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Is there a way to empty an array and if so possibly with .remove()?
For instance,
A = [1,2,3,4];
How can I empty that?
Ways to clear an existing array A:
Method 1
(this was my original answer to the question)
A = [];
This code will set the variable A to a new empty array. This is perfect if you don't have references to the original array A anywhere else because this actually creates a brand new (empty) array. You should be careful with this method because if you have referenced this array from another variable or property, the original array will remain unchanged. Only use this if you only reference the array by its original variable A.
This is also the fastest solution.
This code sample shows the issue you can encounter when using this method:
var arr1 = ['a','b','c','d','e','f'];
var arr2 = arr1; // Reference arr1 by another variable
arr1 = [];
console.log(arr2); // Output ['a','b','c','d','e','f']
Method 2 (as suggested by Matthew Crumley)
A.length = 0
This will clear the existing array by setting its length to 0. It also works when using "strict mode" in ECMAScript 5 because the length property of an array is a read/write property.
Method 3 (as suggested by Anthony)
A.splice(0,A.length)
Using .splice() will work perfectly, but since the .splice() function will return an array with all the removed items, it will actually return a copy of the original array. Benchmarks suggest that this has no effect on performance whatsoever.
Method 4 (as suggested by tanguy_k)
while(A.length > 0) {
A.pop();
}
This solution is not very succinct, and it is also the slowest solution, contrary to earlier benchmarks referenced in the original answer.
Performance
Of all the methods of clearing an existing array, methods 2 and 3 are very similar in performance and are a lot faster than method 4. See this benchmark.
As pointed out by Diadistis in their answer below, the original benchmarks that were used to determine the performance of the four methods described above were flawed. The original benchmark reused the cleared array so the second iteration was clearing an array that was already empty.
The following benchmark fixes this flaw: http://jsben.ch/#/hyj65. It clearly shows that methods #2 (length property) and #3 (splice) are the fastest (not counting method #1 which doesn't change the original array).
This has been a hot topic and the cause of a lot of controversy. There are actually many correct answers and because this answer has been marked as the accepted answer for a very long time, I will include all of the methods here.
If you need to keep the original array because you have other references to it that should be updated too, you can clear it without creating a new array by setting its length to zero:
A.length = 0;
Here the fastest working implementation while keeping the same array ("mutable"):
function clearArray(array) {
while (array.length > 0) {
array.pop();
}
}
FYI it cannot be simplified to while (array.pop()): the tests will fail.
FYI Map and Set define clear(), it would have seem logical to have clear() for Array too.
TypeScript version:
function clearArray<T>(array: T[]) {
while (array.length > 0) {
array.pop();
}
}
The corresponding tests:
describe('clearArray()', () => {
test('clear regular array', () => {
const array = [1, 2, 3, 4, 5];
clearArray(array);
expect(array.length).toEqual(0);
expect(array[0]).toEqual(undefined);
expect(array[4]).toEqual(undefined);
});
test('clear array that contains undefined and null', () => {
const array = [1, undefined, 3, null, 5];
clearArray(array);
expect(array.length).toEqual(0);
expect(array[0]).toEqual(undefined);
expect(array[4]).toEqual(undefined);
});
});
Here the updated jsPerf: http://jsperf.com/array-destroy/32 http://jsperf.com/array-destroy/152
jsPerf offline. Similar benchmark: https://jsben.ch/hyj65
A more cross-browser friendly and more optimal solution will be to use the splice method to empty the content of the array A as below:
A.splice(0, A.length);
The answers that have no less that 2739 upvotes by now are misleading and incorrect.
The question is: "How do you empty your existing array?" E.g. for A = [1,2,3,4].
Saying "A = [] is the answer" is ignorant and absolutely incorrect. [] == [] is false.
This is because these two arrays are two separate, individual objects, with their own two identities, taking up their own space in the digital world, each on its own.
Let's say your mother asks you to empty the trash can.
You don't bring in a new one as if you've done what you've been asked for.
Instead, you empty the trash can.
You don't replace the filled one with a new empty can, and you don't take the label "A" from the filled can and stick it to the new one as in A = [1,2,3,4]; A = [];
Emptying an array object is the easiest thing ever:
A.length = 0;
This way, the can under "A" is not only empty, but also as clean as new!
