Ways to create a Set in JavaScript? - javascript

In Eloquent JavaScript, Chapter 4, a set of values is created by creating an object and storing the values as property names, assigning arbitrary values (e.g. true) as property values. To check if the value is already contained in the set, the in operator is used:
var set = {};
if (!'Tom' in set) {
set.Tom = true;
}
Is this idiomatic JavaScript? Wouldn't be using an array even better?
var set = [];
if (!'Tom' in set) {
set.push = 'Tom';
}

Sets are now available in ES2015 (aka ES6, i.e. ECMAScript 6). ES6 has been the current standard for JavaScript since June 2015.
ECMAScript 6 has the data structure Set which works for arbitrary
values, is fast and handles NaN correctly. -Axel Rauschmayer, Exploring ES6
First two examples from Axel Rauschmayer's book Exploring ES6:
Managing single elements:
> let set = new Set();
> set.add('red')
> set.has('red')
true
> set.delete('red')
true
> set.has('red')
false
Determining the size of a Set and clearing it:
> let set = new Set();
> set.add('red')
> set.add('green')
> set.size
2
> set.clear();
> set.size
0
I would check out Exploring ES6 if you want to learn more about Sets in JavaScript. The book is free to read online, but if you would like to support the author Dr. Axel Rauschmayer you can purchase the book for around $30.
If you want to use Sets and ES6 now you can use Babel, the ES6 to ES5 transpiler, and its polyfills.
Edit: As of June 6th, 2017 most of the major browsers have full Set support in their latest versions (except IE 11). This means you may not need babel if you don't care to support older browsers. If you want to see compatibility in different browsers including your current browser check Kangax's ES6 compatibility table.
EDIT:
Just clarification on initialization. Sets can take any synchronous iterable in their constructor. This means they can take not just arrays but also strings, and iterators. Take for example the following array and string initialization of a set:
const set1 = new Set(['a','a','b','b','c','c']);
console.log(...set1);
console.log(set1.size);
const set2 = new Set("aabbcc");
console.log(...set2);
console.log(set2.size);
Both outputs of the array and string are the same. Note that ...set1 is the spread syntax. It appears that each element of the iterable is added one by one to the set, so since both the array and string have the same elements and since the elements are in the same order the set is created the same. Another thing to note about sets is when iterating over them the iteration order follows the order that the elements were inserted into the set. Here's an example of iterating over a set:
const set1 = new Set(['a','a','b','b','c','c']);
for(const element of set1) {
console.log(element);
}
Since you can use any iterable to initialize a set you could even use a iterator from a generator function. Here is two such examples of iterator initializations that produce the same output:
// a simple generator example
function* getLetters1 () {
yield 'a';
yield 'a';
yield 'b';
yield 'b';
yield 'c';
yield 'c';
}
// a somewhat more commonplace generator example
// with the same output as getLetters1.
function* getLetters2 (letters, repeatTimes) {
for(const letter of letters) {
for(let i = 0; i < repeatTimes; ++i) {
yield letter;
}
}
}
console.log("------ getLetters1 ------");
console.log(...getLetters1());
const set3 = new Set(getLetters1());
console.log(...set3);
console.log(set3.size);
console.log("------ getLetters2 ------");
console.log(...getLetters2('abc', 2));
const set4 = new Set(getLetters2('abc', 2));
console.log(...set4);
console.log(set4.size);
These examples' generator functions could just be written to not repeat, but if the generator function is more complicated and as long as the following doesn't impact performance too negatively you could use the Set method to help get only values from a generator that don't repeat.
If you want to know more about sets without reading Dr. Rauschmayer's chapter of his book you can check out the MDN docs on Set. MDN also has more examples of iterating over a set such as using forEach and using the .keys, .values, and .entries methods. MDN also has examples such as set union, set intersection, set difference, symmetric set difference, and set superset checking. Hopefully most of those operations will become available in JavaScript without needing to build your own functions supporting them. In fact, there is this TC39 proposal for new Set methods which should hopefully add the following methods to Set in JavaScript at some future point in time if the proposal reaches stage 4:
Set.prototype.intersection(iterable) - method creates new Set instance by set intersection operation.
Set.prototype.union(iterable) - method creates new Set instance by set union operation.
Set.prototype.difference(iterable) - method creates new Set without elements present in iterable.
Set.prototype.symmetricDifference(iterable) - returns Set of elements found only in either this or in iterable.
Set.prototype.isSubsetOf(iterable)
Set.prototype.isDisjointFrom(iterable)
Set.prototype.isSupersetOf(iterable)

