 1, 2, 3, 4 ]

It can be best done by javascript reduce function.
var arrays = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"], ["$0"], ["$15"],["$3"], ["$75"], ["$5"], ["$100"], ["$7"], ["$3"], ["$75"], ["$5"]];
arrays = arrays.reduce(function(a, b){
return a.concat(b);
}, []);
Or, with ES2015:
arrays = arrays.reduce((a, b) => a.concat(b), []);
js-fiddle
Mozilla docs

There's a new native method called flat to do this exactly.
(As of late 2019, flat is now published in the ECMA 2019 standard, and core-js#3 (babel's library) includes it in their polyfill library)
const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
// Flatten 2 levels deep
const arr3 = [2, 2, 5, [5, [5, [6]], 7]];
arr3.flat(2);
// [2, 2, 5, 5, 5, [6], 7];
// Flatten all levels
const arr4 = [2, 2, 5, [5, [5, [6]], 7]];
arr4.flat(Infinity);
// [2, 2, 5, 5, 5, 6, 7];

Most of the answers here don't work on huge (e.g. 200 000 elements) arrays, and even if they do, they're slow.
Here is the fastest solution, which works also on arrays with multiple levels of nesting:
const flatten = function(arr, result = []) {
for (let i = 0, length = arr.length; i < length; i++) {
const value = arr[i];
if (Array.isArray(value)) {
flatten(value, result);
} else {
result.push(value);
}
}
return result;
};
Examples
Huge arrays
flatten(Array(200000).fill([1]));
It handles huge arrays just fine. On my machine this code takes about 14 ms to execute.
Nested arrays
flatten(Array(2).fill(Array(2).fill(Array(2).fill([1]))));
It works with nested arrays. This code produces [1, 1, 1, 1, 1, 1, 1, 1].
Arrays with different levels of nesting
flatten([1, [1], [[1]]]);
It doesn't have any problems with flattening arrays like this one.

Update: it turned out that this solution doesn't work with large arrays. It you're looking for a better, faster solution, check out this answer.
function flatten(arr) {
return [].concat(...arr)
}
Is simply expands arr and passes it as arguments to concat(), which merges all the arrays into one. It's equivalent to [].concat.apply([], arr).
You can also try this for deep flattening:
function deepFlatten(arr) {
return flatten( // return shalowly flattened array
arr.map(x=> // with each x in array
Array.isArray(x) // is x an array?
? deepFlatten(x) // if yes, return deeply flattened x
: x // if no, return just x
)
)
}
See demo on JSBin.
References for ECMAScript 6 elements used in this answer:
Spread operator
Arrow functions
Side note: methods like find() and arrow functions are not supported by all browsers, but it doesn't mean that you can't use these features right now. Just use Babel — it transforms ES6 code into ES5.

You can use Underscore:
var x = [[1], [2], [3, 4]];
_.flatten(x); // => [1, 2, 3, 4]

