How to create an array if an array does not exist yet?

Question

How do I create an array if it does not exist yet? In other words how to default a variable to an empty array?

score 93 · Accepted Answer · answered Dec 25 '09 at 17:21

93

If you want to check whether an array x exists and create it if it doesn't, you can do

x = ( typeof x != 'undefined' && x instanceof Array ) ? x : []

answered Dec 25 '09 at 17:21

Rich

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This works in most cases, but it won't cover you if you plan on checking an array instantiated from a different global `Array` constructor - this might occur when scripting across frames. – James Dec 25 '09 at 18:17
1

That is true. How would one handle that case? – Rich Dec 25 '09 at 19:23
2

@Rich: See my answer here: http://stackoverflow.com/questions/1961528/javascript-check-if-array-exist-if-not-create-it/1961653#1961653, it will behave correctly on multi-framed DOM environments. – Christian C. Salvadó Dec 25 '09 at 21:32
11

The correct way to do it nowadays is to use `Array.isArray(x)`: http://stackoverflow.com/a/38670091/271442 – mynameistechno Jul 30 '16 at 04:33
It doesn't work when it is **null**. So a complete check would be `if (Array.isArray(array) && array.length)` You can find more here https://stackoverflow.com/a/24403771/364951 – Claudiu Oct 18 '17 at 19:29
That is not correct. `Array.isArray(null)` will be `false`. Your check is verifying it's a non-empty array. – mynameistechno Mar 05 '20 at 21:18

score 71 · Answer 2 · answered Dec 25 '09 at 17:21

71

var arr = arr || [];

answered Dec 25 '09 at 17:21

Brian Campbell

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But this doesn’t check if *arr* is an array. – Gumbo Dec 25 '09 at 17:23
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You're right. It wasn't clear from the question if checking that it was an array in advance is necessary, or just checking to see if the variable is already defined. This is a common idiom, and is shorter and simpler than the other ones posted, so I figured I'd post it and let the questioner decide if it's sufficient. – Brian Campbell Dec 25 '09 at 17:47
3

If `arr` is already defined in the relevant scope, then why bother with `var ...`? You can just do `arr = arr || [];` – James Dec 25 '09 at 18:15
4

The question was to check whether the array already exists, and create it if not. Thus, you don't know if it's already defined. This idiom is useful if you have several files to be linked together, and don't know which one of them will be loaded first to define some global variable that they will all share. – Brian Campbell Dec 25 '09 at 18:24
@BrianCampbell I might be wrong but unless that's being run in global scope, wouldn't `var` be hoisted and therefore it'd always just become `[]`? Edit: nvm, I am wrong :D – whitfin Aug 26 '15 at 03:49

mynameistechno · Answer 3 · 2018-05-02T04:05:57.213

66

const list = Array.isArray(x) ? x : [x];

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

Or if x could be an array and you want to make sure it is one:

const list = [].concat(x);

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

edited May 02 '18 at 04:05

answered Jul 30 '16 at 03:30

mynameistechno

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Throws `x is not defined` in both cases – huysentruitw Jan 10 '18 at 11:21
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`x` is the variable that could be an array or not. You define that. This is just example code ... – mynameistechno Jan 17 '18 at 22:42

score 12 · Answer 4 · answered Dec 25 '09 at 17:19

12

You can use the typeof operator to test for undefined and the instanceof operator to test if it’s an instance of Array:

if (typeof arr == "undefined" || !(arr instanceof Array)) {
    var arr = [];
}

answered Dec 25 '09 at 17:19

Gumbo

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You can skip the first check, because `undefined` isn't an instanceof Array. – whitfin Aug 26 '15 at 03:52
2

@zackehh If you do so, it can throw an error if `arr` is not yet defined – David Feb 20 '17 at 12:56

score 9 · Answer 5 · answered Dec 25 '09 at 18:24

9

If you want to check if the object is already an Array, to avoid the well known issues of the instanceof operator when working in multi-framed DOM environments, you could use the Object.prototype.toString method:

arr = Object.prototype.toString.call(arr) == "[object Array]" ? arr : [];

answered Dec 25 '09 at 18:24

Christian C. Salvadó

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@kangax Declaration isn't necessary in this context. – Justin Johnson Dec 26 '09 at 15:10
The declaration is necessary, but I thought it was pretty obvious that if `arr` is undeclared a `ReferenceError` will be thrown in the right-hand side of the assignment... – Christian C. Salvadó Dec 26 '09 at 18:24

score 0 · Answer 6 · answered May 02 '18 at 04:33

<script type="text/javascript">

array1  = new Array('apple','mango','banana');
var storearray1 =array1;

if (window[storearray1] && window[storearray1] instanceof Array) {
    alert("exist!");
} else {
    alert('not find: storearray1 = ' + typeof storearray1)
    }

</script>

score -7 · Answer 7 · answered Dec 25 '09 at 17:21

-7

If you are talking about a browser environment then all global variables are members of the window object. So to check:

if (window.somearray !== undefined) {
    somearray = [];
}

answered Dec 25 '09 at 17:21

slebetman

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How to create an array if an array does not exist yet?

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