I use dict objects as sets. This works with strings and numbers, but I suppose would cause problems if you wanted to have a set of objects using custom equality and comparison operators:
Creating a set:
var example_set =
{
'a':true,
'b':true,
'c':true
}
Testing for inclusion in a set
if( example_set['a'] ){
alert('"a" is in set');
}
Adding an element to a set
example_set['d'] = true;
Removing an element from a set
delete example_set['a'];

Sets do not allow duplicate entries and don't typically guarantee predefined ordering. Arrays do both of these, thus violating what it means to be a set (unless you do additional checks).

The first way is idiomatic JavaScript.
Any time you want to store a key/value pair, you must use a JavaScript object. As for arrays, there are several problems:
The index is a numerical value.
No easy way to check to see if a value is in an array without looping through.
A set doesn't allow duplicates. An array does.

If you want to create a set from an array, simply do:
let arr = [1, 1, 2, 1, 3];
let mySet = new Set(arr); // Set { 1, 2, 3 }
This is a sugar syntax that I quite fancied when programming in Python, so glad that ES6 finally made it possible to do the same thing.
NOTE: then I realize what I said didn't directly answer your question. The reason you have this "hack" in ES5 is because lookup time in an object by keys is significantly faster (O(1)) than in an array (O(n)). In performance critical applications, you can sacrifice this bit of readability or intuition for better performance.
But hey, welcome to 2017, where you can use proper Set in all major modern browsers now!

Sets in ES6/ES2015:
ES6/ES2015 now has built in sets. A set is data structure which allows storage of unique values of any type, whether this are primitive values or object references. A set can be declared using the ES6 built in set constructor in the following manner:
const set = new Set([1, 2, 3, 4, 5]);
When creating a set using the Set constructor our newly created set object inherits from the Set.prototype. This has all sorts of auxiliary methods and properties. This allows you to easily do the following things:
Example:
const set = new Set([1, 2, 3, 4, 5]);
// checkout the size of the set
console.log('size is: ' + set.size);
// has method returns a boolean, true if the item is in the set
console.log(set.has(1));
// add a number
set.add(6);
// delete a number
set.delete(1);
// iterate over each element using a callback
set.forEach((el) => {
console.log(el);
});
// remove all the entries from the set
set.clear();
Browser compatibility:
All major browser now fully support sets except IE where some features are missing. For exact reference please refer to the mdn docs.

There are two problems with using bare javascript objects to emulate sets: first, an object can have an inherited property which would screw the "in" operator and second, you can only store scalar values in this way, making a set of objects is not possible. Therefore, a realistic implementation of Sets should provide methods add and contains instead of plain in and property assignments.

You can try Buckets, is a javascript data structure library and has everything you need to manipulate sets.

Basic creation and usage of Set object đź”·
let mySet = new Set()
mySet.add(2) // Set {2}
mySet.add(7) // Set {2, 7}
mySet.add(7) // Set {2, 7}
mySet.add('my text') // Set {2, 7, 'my text'}
let myObj = { a: 1, b: 2 }
mySet.add(myObj) // Set {2, 7, 'my text', {...}}
mySet.has(2) // true
mySet.has(myObj) // true
mySet.size // 4
Iteration
for (let item of mySet) console.log(item) // 2, 7, 'my text', {a:1, b:2}
mySet.forEach(value => console.log(value)) // 2, 7, 'my text', {a:1, b:2}
Convert to array
var myArr = Array.from(mySet) // [2, 7, 'my text', {a:1, b:2}]
âť• The most distinct feature Set offers is every value in Set object must be
unique. So you can not add duplicate values.