Generic procedures mean we don't have to rewrite complexity each time we need to utilize a specific behaviour.
concatMap (or flatMap) is exactly what we need in this situation.
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// your sample data
const data =
[["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]]
console.log (flatten (data))
foresight
And yes, you guessed it correctly, it only flattens one level, which is exactly how it should work
Imagine some data set like this
// Player :: (String, Number) -> Player
const Player = (name,number) =>
[ name, number ]
// team :: ( . Player) -> Team
const Team = (...players) =>
players
// Game :: (Team, Team) -> Game
const Game = (teamA, teamB) =>
[ teamA, teamB ]
// sample data
const teamA =
Team (Player ('bob', 5), Player ('alice', 6))
const teamB =
Team (Player ('ricky', 4), Player ('julian', 2))
const game =
Game (teamA, teamB)
console.log (game)
// [ [ [ 'bob', 5 ], [ 'alice', 6 ] ],
// [ [ 'ricky', 4 ], [ 'julian', 2 ] ] ]
Ok, now say we want to print a roster that shows all the players that will be participating in game …
const gamePlayers = game =>
flatten (game)
gamePlayers (game)
// => [ [ 'bob', 5 ], [ 'alice', 6 ], [ 'ricky', 4 ], [ 'julian', 2 ] ]
If our flatten procedure flattened nested arrays too, we'd end up with this garbage result …
const gamePlayers = game =>
badGenericFlatten(game)
gamePlayers (game)
// => [ 'bob', 5, 'alice', 6, 'ricky', 4, 'julian', 2 ]
rollin' deep, baby
That's not to say sometimes you don't want to flatten nested arrays, too – only that shouldn't be the default behaviour.
We can make a deepFlatten procedure with ease …
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.map(f).reduce(concat, [])
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[0, [1, [2, [3, [4, 5], 6]]], [7, [8]], 9]
console.log (flatten (data))
// [ 0, 1, [ 2, [ 3, [ 4, 5 ], 6 ] ], 7, [ 8 ], 9 ]
console.log (deepFlatten (data))
// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
There. Now you have a tool for each job – one for squashing one level of nesting, flatten, and one for obliterating all nesting deepFlatten.
Maybe you can call it obliterate or nuke if you don't like the name deepFlatten.
Don't iterate twice !
Of course the above implementations are clever and concise, but using a .map followed by a call to .reduce means we're actually doing more iterations than necessary
Using a trusty combinator I'm calling mapReduce helps keep the iterations to a minium; it takes a mapping function m :: a -> b, a reducing function r :: (b,a) ->b and returns a new reducing function - this combinator is at the heart of transducers; if you're interested, I've written other answers about them
// mapReduce = (a -> b, (b,a) -> b, (b,a) -> b)
const mapReduce = (m,r) =>
(acc,x) => r (acc, m (x))
// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = f => xs =>
xs.reduce (mapReduce (f, concat), [])
// concat :: ([a],[a]) -> [a]
const concat = (xs,ys) =>
xs.concat (ys)
// id :: a -> a
const id = x =>
x
// flatten :: [[a]] -> [a]
const flatten =
concatMap (id)
// deepFlatten :: [[a]] -> [a]
const deepFlatten =
concatMap (x =>
Array.isArray (x) ? deepFlatten (x) : x)
// your sample data
const data =
[ [ [ 1, 2 ],
[ 3, 4 ] ],
[ [ 5, 6 ],
[ 7, 8 ] ] ]
console.log (flatten (data))
// [ [ 1. 2 ], [ 3, 4 ], [ 5, 6 ], [ 7, 8 ] ]
console.log (deepFlatten (data))
// [ 1, 2, 3, 4, 5, 6, 7, 8 ]

To flatten an array of single element arrays, you don't need to import a library, a simple loop is both the simplest and most efficient solution :
for (var i = 0; i < a.length; i++) {
a[i] = a[i][0];
}
To downvoters: please read the question, don't downvote because it doesn't suit your very different problem. This solution is both the fastest and simplest for the asked question.

Another ECMAScript 6 solution in functional style:
Declare a function:
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
and use it:
flatten( [1, [2,3], [4,[5,[6]]]] ) // -> [1,2,3,4,5,6]
const flatten = arr => arr.reduce(
(a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);
console.log( flatten([1, [2,3], [4,[5],[6,[7,8,9],10],11],[12],13]) )
Consider also a native function Array.prototype.flat() (proposal for ES6) available in last releases of modern browsers. Thanks to #(Константин Ван) and #(Mark Amery) mentioned it in the comments.
The flat function has one parameter, specifying the expected depth of array nesting, which equals 1 by default.
[1, 2, [3, 4]].flat(); // -> [1, 2, 3, 4]
[1, 2, [3, 4, [5, 6]]].flat(); // -> [1, 2, 3, 4, [5, 6]]
[1, 2, [3, 4, [5, 6]]].flat(2); // -> [1, 2, 3, 4, 5, 6]
[1, 2, [3, 4, [5, 6]]].flat(Infinity); // -> [1, 2, 3, 4, 5, 6]
let arr = [1, 2, [3, 4]];
console.log( arr.flat() );
arr = [1, 2, [3, 4, [5, 6]]];
console.log( arr.flat() );
console.log( arr.flat(1) );
console.log( arr.flat(2) );
console.log( arr.flat(Infinity) );

You can also try the new Array.flat() method. It works in the following manner:
let arr = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]].flat()
console.log(arr);
The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the 1 layer of depth (i.e. arrays inside arrays)
If you want to also flatten out 3 dimensional or even higher dimensional arrays you simply call the flat method multiple times. For example (3 dimensions):
let arr = [1,2,[3,4,[5,6]]].flat().flat().flat();
console.log(arr);
Be careful!
Array.flat() method is relatively new. Older browsers like ie might not have implemented the method. If you want you code to work on all browsers you might have to transpile your JS to an older version. Check for MDN web docs for current browser compatibility.

