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up vote 615 down vote favorite
126

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?

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1  
Without more context from the OP, who knows which answer is best. Questioners, please make a bit more effort! 'Is it possible to empty with .remove()?' What? And what is the use case? Everyone is arguing over the answer; I have no idea why the question has so many upvotes. –  la quinta huésped Jul 7 '13 at 2:19
2  
This question (as well as the answer) has so many upvotes because people try the accepted answer and it seems to work, even though it most certainly does not do what the question requests. Because it seems to work the newer programmers are delighted to have found what seems to be the answer so easily and quickly that they click upvote on both the question and the answer. Make no mistake, Daniel Baulig is right in his highly voted comment in the accepted answer. This is why I don't always go for the accepted answer, but the highest voted one, which is clearly Mathew Crumley's answer. –  VoidKing Oct 16 '13 at 15:36
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13 Answers

up vote 446 down vote accepted

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 vote for this answer, please upvote the other answers that I have referenced as well. (most of the comments on this answer are about method 1, which was my original answer)

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.

Method 2 (as suggested by Matthew Crumley)

A.length = 0

This will clear the existing array by setting its length to 0. Some have argued that this may not work in all implementations of Javascript but it turns out that this is not the case. 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 it's not very efficient because the .splice() function will return an array with all the removed items, so it will actually return a copy of the original array in some (most?) Javascript implementations.

Method 4 (as suggested by tanguy_k)

while(A.length > 0) {
    A.pop();
}

This solution is not very succinct but it is by far the fastest solution (apart from setting the array to a new array). If you care about performance, consider using this method to clear an array. Benchmarks show that this is at least 10 times faster than setting the length to 0 or using splice().

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475  
this will NOT empty the array, but create a new, empty array. This might cause problems, especially if there are other references to the array. OP: Please consider to accept Matthew's answer instead. It is the cleaner and formally correct approach. –  Daniel Baulig Jan 19 '11 at 13:08
22  
To all commenters and downvoters: you are all right, and the answer below (setting the length to 0) is a better answer, but in many cases the solution I presented works perfectly and is more succinct –  Philippe Leybaert Oct 14 '11 at 19:12
8  
arr = new Array() is exactly the same as arr = [] –  Philippe Leybaert May 16 '12 at 12:32
27  
-1: There is a significant difference between emptying an array and creating a new one (as stated by @DanielBaulig). –  David Faivre Aug 19 '12 at 23:16
8  
This does not clear the array. Simply replaces it with a blank one. The problems with this is that any other reference to this variable will still retain the uncleared Array. –  Shamasis Bhattacharya Sep 26 '12 at 12:22
show 9 more comments
up vote 1475 down vote

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;
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5  
@Acorn Yes, it will work in all browsers. –  Matthew Crumley May 10 '11 at 22:01
9  
what does ECMAScript 5 Standard says about this? –  Pacerier Jun 21 '11 at 7:00
116  
@Pacerier: It still works in ES5. From section 15.4: "...whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted" –  Matthew Crumley Jun 21 '11 at 7:43
5  
@LosManos Even in strict mode, length is a special property, but not read only, so it will still work. –  Matthew Crumley Jan 4 '13 at 14:18
7  
@MattewCrumley I done some test, and it seems like a.length = 0 is not to efficient clearing whole array. jsperf.com/length-equal-0-or-new-array I think if you have one refence (and you haven't added extra properties that you want to keep), it is better to create new array, and leaves old to the garbage collector, that will run when appropriate. –  bluesm Nov 16 '13 at 19:08
show 19 more comments
up vote 66 down vote

Here the fastest working implementation while keeping the same array:

Array.prototype.clear = function() {
  while (this.length > 0) {
    this.pop();
  }
};

Or as an Underscore.js mixin:

_.mixin({
  clearArray: function(array) {
    while (array.length > 0) {
      array.pop();
    }
  }
});

And the test that goes with it:

describe('Array', function() {
  it('should clear the array', function() {
    var array = [1, 2, 3, 4, 5];
    array.clear();
    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

Answers from Jan and David McCurley are incorrect.

As noted by 755 in a comment, the algorithm provided by Jan only removes half of the items from the array.