Related

Cloning arrays of objects with Object.assign

I discovered a bug on a project I'm working on that can be replicated by this snippet:
const original = [ { value: 1 } ];
function test() {
const copy = Object.assign([], original);
copy.forEach(obj => obj.value = obj.value + 1);
}
console.log(original[0].value); // -> 1, expected 1
test();
console.log(original[0].value); // -> 2, expected 1
test();
console.log(original[0].value); // -> 3, expected 1
I do not understand why this is the case. In the MDN web docs, the following statements can be found in the deep copy warning section:
For deep cloning, we need to use alternatives, because Object.assign() copies property values.
If the source value is a reference to an object, it only copies the reference value.
How do these notes apply to arrays / in this case? Are array values somehow considered as properties?
Looking back now, the method was probably not intended to work with arrays, so I guess I reap what I sow... but I'd still like to understand what's going on here. The intent was to deep copy the array in order to mutate the objects inside while keeping the original intact.
Are array values somehow considered as properties?
Yes. In JavaScript, arrays are objects (which is why Object.assign works with them), and properties with a special class of names called array indexes (strings defining decimal numbers in standard form with numeric values < 232 - 1) represent the elements of the array. (Naturally, JavaScript engines optimize them into true arrays when they can, but they're defined as objects and performing object operations on them is fully supported.) I found this sufficiently surprising when getting deep into JavaScript that I wrote it up on my anemic old blog.
Given:
const obj = {a: 1};
const arr = [1];
these two operations are the same from a specification viewpoint:
console.log(obj["a"]);
console.log(arr["0"]); // Yes, in quotes
Of course, we don't normally write the quotes when accessing array elements by index, normally we'll just do arr[0], but in theory, the number is converted to a string and then the property is looked up by name — although, again, modern JavaScript engines optimize.
const obj = {a: 1};
const arr = [1];
console.log(obj["a"]);
console.log(arr["0"]); // Yes, in quotes
console.log(arr[0]);
If you need to clone an array and the objects in it, map + property spread is a useful way to do that, but note the objects are only cloned shallowly (which is often sufficient, but not always):
const result = original.map((value) => ({...value}));
For a full deep copy, see this question's answers.
Here we can use structuredClone for deep copy.

How to update an array after splice in Svelte?

I'm learning Svelte, and read in the documentation that arrays need to be reassigned in order for a component or page to update it. For that they devised a more idiomatic solution. Instead of writing:
messages.push('hello');
messages = messages;
you can write instead:
messages = [...messages, 'hello'];
Alright, makes sense. But then the documentation says:
You can use similar patterns to replace pop, shift, unshift and splice.
But how? I cannot see how you can remove items from an array. More to the point, how could I write the following more idiomatically?
messages.splice(messages.indexOf('hello'), 1);
messages = messages;
You could e.g. use the filter array method to create a new array without the element 'hello':
messages = messages.filter(m => m !== 'hello');
As mentioned, Svelte's reactivity is triggered by assignments. The current Svelte tutorial uses JavaScript's (ES6) spread syntax (three dots) to add the next-higher number to an array, providing a more idiomatic solution than a redundant assignment using push:
function pushNumber() {
numbers = [...numbers, lastnumber]; // 1, 2, 3, 4, 5
}
You could use spread syntax to replace pop, shift, unshift and splicethough it might increase the time and complexity of the operation in some cases:
function unshiftNumber() {
numbers = [firstnumber, ...numbers]; // 0, 1, 2, 3, 4
}
function popNumber() {
numbers = [...numbers.slice(0,numbers.length - 1)]; // 1, 2, 3
}
function shiftNumber() {
numbers = [...numbers.slice(1,numbers.length)]; // 2, 3, 4
}
function spliceNumber() {
numbers = [firstnumber, ...numbers.slice(0,numbers.length-1)];// 0, 1, 2, 3
}
Spread is just one way to do it, though. The purpose behind not using pop/push etc is to encourage immutability. So any removal can just be a filter, for example.
There are several things to consider here.
Given this code:
messages.splice(messages.indexOf('hello'), 1);
messages = messages;
What's happening here is:
Looking for the first occurrence of the string "hello" in the array
Removing such element from the array, based on the index found.
The assumption here is that "hello" needs to exists, otherwise the could would remove the last item from the array (since indexOf returns -1).
The original array is therefore mutate: depends by the context, that sometimes can be preferable instead of copying the whole array into a new one; otherwise it's generally a better practice avoid such mutation.
So. If you want to have this behavior exactly, probably this is the best code you can have. For example, takes the filter example:
messages = messages.filter(message => message !== "hello")
What's happening here is:
Filter out any element equals to "hello"
Returns a new array without such element
So it's quite different from the original code: first of all, it always loop the whole array. If you have thousands of element, even if you have only one "hello" at the second index, it would always iterate all of them. Maybe it's what you want, maybe not. If the element is unique, such as an id, maybe you want to stop once you find it.
Second, it returns a new array. Again, that usually a better practice than mutate the array, but in some context it's preferable mutate it instead of create a new one.
So, if you want to mutate the original array, it's probably better to stick to your original code.
If, instead, you don't care (such as the example of push), I believe that in the intention of svelte's developers, your code would be roughly translate to:
let i = messages.indexOf("hello");
messages = [...messages.slice(0, i), ...messages.slice(i + 1)];
(Still assuming there is a "hello" message and you're interested only in the first occurrence).
It's unfortunate that JS doesn't have a better syntax to handles slices.
In case you're wandering, filter can also be used to remove elements using a given index:
let elements = ['a','b', 'c'];
let idx = 1;
elements = elements.filter( (e,i) => i !== idx );
// => ['a', 'c']
You can perform the usual push and pop or `splice on your Array
But because Svelte's reactivity is triggered by assignments, using array methods like push and splice won't automatically cause updates.
According to All about Immutable Arrays and Objects in JavaScript you can do it this way...
let messages = ['something', 'another', 'hello', 'word', 'another', 'again'];
const indexOfHello = messages.indexOf('hello');
messages = [...messages.slice(0, indexOfHello), ...messages.slice(indexOfHello + 1)];
Note the difference between splice and slice
The splice() method adds/removes items to/from an array, and returns
the removed item(s). Note: This method changes the original array.
Syntax: array.splice(start, deleteCount, itemstoAdd, addThisToo);
But
The slice() method returns the selected elements in an array, as a new array object. The slice() method selects the elements starting at the given start argument, and ends at, but does not include, the given end argument.
Note: The original array will not be changed.
In order words
It return a shallow copy of a portion of an array into a new array
object selected from begin to end (end not included). The original
array will not be modified.
Syntax: array.slice(start, end);
You can try this: https://svelte.dev/repl/0dedb37665014ba99e05415a6107bc21?version=3.53.1
use a library called svelox. It allows you to use the Array native api(push/splice...etc.) without reassignment statements.
Spread the spliced array to reassign it to itself ;)
messages = [...messages.splice(messages.indexOf('hello'), 1)];
The goal is to make Svelte detecting that array messages (a property of your component or a variable in the Svelte store) has changed. This is why the array messages must be declared with let or var keyword, not const. This way you're allowed to reassign it. And the reassign operation itself is sufficient to make Svelte detecting that the array has changed.
Perhaps even, simply by doing so works too:
messages = messages.splice(messages.indexOf('hello'), 1);