A solution for the more general case, when you may have some non-array elements in your array.
function flattenArrayOfArrays(a, r){
if(!r){ r = []}
for(var i=0; i<a.length; i++){
if(a[i].constructor == Array){
flattenArrayOfArrays(a[i], r);
}else{
r.push(a[i]);
}
}
return r;
}

What about using reduce(callback[, initialValue]) method of JavaScript 1.8
list.reduce((p,n) => p.concat(n),[]);
Would do the job.

const common = arr.reduce((a, b) => [...a, ...b], [])

You can use Array.flat() with Infinity for any depth of nested array.
var arr = [ [1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]], [[1,2,3,4], [1,2,[1,2,3]], [1,2,3,4,5,[1,2,3,4,[1,2,3,4]]]] ];
let flatten = arr.flat(Infinity)
console.log(flatten)
check here for browser compatibility

Please note: When Function.prototype.apply ([].concat.apply([], arrays)) or the spread operator ([].concat(...arrays)) is used in order to flatten an array, both can cause stack overflows for large arrays, because every argument of a function is stored on the stack.
Here is a stack-safe implementation in functional style that weighs up the most important requirements against one another:
reusability
readability
conciseness
performance
// small, reusable auxiliary functions:
const foldl = f => acc => xs => xs.reduce(uncurry(f), acc); // aka reduce
const uncurry = f => (a, b) => f(a) (b);
const concat = xs => y => xs.concat(y);
// the actual function to flatten an array - a self-explanatory one-line:
const flatten = xs => foldl(concat) ([]) (xs);
// arbitrary array sizes (until the heap blows up :D)
const xs = [[1,2,3],[4,5,6],[7,8,9]];
console.log(flatten(xs));
// Deriving a recursive solution for deeply nested arrays is trivially now
// yet more small, reusable auxiliary functions:
const map = f => xs => xs.map(apply(f));
const apply = f => a => f(a);
const isArray = Array.isArray;
// the derived recursive function:
const flattenr = xs => flatten(map(x => isArray(x) ? flattenr(x) : x) (xs));
const ys = [1,[2,[3,[4,[5],6,],7],8],9];
console.log(flattenr(ys));
As soon as you get used to small arrow functions in curried form, function composition and higher order functions, this code reads like prose. Programming then merely consists of putting together small building blocks that always work as expected, because they don't contain any side effects.

ES6 One Line Flatten
See lodash flatten, underscore flatten (shallow true)
function flatten(arr) {
return arr.reduce((acc, e) => acc.concat(e), []);
}
or
function flatten(arr) {
return [].concat.apply([], arr);
}
Tested with
test('already flatted', () => {
expect(flatten([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats first level', () => {
expect(flatten([1, [2, [3, [4]], 5]])).toEqual([1, 2, [3, [4]], 5]);
});
ES6 One Line Deep Flatten
See lodash flattenDeep, underscore flatten
function flattenDeep(arr) {
return arr.reduce((acc, e) => Array.isArray(e) ? acc.concat(flattenDeep(e)) : acc.concat(e), []);
}
Tested with
test('already flatted', () => {
expect(flattenDeep([1, 2, 3, 4, 5])).toEqual([1, 2, 3, 4, 5]);
});
test('flats', () => {
expect(flattenDeep([1, [2, [3, [4]], 5]])).toEqual([1, 2, 3, 4, 5]);
});

Using the spread operator:
const input = [["$6"], ["$12"], ["$25"], ["$25"], ["$18"], ["$22"], ["$10"]];
const output = [].concat(...input);
console.log(output); // --> ["$6", "$12", "$25", "$25", "$18", "$22", "$10"]

I recommend a space-efficient generator function:
function* flatten(arr) {
if (!Array.isArray(arr)) yield arr;
else for (let el of arr) yield* flatten(el);
}
// Example:
console.log(...flatten([1,[2,[3,[4]]]])); // 1 2 3 4
If desired, create an array of flattened values as follows:
let flattened = [...flatten([1,[2,[3,[4]]]])]; // [1, 2, 3, 4]

If you only have arrays with 1 string element:
[["$6"], ["$12"], ["$25"], ["$25"]].join(',').split(',');
will do the job. Bt that specifically matches your code example.