Furthermore, you are not required to remove the trash by hand until the can is empty! You were asked to empty the existing one, completely, in one turn, not to pick up the trash until the can gets empty, as in:
while(A.length > 0) {
A.pop();
}
Nor, to put your left hand at the bottom of the trash, holding it with your right at the top to be able to pull its content out as in:
A.splice(0, A.length);
No, you were asked to empty it:
A.length = 0;
This is the only code that correctly empties the contents of a given JavaScript array.
Performance test:
http://jsperf.com/array-clear-methods/3
a = []; // 37% slower
a.length = 0; // 89% slower
a.splice(0, a.length) // 97% slower
while (a.length > 0) {
a.pop();
} // Fastest
You can add this to your JavaScript file to allow your arrays to be "cleared":
Array.prototype.clear = function() {
this.splice(0, this.length);
};
Then you can use it like this:
var list = [1, 2, 3];
list.clear();
Or if you want to be sure you don't destroy something:
if (!Array.prototype.clear) {
Array.prototype.clear = function() {
this.splice(0, this.length);
};
}
Lots of people think you shouldn't modify native objects (like Array), and I'm inclined to agree. Please use caution in deciding how to handle this.
You can easily create a function to do that for you, change the length or even add it to native Array as remove() function for reuse.
Imagine you have this array:
var arr = [1, 2, 3, 4, 5]; //the array
OK, just simply run this:
arr.length = 0; //change the length
and the result is:
[] //result
easy way to empty an array...
Also using loop which is not necessary but just another way to do that:
/* could be arr.pop() or arr.splice(0)
don't need to return as main array get changed */
function remove(arr) {
while(arr.length) {
arr.shift();
}
}
There are also tricky way which you can think about, for example something like this:
arr.splice(0, arr.length); //[]
So if arr has 5 items, it will splice 5 items from 0, which means nothing will remain in the array.
Also other ways like simply reassign the array for example:
arr = []; //[]
If you look at the Array functions, there are many other ways to do this, but the most recommended one could be changing the length.
As I said in the first place, you can also prototype remove() as it's the answer to your question. you can simply choose one of the methods above and prototype it to Array object in JavaScript, something like:
Array.prototype.remove = Array.prototype.remove || function() {
this.splice(0, this.length);
};
and you can simply call it like this to empty any array in your javascript application:
arr.remove(); //[]
If you are using
a = [];
Then you are assigning new array reference to a, if reference in a is already assigned to any other variable, then it will not empty that array too and hence garbage collector will not collect that memory.
For ex.
var a=[1,2,3];
var b=a;
a=[];
console.log(b);// It will print [1,2,3];
or
a.length = 0;
When we specify a.length, we are just resetting boundaries of the array and memory for rest array elements will be connected by garbage collector.
Instead of these two solutions are better.
a.splice(0,a.length)
and
while(a.length > 0) {
a.pop();
}
As per previous answer by kenshou.html, second method is faster.
There is a lot of confusion and misinformation regarding the while;pop/shift performance both in answers and comments. The while/pop solution has (as expected) the worst performance. What's actually happening is that setup runs only once for each sample that runs the snippet in a loop. eg:
var arr = [];
for (var i = 0; i < 100; i++) {
arr.push(Math.random());
}
for (var j = 0; j < 1000; j++) {
while (arr.length > 0) {
arr.pop(); // this executes 100 times, not 100000
}
}
I have created a new test that works correctly :
http://jsperf.com/empty-javascript-array-redux
Warning: even in this version of the test you can't actually see the real difference because cloning the array takes up most of the test time. It still shows that splice is the fastest way to clear the array (not taking [] into consideration because while it is the fastest it's not actually clearing the existing array).
Array.prototype.clear = function() {
this.length = 0;
};
And call it: array.clear();
In case you are interested in the memory allocation, you may compare each approach using something like this jsfiddle in conjunction with chrome dev tools' timeline tab. You will want to use the trash bin icon at the bottom to force a garbage collection after 'clearing' the array. This should give you a more definite answer for the browser of your choice. A lot of answers here are old and I wouldn't rely on them but rather test as in #tanguy_k's answer above.
(for an intro to the aforementioned tab you can check out here)
Stackoverflow forces me to copy the jsfiddle so here it is:
<html>
<script>
var size = 1000*100
window.onload = function() {
document.getElementById("quantifier").value = size
}
function scaffold()
{
console.log("processing Scaffold...");
a = new Array
}
function start()
{
size = document.getElementById("quantifier").value
console.log("Starting... quantifier is " + size);
console.log("starting test")
for (i=0; i<size; i++){
a[i]="something"
}
console.log("done...")