Most peformant set structure in JS

I want to have a list of strings that is unique and so everytime I get a new string that I should push onto the list I need to check if the list contains the item before pushing it on the list. This seems unperformant.
However, if I use a hash structure and store the items as keys, is there some way to make this more performant than a simple array?
I guess I am simply wondering what the most performant set data structure exists in JavaScript.
Yes, using a Set will be much faster than checking for an existing value (O(1) for set vs. O(n) for an array).
var s = Set();
s.add(1); // s is (1)
s.add(2);
s.add(3);
s.add(1)
s.add(1)
// s is now (1, 2, 3)
In modern browsers (Chrome 38+, IE11+) the Set type is defined, it is documented here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
Otherwise, in JavaScript, Object values (a fundamental type in ECMAScript) are internally implemented as hashtables - so the fastest conceptual "HashSet" structure would exist as a generalisation of of a hashtable with a disregarded value-type.
Here's how I'd do it (if Set was unavailable):
function StringSet() {
this.values = {};
this.add = function(value) {
value = value.toUpperCase(); // use UpperCase for string normalization because of how casing rules work in different languages, especially Turkish
this.values[ value ] = true; // use bool values as stubs
};
this.contains = function(value) {
value = value.toUpperCase();
return value in this.values; // JavaScript has a fast `in` operator which runs in `O(1)` time
}
}
var foo = new StringSet();
foo.add("bar");
assert( foo.contains("bar") );

What’s the difference between "Array()" and "[]" while declaring a JavaScript array?