I have done it using recursion and closures
function flatten(arr) {
var temp = [];
function recursiveFlatten(arr) {
for(var i = 0; i < arr.length; i++) {
if(Array.isArray(arr[i])) {
recursiveFlatten(arr[i]);
} else {
temp.push(arr[i]);
}
}
}
recursiveFlatten(arr);
return temp;
}

A Haskellesque approach
function flatArray([x,...xs]){
return x ? [...Array.isArray(x) ? flatArray(x) : [x], ...flatArray(xs)] : [];
}
var na = [[1,2],[3,[4,5]],[6,7,[[[8],9]]],10];
fa = flatArray(na);
console.log(fa);

ES6 way:
const flatten = arr => arr.reduce((acc, next) => acc.concat(Array.isArray(next) ? flatten(next) : next), [])
const a = [1, [2, [3, [4, [5]]]]]
console.log(flatten(a))
ES5 way for flatten function with ES3 fallback for N-times nested arrays:
var flatten = (function() {
if (!!Array.prototype.reduce && !!Array.isArray) {
return function(array) {
return array.reduce(function(prev, next) {
return prev.concat(Array.isArray(next) ? flatten(next) : next);
}, []);
};
} else {
return function(array) {
var arr = [];
var i = 0;
var len = array.length;
var target;
for (; i < len; i++) {
target = array[i];
arr = arr.concat(
(Object.prototype.toString.call(target) === '[object Array]') ? flatten(target) : target
);
}
return arr;
};
}
}());
var a = [1, [2, [3, [4, [5]]]]];
console.log(flatten(a));

if you use lodash, you can just use its flatten method: https://lodash.com/docs/4.17.14#flatten
The nice thing about lodash is that it also has methods to flatten the arrays:
i) recursively: https://lodash.com/docs/4.17.14#flattenDeep
ii) upto n levels of nesting: https://lodash.com/docs/4.17.14#flattenDepth
For example
const _ = require("lodash");
const pancake = _.flatten(array)

I was goofing with ES6 Generators the other day and wrote this gist. Which contains...
function flatten(arrayOfArrays=[]){
function* flatgen() {
for( let item of arrayOfArrays ) {
if ( Array.isArray( item )) {
yield* flatten(item)
} else {
yield item
}
}
}
return [...flatgen()];
}
var flatArray = flatten([[1, [4]],[2],[3]]);
console.log(flatArray);
Basically I'm creating a generator that loops over the original input array, if it finds an array it uses the yield* operator in combination with recursion to continually flatten the internal arrays. If the item is not an array it just yields the single item. Then using the ES6 Spread operator (aka splat operator) I flatten out the generator into a new array instance.
I haven't tested the performance of this, but I figure it is a nice simple example of using generators and the yield* operator.
But again, I was just goofing so I'm sure there are more performant ways to do this.

just the best solution without lodash
let flatten = arr => [].concat.apply([], arr.map(item => Array.isArray(item) ? flatten(item) : item))

I would rather transform the whole array, as-is, to a string, but unlike other answers, would do that using JSON.stringify and not use the toString() method, which produce an unwanted result.
With that JSON.stringify output, all that's left is to remove all brackets, wrap the result with start & ending brackets yet again, and serve the result with JSON.parse which brings the string back to "life".
Can handle infinite nested arrays without any speed costs.
Can rightly handle Array items which are strings containing commas.
var arr = ["abc",[[[6]]],["3,4"],"2"];
var s = "[" + JSON.stringify(arr).replace(/\[|]/g,'') +"]";
var flattened = JSON.parse(s);
console.log(flattened)
Only for multidimensional Array of Strings/Numbers (not Objects)

Ways for making flatten array
using Es6 flat()
using Es6 reduce()
using recursion
using string manipulation
[1,[2,[3,[4,[5,[6,7],8],9],10]]] - [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
// using Es6 flat()
let arr = [1,[2,[3,[4,[5,[6,7],8],9],10]]]
console.log(arr.flat(Infinity))
// using Es6 reduce()
let flatIt = (array) => array.reduce(
(x, y) => x.concat(Array.isArray(y) ? flatIt(y) : y), []
)
console.log(flatIt(arr))
// using recursion
function myFlat(array) {
let flat = [].concat(...array);
return flat.some(Array.isArray) ? myFlat(flat) : flat;
}
console.log(myFlat(arr));
// using string manipulation
let strArr = arr.toString().split(',');
for(let i=0;i<strArr.length;i++)
strArr[i]=parseInt(strArr[i]);
console.log(strArr)

I think array.flat(Infinity) is a perfect solution. But flat function is a relatively new function and may not run in older versions of browsers. We can use recursive function for solving this.
const arr = ["A", ["B", [["B11", "B12", ["B131", "B132"]], "B2"]], "C", ["D", "E", "F", ["G", "H", "I"]]]
const flatArray = (arr) => {
const res = []
for (const item of arr) {
if (Array.isArray(item)) {
const subRes = flatArray(item)
res.push(...subRes)
} else {
res.push(item)
}
}
return res
}
console.log(flatArray(arr))