}
function tearDown()
{
console.log("processing teardown");
a.length=0
}
</script>
<body>
<span style="color:green;">Quantifier:</span>
<input id="quantifier" style="color:green;" type="text"></input>
<button onclick="scaffold()">Scaffold</button>
<button onclick="start()">Start</button>
<button onclick="tearDown()">Clean</button>
<br/>
</body>
</html>
And you should take note that it may depend on the type of the array elements, as javascript manages strings differently than other primitive types, not to mention arrays of objects. The type may affect what happens.
Use a modified version of Jan's initial suggestion:
var originalLength = A.length;
for (var i = originalLength; i > 0; i--) {
A.pop();
}
Terser:
for (let i = A.length; i > 0;A.pop(),i--) {}
Or here's another take:
while(!A[Symbol.iterator]().next().done)A.shift()
A.splice(0);
I just did this on some code I am working on. It cleared the array.
If you use constants then you have no choice:
const numbers = [1, 2, 3]
You can not reasign:
numbers = []
You can only truncate:
numbers.length = 0
To Empty a Current memory location of an array use: 'myArray.length = 0' or 'myArray.pop() UN-till its length is 0'
length : You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases.
pop() : The pop method removes the last element from an array and returns that returns the removed value.
shift() : The shift method removes the element at the zeroeth index and shifts the values at consecutive indexes down, then returns the removed value.
Example:
var arr = ['77'];
arr.length = 20;
console.log("Increasing : ", arr); // (20) ["77", empty × 19]
arr.length = 12;
console.log("Truncating : ", arr); // (12) ["77", empty × 11]
var mainArr = new Array();
mainArr = ['1', '2', '3', '4'];
var refArr = mainArr;
console.log('Current', mainArr, 'Refered', refArr);
refArr.length = 3;
console.log('Length: ~ Current', mainArr, 'Refered', refArr);
mainArr.push('0');
console.log('Push to the End of Current Array Memory Location \n~ Current', mainArr, 'Refered', refArr);
mainArr.poptill_length(0);
console.log('Empty Array \n~ Current', mainArr, 'Refered', refArr);
Array.prototype.poptill_length = function (e) {
while (this.length) {
if( this.length == e ) break;
console.log('removed last element:', this.pop());
}
};
new Array() | [] Create an Array with new memory location by using Array constructor or array literal.
mainArr = []; // a new empty array is addressed to mainArr.
var arr = new Array('10'); // Array constructor
arr.unshift('1'); // add to the front
arr.push('15'); // add to the end
console.log("After Adding : ", arr); // ["1", "10", "15"]
arr.pop(); // remove from the end
arr.shift(); // remove from the front
console.log("After Removing : ", arr); // ["10"]
var arrLit = ['14', '17'];
console.log("array literal « ", indexedItem( arrLit ) ); // {0,14}{1,17}
function indexedItem( arr ) {
var indexedStr = "";
arr.forEach(function(item, index, array) {
indexedStr += "{"+index+","+item+"}";
console.log(item, index);
});
return indexedStr;
}
slice() : By using slice function we get an shallow copy of elements from the original array, with new memory address, So that any modification on cloneArr will not affect to an actual|original array.
var shallowCopy = mainArr.slice(); // this is how to make a copy
var cloneArr = mainArr.slice(0, 3);
console.log('Main', mainArr, '\tCloned', cloneArr);
cloneArr.length = 0; // Clears current memory location of an array.
console.log('Main', mainArr, '\tCloned', cloneArr);
I'm surprised no one has suggested this yet:
let xs = [1,2,3,4];
for (let i in xs)
delete xs[i];
This yields an array in quite a different state from the other solutions. In a sense, the array has been 'emptied':
xs
=> Array [ <4 empty slots> ]
[...xs]
=> Array [ undefined, undefined, undefined, undefined ]
xs.length
=> 4
xs[0]
=> ReferenceError: reference to undefined property xs[0]
You can produce an equivalent array with [,,,,] or Array(4)
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
What is the difference between an array and an object?
The item exists in the array but it says that the array is 0 length?
I m a bit confused in object and associative array in javascript. I read this :question but this question says that there is not much of a difference in both. I wrote this in the console:
var a = [];
a["A"] = 1;
var b = {};
b["B"] = 2;
var c = new Object();
c["C"] = 3;
output for above are as:
a gives {A : 1}
b gives {B : 2}
c gives {C : 3}
All above three cases gives same reault as in they all gives an object. Question is how all above 3 cases are treated in javascript.
An array is also an object, that's why you can use non-numeric indices also. Those will be added as properties to the array object, not part of the array data.