What's the real difference between declaring an array like this:
var myArray = new Array();
and
var myArray = [];
There is a difference, but there is no difference in that example.
Using the more verbose method: new Array() does have one extra option in the parameters: if you pass a number to the constructor, you will get an array of that length:
x = new Array(5);
alert(x.length); // 5
To illustrate the different ways to create an array:
var a = [], // these are the same
b = new Array(), // a and b are arrays with length 0
c = ['foo', 'bar'], // these are the same
d = new Array('foo', 'bar'), // c and d are arrays with 2 strings
// these are different:
e = [3] // e.length == 1, e[0] == 3
f = new Array(3), // f.length == 3, f[0] == undefined
;
Another difference is that when using new Array() you're able to set the size of the array, which affects the stack size. This can be useful if you're getting stack overflows (Performance of Array.push vs Array.unshift) which is what happens when the size of the array exceeds the size of the stack, and it has to be re-created. So there can actually, depending on the use case, be a performance increase when using new Array() because you can prevent the overflow from happening.
As pointed out in this answer, new Array(5) will not actually add five undefined items to the array. It simply adds space for five items. Be aware that using Array this way makes it difficult to rely on array.length for calculations.
The difference between creating an array with the implicit array and the array constructor is subtle but important.
When you create an array using
var a = [];
You're telling the interpreter to create a new runtime array. No extra processing necessary at all. Done.
If you use:
var a = new Array();
You're telling the interpreter, I want to call the constructor "Array" and generate an object. It then looks up through your execution context to find the constructor to call, and calls it, creating your array.
You may think "Well, this doesn't matter at all. They're the same!". Unfortunately you can't guarantee that.
Take the following example:
function Array() {
this.is = 'SPARTA';
}
var a = new Array();
var b = [];
alert(a.is); // => 'SPARTA'
alert(b.is); // => undefined
a.push('Woa'); // => TypeError: a.push is not a function
b.push('Woa'); // => 1 (OK)
In the above example, the first call will alert 'SPARTA' as you'd expect. The second will not. You will end up seeing undefined. You'll also note that b contains all of the native Array object functions such as push, where the other does not.
While you may expect this to happen, it just illustrates the fact that [] is not the same as new Array().
It's probably best to just use [] if you know you just want an array. I also do not suggest going around and redefining Array...
There is an important difference that no answer has mentioned yet.
From this:
new Array(2).length // 2
new Array(2)[0] === undefined // true
new Array(2)[1] === undefined // true
You might think the new Array(2) is equivalent to [undefined, undefined], but it's NOT!
Let's try with map():
[undefined, undefined].map(e => 1) // [1, 1]
new Array(2).map(e => 1) // "(2) [undefined Ă— 2]" in Chrome
See? The semantics are totally different! So why is that?
According to ES6 Spec 22.1.1.2, the job of Array(len) is just creating a new array whose property length is set to the argument len and that's it, meaning there isn't any real element inside this newly created array.
Function map(), according to spec 22.1.3.15 would firstly check HasProperty then call the callback, but it turns out that:
new Array(2).hasOwnProperty(0) // false
[undefined, undefined].hasOwnProperty(0) // true
And that's why you can not expect any iterating functions working as usual on arrays created from new Array(len).
BTW, Safari and Firefox have a much better "printing" to this situation:
// Safari
new Array(2) // [](2)
new Array(2).map(e => 1) // [](2)
[undefined, undefined] // [undefined, undefined] (2)
// Firefox
new Array(2) // Array [ <2 empty slots> ]
new Array(2).map(e => 1) // Array [ <2 empty slots> ]
[undefined, undefined] // Array [ undefined, undefined ]
I have already submitted an issue to Chromium and ask them to fix this confusing printing:
https://bugs.chromium.org/p/chromium/issues/detail?id=732021
UPDATE: It's already fixed. Chrome now printed as:
new Array(2) // (2) [empty × 2]
Oddly enough, new Array(size) is almost 2x faster than [] in Chrome, and about the same in FF and IE (measured by creating and filling an array). It only matters if you know the approximate size of the array. If you add more items than the length you've given, the performance boost is lost.
More accurately: Array( is a fast constant time operation that allocates no memory, wheras [] is a linear time operation that sets type and value.
For more information, the following page describes why you never need to use new Array()
You never need to use new Object() in
JavaScript. Use the object literal {}
instead. Similarly, don’t use new Array(),
use the array literal []
instead. Arrays in JavaScript work
nothing like the arrays in Java, and
use of the Java-like syntax will
confuse you.
Do not use new Number, new String, or
new Boolean. These forms produce
