best way to test if a variable is an array

Question

Possible Duplicate:
How to detect if a variable is an array

When I need to test if variable is an array (for example input argument in a function that could be object or array) I usually use this piece of code

typeof(myVar) === 'object' && myVar.length !== undefined;

Is this the correct way or is there a more efficient way, considering that even if myVar instanceof Array is faster it should be avoided due to iframe problem?

http://stackoverflow.com/questions/767486/how-do-you-check-if-a-variable-is-an-array-in-javascript — Anders, Jul 23 '12 at 22:51
zepto.js (has good source code) uses instanceof: `function isArray(value) { return value instanceof Array }` [src](https://github.com/madrobby/zepto/blob/master/src/zepto.js) — bokonic, Jul 23 '12 at 22:52
@bokonic : The answer in the possible duplicate clearly tells that this will fail if the object is passed across frame boundaries as each frame has its own Array object. — Anmol Saraf, Jul 24 '12 at 19:43

score 6 · Accepted Answer · answered Jul 23 '12 at 22:54

Array.isArray is now available with ECMAScript 5, so you can use it with a polyfill for older browsers:

if(!Array.isArray) {
  Array.isArray = function (vArg) {
    return Object.prototype.toString.call(vArg) === "[object Array]";
  };
}

Array.isArray(myVar);

score 0 · Answer 2 · answered Jul 23 '12 at 22:50

0

The "iframe problem` can be avoided simply by not using the same name for a frame and for an array. Not hard to do, in my opinion. That said, I've never had a need to assert whether or not something is an array...

answered Jul 23 '12 at 22:50

Niet the Dark Absol

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The "iframe problem" is that an array created in an iframe is only instanceof the Array constructor of the iframe in which it was created, so if you pass it between frames then instanceof "fails". – Neil Jul 23 '12 at 23:22

score 0 · Answer 3 · answered Jul 23 '12 at 22:50

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If you are already using jQuery within your code, you may use jQuery.isArray(). Here is the documentation:

http://api.jquery.com/jQuery.isArray/

answered Jul 23 '12 at 22:50

Silver Gonzales

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score 0 · Answer 4 · answered Jul 23 '12 at 22:59

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You can try - Object.prototype.toString.call
An example -

var a = [1,2]
console.log(Object.prototype.toString.call(a))

This returns [object Array]
which can be checked with string slice method in following way

console.log(Object.prototype.toString.call(a).slice(8,-1)) <br />

which returns "Array"

var a = [1,2]
console.log(Object.prototype.toString.call(a).slice(8,-1) == "Array") // true

answered Jul 23 '12 at 22:59

Anmol Saraf

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or for less overhead you can just compare to the full string: `'[object Array]'` (like below) – bokonic Jul 23 '12 at 23:00
Well I guess Jonathan has already answered this, I must say in a better way :) – Anmol Saraf Jul 23 '12 at 23:04
Yes you are right Bokonic. I completely agree with you. – Anmol Saraf Jul 23 '12 at 23:05
One more way to check this is by a.constructor == Array // returns true ( only if a is true array and a is not array-like ) – Anmol Saraf Jul 24 '12 at 19:45

best way to test if a variable is an array

4 Answers4