The difference between an array and an object, is that the array counts properties with a numeric index as being part of the array data, and updates the length property accordingly. (Also the Array object has some specific methods to work with the array data.)
var a = [];
a[42] = 1337;
Now the length has changed to include the item:
alert(a.length); // shows 43
Using strings or numbers as index doesn't matter, a numeric index counts even if it's a string:
alert(a[42]); // shows 1337
alert(a["42"]); // shows 1337
Reducing the length removes the properties outside it:
a.length = 10;
alert(a[42]); // shows undefined
In your example, b and c are essentially the same thing, because {} is the equivalent of new Object().
Coming back to a, it's defined as an Array which is a special kind of Object in the sense that numeric properties (based on uint32) are treated differently. Its length property gets updated when those properties are added and removed.
When you use 'A' as an index, it gets treated as a regular property, defined by how Object works with properties, so you can access it as a.A or a['A'] whereas an index of [5] can only be accessed using a[5].
Normally, the debug output of an array is always [] unless the length property is non-zero, so the output you've shown is somewhat irregular.
Trivia
According to the ECMAScript documentation, a particular value p can only be an array index if and only if:
(p >>> 0 === p) && (p >>> 0 !== Math.pow(2, 32) - 1)
See also:
The item exists in the array but it says that the array is 0 length?
Let's start by clarifying something:
new Object() is the same as {}
new Array() is the same as []
The latter are just shortened forms of the former.
Behind the scenes, everything in javascript is basically an object (this is an exaggeration but fairly accurate). Arrays are simply derived from objects. Here's a fairly rudimentary example of what an Array REALLY looks like:
var myArray = {};
myArray[0] = 'value1';
myArray[1] = 'value2';
myArray[2] = 'value3';
myArray[length] = 3;
The prototype of an array contains all the methods. For example:
// myArray#push
myArray.prototype.push = function(val){
myArray[this.length] = val;
this.length++;
}
Another way to illustrate this is to take an array and add keys that are not numeric:
var example = [];
example.push(0);
example.push(1);
example.push(2);
example['a'] = 'a';
example['b'] = 'b';
example['c'] = 'c';
example.log = function(){
for(var i = 0, l = this.length; i < l; i++){
console.log( example[i] );
}
console.log(this['a']);
console.log(this['b']);
console.log(this['c']);
}
// example[0] is 0
// example[1] is 1
// example[2] is 2
// example.log is my custom function
example.log(); // results in
// 0
// 1
// 2
// a
// b
// c
Also, don't always believe everything the console tells you. For example, if you do this:
var console_confusion = {};
console_confusion.length = 100;
console_confusion.splice = function(){ /* do absolutely nothing */ };
console.log( console_confusion );//results in
//
// in Chrome:
// [undefined × 100]
//
Chrome will interprut anything with a numeric length property and a splice function as an Array. This is why jQuery objects look like Arrays in the console.
Please read the following article – http://www.2ality.com/2012/12/arrays.html
In short, “regular arrays”, denoted as [] in JavaScript, are also objects, just like {}, but they have length property and “numeric” keys (“indicies”), plus their internal __proto__ property points at Array.prototype object, which holds all Array methods, such as push or forEach etc.
first is an array
second is an object array
third is an array object
I am making an app in which an algorithm is run on an array. Because the contents of the array will change during the execution of the algorithm, I'm storing the array contents to another array beforehand - performing the 'if' statements on the source array but updating the temporary array, then equating them afterwards.
The problem is that after running the algorithm, the two arrays are still identical. It seems that updating the temporary array automatically updates the source array.
I've created this jsfiddle to demonstrate:
var a = new Array( 0 , 1 , 2 );
var b = a;
b[1]=3;
document.write( (a[1]==b[1]) );
//Should show 'false' as this will not be correct
The code above returns "True". Is this normal behaviour? How can I overcome this?
That is normal behaviour, when copying array you are actually making a reference, so when you do var b = a that means instead of copying value you are just copying the reference
if you want to shallow copy an array (array with only one depth level) then you can use simple method like this:
var b = a.slice(0);
But if you want to deep copy an array (array with 2 or more depth level) then you can use below method for that:
var objClone = function(srcObj) {
var destObj = (this instanceof Array) ? [] : {};
for (var i in srcObj)
{
if (srcObj[i] && typeof srcObj[i] == "object") {
destObj[i] = objClone(srcObj[i]);
} else {
destObj[i] = srcObj[i];
}
}
return destObj;
};
For use of both of these methods check this jsFiddle
b = a doesn't copy the array, but a reference. Use b = a.slice() to make a copy.
