Testing if the element is an array, in Javascript

Question

To check if an element is an array in JavaScript, I have always used Crockford's function (pg 61 of The Good Parts):

var is_array = function (value) {
    return value &&
        typeof value === 'object' &&
        typeof value.length === 'number' &&
        typeof value.splice === 'function' &&
        !(value.propertyIsEnumerable('length'));
}

But if I'm not mistaken, recently some guy from Google had found a new way on how to test for a JavaScript array, but I just can't remember from where I read it and how the function went.

Can anyone point me to his solution please?

[Update]
The person from Google who apparently discovered this is called Mark Miller.

Now I've also read that from this post that his solution can easily break as well:

// native prototype overloaded, some js libraries extends them
Object.prototype.toString= function(){
  return  '[object Array]';
}

function isArray ( obj ) {
  return Object.prototype.toString.call(obj) === '[object Array]';
}

var a = {};
alert(isArray(a)); // returns true, expecting false;

So, I ask, is there any way that we can truly check for array validity?

Possible duplicates: stackoverflow.com/questions/1202841 stackoverflow.com/questions/1058427 — Christian C. Salvadó
– Christian C. Salvadó, Commented Nov 18, 2009 at 22:13
@Andreas: if you stab yourself in the eye, don't wonder why you're going blind... — Christoph
– Christoph, Commented Nov 18, 2009 at 23:00
ah haha; ure talking about messing with the Object.prototype ? — Andreas Grech
– Andreas Grech, Commented Nov 18, 2009 at 23:00
@Andreas: yes; extending Object.prototype is (or would be) ok as long as everyone remembers to check hasOwnProperty(), but overwriting native methods definitely is a big NO-NO; the only valid reason to do this is to fix bugs/non-standard behaviour (cough JScript cough) — Christoph
– Christoph, Commented Nov 18, 2009 at 23:13
I agree with you completely, but the problem is that certain libraries tend to extend the Object prototype...which will then screw up your code — Andreas Grech
– Andreas Grech, Commented Nov 18, 2009 at 23:16

Russ Cam · Accepted Answer · 2009-11-18 22:30:35Z

7

I believe you are looking for

Object.prototype.toString.call(value) === "[object Array]";

This is the method that jQuery uses to check whether a passed parameter value is a function or array object. There are browser specific instances where using typeof does not yield the correct result

edited Nov 18, 2009 at 22:30

answered Nov 18, 2009 at 22:00

Russ Cam

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4 Comments

Crescent Fresh Over a year ago

FYI the second example is not a valid shorthand. jQuery creates a local toString reference that simply points to Object.prototype.toString.

Russ Cam Over a year ago

Doesn't jQuery define it as a local reference to resolve the call quicker? Since all objects derive from Object, they should all have the toString function

Crescent Fresh Over a year ago

@Russ: heh, sorry I think we're just saying the same thing. You're right, jQuery simply does var toString = Object.prototype.toString;. My point was Object.prototype.toString !== window.toString, which invalidates your statement that the second example is equivalent to the first example, that's all :) Cheers.

Russ Cam Over a year ago

You could of course, define a toString function on the constructor's prototype and break the shorthand, so duly noted, will remove the shorthand :)

Aishwar · Accepted Answer · 2009-11-18 22:46:21Z

-1

You could do this:

t = [1,2];
// Now to check if this is an array
if (t.constructor == Array)
{
    alert('t is an array');
}
else
{
    alert('t is NOT an array');
}

Basically, variable.constructor == Array

answered Nov 18, 2009 at 22:46

Aishwar

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1 Comment

Andreas Grech Over a year ago

That fails as well, for the same reasons I told @tyranid. Read the section 'Detecting Object and Array' at this post: karmagination.com/blog/2009/07/29/…

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