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2

how do i check if a specific array exists, and if not it will be created?

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5 Answers

up vote 16 down vote accepted

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 : []
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1  
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
That is true. How would one handle that case? – Rich Dec 25 '09 at 19:23
1  
@Rich: See my answer here: stackoverflow.com/questions/1961528/…, it will behave correctly on multi-framed DOM environments. – CMS Dec 25 '09 at 21:32
Oh, that's quite clever! – Rich Dec 26 '09 at 17:41
up vote 5 down vote 
var arr = arr || [];
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5  
But this doesn’t check if arr is an array. – Gumbo Dec 25 '09 at 17:23
1  
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
1  
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
1  
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
up vote 2 down vote

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 = [];
}
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You could use alse instanceof operator. – el.pescado Dec 25 '09 at 17:21
To my eye, this is easier to read than Rich's version. – DOK Dec 25 '09 at 17:29
I've just become totally obsessed with the "? :" ternary operator as I've been working on code that has large expressions everywhere. – Rich Dec 25 '09 at 17:36
up vote 2 down vote

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 : [];
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Need to declare arr – kangax Dec 26 '09 at 14:57
@kangax Declaration isn't necessary in this context. – Justin Johnson Dec 26 '09 at 15:10
@Justin Johnson How so? – kangax Dec 26 '09 at 16:09
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 Dec 26 '09 at 18:24
up vote -1 down vote

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 = [];
}
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Sorry, bad answer. See Gumbo's answer for better code. – slebetman Dec 25 '09 at 17:22

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