See also ...
As others have mentioned a = b does not copy the array. However, if you do end up using the .slice() method to copy arrays please note that objects within an array will be copied by reference.
Take the following example
var obj = { greeting: "hello world" };
var a = [ obj ];
var b = a;
a[0].greeting = "foo bar";
b[0].greeting // => "foo bar";
Other than that you are good to go :)
Note: this isn't one of the millions dups of the common array copy 'problem' where using arr.slice(0) fixes this 'problem'
That said, I want to understand why I am getting this unexpected result:
var oldArr = [[1,2],[3,4]];
var find = oldArr[1];
var newArr = oldArr.slice(0);
console.log(newArr.indexOf(find)); //1?
//proof that newArr is NOT referenced to oldArr
newArr[0] = "Hi";
newArr[1] = "How are you?";
console.log(oldArr+" "+newArr); //"1,2,3,4 Hi,How are you?"
jsFildde Demo
If you replace find with any of the following alternatives, it returns the expected -1:
Use [3,4] directly
Use a variable holding [3,4]
Use a variable with reference of another array holding [3,4]
I can't find any explanation on why there is any difference between these last three methods and the first example. As far as I know, there shouldn't be any.
Any ideas?
In:
[[1,2],[3,4]]
There are three array objects created.
Only the outer one is being slice'd. This results in a "shallow copy".
Consider this:
var a = [1,2]
var b = [3,4]
var c = [a,b]
var d = c.slice(0)
d[0] === a // true, which means it is the /same/ object
d[0][0] = "Hi!"
a // ["Hi!", 2]
Happy coding!
Everything is working as expected
indexOf searches by reference and not value, therefore if two objects do not === each other, indexOf will not find them
In your case:
var oldArr = [[1,2],[3,4]];
var find = oldArr[1];
var newArr = oldArr.slice(0);
find points to the array [3,4];
so newArray contains 2 objects (the two arrays).
If you want to see what is really going on try doing find.push(5)
In the example below, the array2.length is only 10, while in my mind, it should be 13.
Why does the "string keyed" indexes not increase the length of the array?
I can store things and still access it, and the VS debugger shows that those arrays are being stored properly. So why is the length not increased?
var array2 = new Array();
array2["a"] = new Array();
array2["b"] = new Array();
array2["c"] = new Array();
for (var i = 0; i < 10; ++i)
array2[i] = new Array();
var nothing = "";
for (var i = 0; i < array2.length; ++i)
nothing = "";
Javascript arrays cannot have "string indexes". A Javascript Array is exclusively numerically indexed. When you set a "string index", you're setting a property of the object. These are equivalent:
array.a = 'foo';
array['a'] = 'foo';
Those properties are not part of the "data storage" of the array.
If you want "associative arrays", you need to use an object:
var obj = {};
obj['a'] = 'foo';
Maybe the simplest visualization is using the literal notation instead of new Array:
// numerically indexed Array
var array = ['foo', 'bar', 'baz'];
// associative Object
var dict = { foo : 42, bar : 'baz' };
Because the length is defined to be one plus the largest numeric index in the array.
var xs = [];
xs[10] = 17;
console.log( xs.length ); //11
For this reason, you should only use arrays for storing things indexed by numbers, using plain objects instead if you want to use strings as keys. Also, as a sidenote, it is a better practice to use literals like [] or {} instead of new Array and new Object.
You're not adding items to the array; you're adding properties to the Array object.
As said above, use object for associative arrays.
If you don't you won't necessarily notice you're doing it wrong, until you innocently use "length" as an array index :
var myArray = [];
myArray["foo"] = "bar"; //works
console.log(myArray["foo"]) //print "bar"
myArray["length"] = "baz" //crash with a "RangeError: Invalid array length"
That is because you are replacing the length attribute of an array with a String, which is invalid.
"string keyed" indexes are not indexes at all, but properties. array2["a"] is the same as saying array2.a. Remember that you can set properties on any kind of variable in javascript, which is exactly what you're doing here.
You can push object to array, it will automatically get indexed (integer). If you want to add index as you want then you want to make it as object
If you want to use an object's properties as if they were like instances of a string indexed array, the work around for the length is:
var myArray = new Array();
myArray["a"] = 'foo';
myArray["b"] = 'bar';
myArray["c"] = 'baz';
let theLength = Object.keys(myArray).length