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javascript: check if array exist, if not create it

Answer 1

+2 A:

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 = [];
}

Gumbo 2009-12-25 17:19:36

You could use alse instanceof operator.

el.pescado 2009-12-25 17:21:06

To my eye, this is easier to read than Rich's version.

DOK 2009-12-25 17:29:20

I've just become totally obsessed with the "? :" ternary operator as I've been working on code that has large expressions everywhere.

Rich 2009-12-25 17:36:15

Answer 2

A:

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 = [];
}

slebetman 2009-12-25 17:21:00

Sorry, bad answer. See Gumbo's answer for better code.

slebetman 2009-12-25 17:22:28

Answer 3

+4 A:

var arr = arr || [];

Brian Campbell 2009-12-25 17:21:03

But this doesn’t check if *arr* is an array.

Gumbo 2009-12-25 17:23:28

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 2009-12-25 17:47:42

If `arr` is already defined in the relevant scope, then why bother with `var ...`? You can just do `arr = arr || [];`

J-P 2009-12-25 18:15:39

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 2009-12-25 18:24:26

Answer 4

+8 A:

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 : []

Rich 2009-12-25 17:21:10

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.

J-P 2009-12-25 18:17:04

That is true. How would one handle that case?

Rich 2009-12-25 19:23:53

@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.

CMS 2009-12-25 21:32:15

Oh, that's quite clever!

Rich 2009-12-26 17:41:59

Answer 5

+2 A:

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 : [];

CMS 2009-12-25 18:24:44

Need to declare `arr`

kangax 2009-12-26 14:57:20

@kangax Declaration isn't necessary in this context.

Justin Johnson 2009-12-26 15:10:56

@Justin Johnson How so?

kangax 2009-12-26 16:09:53

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...

CMS 2009-12-26 18:24:03

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