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1  
TT your answer is the only one that correct and fast ( at the same time ) but have some much less "upvotes". Well, it seems that people like pretty solutions that are slow :/ –  Ai_boy Jun 26 '13 at 5:09
1  
This is the correct and fastest solution, referred to you in my answer. –  Jan Jun 27 '13 at 9:08
    
Wow, never though I'd see people waging wars over it. I'm going with your solution, thanks. –  S.O. Jul 3 '13 at 14:23
3  
@thefourtheye Good solution for performance, though I agree with @naomik, you should not modify native objects. Saying that it should be there is beside the point, the problem is you're modifying globals, which is bad. If you're providing your code for others to use, then it should have no unforeseen side effects. Imagine if another library also modified the Array.prototype and it was doing something slightly different, then all throughout your code [].clear() was slightly wrong. This would not be fun to debug. So, the general message is: Don't modify globals. –  jpillora Sep 14 '13 at 10:39
1  
@thefourtheye The whole point of not modifying global scope is because you won't know if someone else's code is already (or will be) using the name. I suggest a function inside local scope. So, inside your application's/library's IIFE, do function clear(arr) { while(arr.length) arr.pop(); }, then clear arrays with clear(arr) instead of arr.clear(). –  jpillora Sep 15 '13 at 4:58
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up vote 38 down vote

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);

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21  
Why is this more cross-browser friendly? What browsers have issues with A.length? –  stricjux Nov 21 '11 at 15:12
1  
This is the most correct answer since this actually "clears the array content and retains the reference to the original array object. –  Shamasis Bhattacharya May 23 '12 at 8:32
3  
@jm2 what you are saying is not entirely true. It actually modifies the array in question and subsequently all references get affected. See the test on my jsFiddle: jsfiddle.net/shamasis/dG4PH –  Shamasis Bhattacharya Sep 26 '12 at 12:38
2  
@alex no it does not, splice modifies the array and returns the deleted entries. Read the docs first: developer.mozilla.org/en-US/docs/JavaScript/Reference/… –  David Oct 29 '12 at 16:22
1  
We could prevent the resulting array from being returned by using the comma operator: A.splice(0, A.length),0;. This would leave a return value of 0 just as A.length = 0; would. The resulting array is still created and should cause the script to run slower: (jsperf ~56% slower). Browser implementation will affect this although I see no reason why splice would be faster than setting length. –  Evan Kennedy Aug 18 '13 at 3:47
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up vote 11 down vote

You can add this to your JavaScript file to allow your arrays to be "cleared":

Array.prototype.clear = function() {
    this.splice(0, this.length);
};

list.clear();
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1  
@naomik Can you explain your reasoning why doing such a thing is frowned upon? –  Sam Sep 16 '13 at 16:24
1  
It is "frowned upon" to modify javascript primitive functions like Array and String. You could possibly be overloading an already existing function and trash the object class. There might be an obscure javascript engine that already has clear() and expects it to behave a different way. Tread carefully is all I say. –  Design by Adrian Mar 25 at 15:20
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up vote 7 down vote 
Array.prototype.clear = function()
{
    this.length = 0;
};

and call it: array.clear();

=)

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14  
Please don't encourage modification of the native objects. –  naomik Jul 7 '13 at 6:53
2  
why do people have this tendency to grab the accepted answer and put it into a prototype function? Do you actually do this in your projects? Do you have a huge library of prototype additions that you include in every project? –  nurettin Nov 29 '13 at 11:05
1  
Why not just type array.length = 0? –  Design by Adrian Mar 25 at 15:24
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up vote 2 down vote

How about the below modified version of Jan's initial suggestion?

var originalLength = A.length;
for(var i = originalLength; i > 0; i--) {
     A.pop();
}
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up vote 2 down vote

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.

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up vote 1 down vote

The simplest way to delete all content of Array is to set length to zero.

Eg. var myArray = ['A','B','C'];

myArray.length = 0; // Delete all content of Array
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10  
Cool! Just like this answer dated Aug 5 '09. –  Scotty.NET Nov 27 '13 at 11:43
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up vote 0 down vote

A.splice(0);

I just did this on some code I am working on. It cleared the array.

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up vote 0 down vote 
A.length = 0;

or 

A = [];

or 

A = new Array();
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1  
Already answered Aug 2009 ;) –  Design by Adrian Mar 25 at 15:26
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up vote -2 down vote 
delete A;

Will delete the Array then simply re-create it.

A = [];
share|improve this answer
    
There is no point deleting it if you are re-referencing it. –  Design by Adrian Mar 25 at 15:25
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up vote -6 down vote

The fastest solution for all current browsers is to implement the pop or shift method. Combining answers from leech and jan, we can come up with a method that is declared once and makes it easy to clear the array:

Array.prototype.clear = function()  //Add a new method to the Array Object
{
    var i = this.length;
    for(var i=0;i<ii;i++)
    {
        this.pop();
    }
}

var NumberList = new Array();  //Declare the variable
NumberList.push(111);          //Add number to the end of the list
NumberList.clear;              //Clear the list

Or you could just use a while loop:

while(your_array_name_here.length > 0)
    your_array_name_here.pop();

But the best answer as found above would be to use the length property straight up.

your_array_name_here.length = 0;
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2  
This answer is wrong since Jan implementation is, as 755 said: "only pops half of the elements in the array, since this.length is decreasing as i is increasing" and I've verified this myself –  tanguy_k Jun 25 '13 at 20:09
4  
Please don't encourage modification of the native objects. –  naomik Jul 7 '13 at 6:54
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