unnecessary object wrappers. Just use
simple literals instead.
Also check out the comments - the new Array(length) form does not serve any useful purpose (at least in today's implementations of JavaScript).
In order to better understand [] and new Array():
> []
[]
> new Array()
[]
> [] == []
false
> [] === []
false
> new Array() == new Array()
false
> new Array() === new Array()
false
> typeof ([])
"object"
> typeof (new Array())
"object"
> [] === new Array()
false
> [] == new Array()
false
The above result is from Google Chrome console on Windows 7.
The first one is the default object constructor call. You can use it's parameters if you want.
var array = new Array(5); //initialize with default length 5
The second one gives you the ability to create not empty array:
var array = [1, 2, 3]; // this array will contain numbers 1, 2, 3.
I can explain in a more specific way starting with this example that's based on Fredrik's good one.
var test1 = [];
test1.push("value");
test1.push("value2");
var test2 = new Array();
test2.push("value");
test2.push("value2");
alert(test1);
alert(test2);
alert(test1 == test2);
alert(test1.value == test2.value);
I just added another value to the arrays, and made four alerts:
The first and second are to give us the value stored in each array, to be sure about the values. They will return the same!
Now try the third one, it returns false, that's because
JS treats test1 as a VARIABLE with a data type of array, and it treats test2 as an OBJECT with the functionality of an array, and
there are few slight differences here.
The first difference is when we call test1 it calls a variable without thinking, it just returns the values that are stored in this variable disregarding its data type!
But, when we call test2 it calls the Array() function and then it stores our "Pushed" values in its "Value" property, and the same happens when we alert test2, it returns the "Value" property of the array object.
So when we check if test1 equals test2 of course they will never return true, one is a function and the other is a variable (with a type of array), even if they have the same value!
To be sure about that, try the 4th alert, with the .value added to it; it will return true. In this case we tell JS "Disregarding the type of the container, whether was it function or variable, please compare the values that are stored in each container and tell us what you've seen!" that's exactly what happens.
I hope I said the idea behind that clearly, and sorry for my bad English.
There is no difference when you initialise array without any length. So var a = [] & var b = new Array() is same.
But if you initialise array with length like var b = new Array(1);, it will set array object's length to 1. So its equivalent to var b = []; b.length=1;.
This will be problematic whenever you do array_object.push, it add item after last element & increase length.
var b = new Array(1);
b.push("hello world");
console.log(b.length); // print 2
vs
var v = [];
a.push("hello world");
console.log(b.length); // print 1
There's more to this than meets the eye. Most other answers are correct BUT ALSO..
new Array(n)
Allows engine to reallocates space for n elements
Optimized for array creation
Created array is marked sparse which has the least performant array operations, that's because each index access has to check bounds, see if value exists and walk the prototype chain
If array is marked as sparse, there's no way back (at least in V8), it'll always be slower during its lifetime, even if you fill it up with content (packed array) 1ms or 2 hours later, doesn't matter
[1, 2, 3] || []
Created array is marked packed (unless you use delete or [1,,3] syntax)
Optimized for array operations (for .., forEach, map, etc)
Engine needs to reallocate space as the array grows
This probably isn't the case for older browser versions/browsers.
The first one is the default object constructor call.mostly used for dynamic values.
var array = new Array(length); //initialize with default length
the second array is used when creating static values
var array = [red, green, blue, yellow, white]; // this array will contain values.
The difference of using
var arr = new Array(size);
Or
arr = [];
arr.length = size;
As been discussed enough in this question.
I would like to add the speed issue - the current fastest way, on google chrome is the second one.
But pay attention, these things tend to change a lot with updates. Also the run time will differ between different browsers.
For example - the 2nd option that i mentioned, runs at 2 million [ops/second] on chrome, but if you'd try it on mozilla dev. you'd get a surprisingly higher rate of 23 million.
Anyway, I'd suggest you check it out, every once in a while, on different browsers (and machines), using site as such
As I know the diference u can find the slice(or the other funcitons of Array) like code1.and code2 show u Array and his instances:
code1:
[].slice; // find slice here
var arr = new Array();
arr.slice // find slice here
Array.prototype.slice // find slice here
code2:
[].__proto__ == Array.prototype; // true
var arr = new Array();
arr.__proto__ == Array.prototype; // true
conclusion:
as u can see [] and new Array() create a new instance of Array.And they all get the prototype functions from Array.prototype
They are just different instance of Array.so this explain why
[] != []
:)
There is no big difference, they basically do the same thing but doing them in different ways, but read on, look at this statement at W3C:
var cars = ["Saab", "Volvo","BMW"];
and
var cars = new Array("Saab", "Volvo", "BMW");
The two examples above do exactly the same. There is no need to use
new Array(). For simplicity, readability and execution speed, use the
first one (the array literal method).
But at the same time, creating new array using new Array syntax considered as a bad practice:
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor
new Array().
Use [] instead.
These two different statements both create a new empty array named
points:
var points = new Array(); // Bad
var points = []; // Good
These two different statements both create a new array containing 6
numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
The new keyword only complicates the code. It can also produce some
unexpected results:
var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!
So basically not considered as the best practice, also there is one minor difference there, you can pass length to new Array(length) like this, which also not a recommended way.
I've incurred in a weird behaviour using [].
We have Model "classes" with fields initialised to some value. E.g.:
require([
"dojo/_base/declare",
"dijit/_WidgetBase",
], function(declare, parser, ready, _WidgetBase){
declare("MyWidget", [_WidgetBase], {
field1: [],
field2: "",
function1: function(),
function2: function()
});
});
I found that when the fields are initialised with [] then it would be shared by all Model objects. Making changes to one affects all others.
This doesn't happen initialising them with new Array(). Same for the initialisation of Objects ({} vs new Object())
TBH I am not sure if its a problem with the framework we were using (Dojo)
Well, var x = new Array() is different than var x = [] is different in some features I'll just explain the most useful two (in my opinion) of them.
Before I get into expalining the differences, I will set a base first; when we use x = [] defines a new variable with data type of Array, and it inherits all the methods that belong to the array prototype, something pretty similar (but not exactly) to extending a class. However, when we use x = new Array() it initilizes a clone of the array prototype assigned to the variable x.
Now let's see what are the difference
The First Difference is that using new Array(x) where x is an integer, initilizes an array of x undefined values, for example new Array(16) will initialize an array with 16 items all of them are undefined. This is very useful when you asynchronously fill an array of a predefined length.
For example (again :) ) let's say you are getting the results of 100 competitiors, and you're receiving them asynchronously from a remote system or db, then you'll need to allocate them in the array according to the rank once you receive each result. In this very rare case you will do something like myArray[result.rank - 1] = result.name, so the rank 1 will be set to the index 0 and so on.
The second difference is that using new Array() as you already know, instanciates a whole new clone of the array prototype and assigns it to your variable, that allows you to do some magic (not recommended btw). This magic is that you can overwrite a specific method of the legacy array methods. So, for example you can set the Array.push method to push the new value to the beginning of the array instead of the end, and you can also add new methods (this is better) to this specific clone of the Array Prototype. That will allow you to define more complex types of arrays throughout your project with your own added methods and use it as a class.
Last thing, if you're from the very few people (that I truly love) that care about processing overhead and memory consumption of your app, you'd never tough new Array() without being desperate to use it :).
I hope that has explained enough about the beast new Array() :)
I found a difference while using promises. While using array of promises (say arr, initialised as arr=[]), got an error in Promise.all(arr). Whereas when declared as arr = Array(), did not get compilation issues. Hope this helps.
I've found one difference between the two constructions that bit me pretty hard.
Let's say I have:
function MyClass(){
this.property1=[];
this.property2=new Array();
};
var MyObject1=new MyClass();
var MyObject2=new MyClass();
In real life, if I do this:
MyObject1.property1.push('a');
MyObject1.property2.push('b');
MyObject2.property1.push('c');
MyObject2.property2.push('d');
What I end up with is this:
MyObject1.property1=['a','c']
MyObject1.property2=['b']
MyObject2.property1=['a','c']
MyObject2.property2=['d']
I don't know what the language specification says is supposed to happen, but if I want my two objects to have unique property arrays in my objects, I have to use new Array().
Using the Array constructor makes a new array of the desired length and populates each of the indices with undefined, the assigned an array to a variable one creates the indices that you give it info for.

How are JavaScript arrays implemented?

Namely, how does the following code:
var sup = new Array(5);
sup[0] = 'z3ero';
sup[1] = 'o3ne';
sup[4] = 'f3our';
document.write(sup.length + "<br />");
output '5' for the length, when all you've done is set various elements?
My 'problem' with this code is that I don't understand how length changes without calling a getLength() or a setLength() method. When I do any of the following:
a.length
a['length']
a.length = 4
a['length'] = 5
on a non-array object, it behaves like a dict / associative array. When I do this on the array object, it has special meaning. What mechanism in JavaScript allows this to happen? Does JavaScript have some type of property system which translates
a.length
a['length']
into "get" methods and
a.length = 4
a['length'] = 5
into "set" methods?
Everything in JavaScript is an object. In the case of an Array, the length property returns the size of the internal storage area for indexed items of the array. Some of the confusion may come into play in that the [] operator works for both numeric and string arguments. For an array, if you use it with a numeric index, it returns/sets the expected indexed item. If you use it with a string, it returns/sets the named property on the array object - unless the string corresponds to a numeric value, then it returns the indexed item. This is because in JavaScript array indexes are coerced to strings by an implicit toString() call. Frankly, this is just one more of those things that makes you scratch your head and say "JavaScript, this, this is why they laugh at you."
The actual underlying representation may differ between browsers (or it may not). I wouldn't rely on anything other than the interface that is supplied when working with it.
You can find out more about JavaScript arrays at MDN.
Characteristics of a JavaScript array
Dynamic - Arrays in JavaScript can grow dynamically .push
Can be sparse - for example, array[50000] = 2;
Can be dense - for example, array = [1, 2, 3, 4, 5]
In JavaScript, it is hard for the runtime to know whether the array is going to be dense or sparse. So all it can do is take a guess. All implementations use a heuristic to determine if the array is dense or sparse.
For example, code in point 2 above, can indicate to the JavaScript runtime that this is likely a sparse array implementation. If the array is initialised with an initial count, this could indicate that this is likely a dense array.
When the runtime detects that the array is sparse, it is implemented in a similar way to an object. So instead of maintaining a contiguous array, a key/value map is built.
For more references, see How are JavaScript arrays implemented internally?
This really depends on what you intend to do with it.
[].length is "magical".
It doesn't actually return the number of items in the array. It returns the largest instated index in the array.
var testArr = []; testArr[5000] = "something"; testArr.length; // 5001
But the method behind the setter is hidden in the engine itself.
Some engines in some browsers will give you access to their implementations of those magic-methods.
Others will keep everything completely locked down.
So don't rely on defineGetter and defineSetter methods, or even, really, __proto__ methods, unless you know which browsers you know you're targeting, and which you aren't.
This will change in the future, where opt-in applications written in ECMAScript Next/6 will have access to more.
ECMAScript 5-compliant browsers are already starting to offer get and set magic methods in objects and there's more to come... ...but it's probably a while away before you can dump support for oldIE and a tonne of smartphones, et cetera...
It is important to know that when you do sup['look'] = 4; you are not using an associative array, but rather modify properties on the object sup.
It is equivalent to sup.look = 4; since you can dynamically add properties on JavaScript objects at any time. sup['length'] would for an instance output 5 in your first example.
To add to tvanfosson's answer: In ECMA-262 (the 3.0 specification, I believe), arrays are simply defined as having this behavior for setting properties (See 15.4.5.1). There's no general mechanism underlying it (at least as of now) - this is just how it's defined, and how JavaScript interpreters must behave.
As other people have mentioned, a property in JavaScript can basically act as both as getter and a setter of your array (or string or other inputs).
As a matter of fact, you might try this yourself:
const test = [1, 2, 3, 4, 5]
test.length = 3
console.log(test) // [1, 2, 3]
test.length = 5
console.log(test) // Guess what happens here!
As far as I know, arrays in JavaScript do not work exactly like associative arrays and you have elements which are put in memory as contiguously as possible (given that you can have arrays of mixed objects), depending on the JavaScript engine you are considering.
As a side note, I am a bit baffled that the most voted answer keeps spreading the over-simplified myth (or half-truth) of "everything being an object in JavaScript"; that is not exactly true, otherwise you will never study primitives, for example.
Try to do this:
const pippi = "pippi"
pippi.cat = "cat"
console.log(pippi.cat) // Will it work? Throw an error? Guess why again
Spoiler: the string is wrapped in a throwaway object for that specific operation on the second line, and then in the following one you are just going to access a property of the primitive which is not there (provided you did not play with String.prototype or the like), so you get undefined.
Array object inherits caller, constructor, length, and name properties from Function.prototype.
A JavaScript array is an object just like any other object, but JavaScript gives it special syntax.
arr[5] = "yo"
The above is syntactic sugar for
arr.insert(5,"yo")
which is how you would add stuff to a regular object. It's what is inside the insert method that changes the value of arr.length
See my implementation of a customArray type here: http://jsfiddle.net/vfm3vkxy/4/

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