How to detect if a variable is an array

Type checking of objects in JS is done via instanceof, ie

obj instanceof Array

This won't work if the object is passed across frame boundaries as each frame has its own Array object. You can work around this by checking the internal [[Class]] property of the object. To get it, use Object.prototype.toString() (this is guaranteed to work by ECMA-262):

Object.prototype.toString.call(obj) === '[object Array]'

Both methods will only work for actual arrays and not array-like objects like the arguments object or node lists. As all array-like objects must have a numeric length property, I'd check for these like this:

typeof obj !== 'undefined' && obj !== null && typeof obj.length === 'number'

Please note that strings will pass this check, which might lead to problems as IE doesn't allow access to a string's characters by index. Therefore, you might want to change typeof obj !== 'undefined' to typeof obj === 'object' to exclude primitives and host objects with types distinct from 'object' alltogether. This will still let string objects pass, which would have to be excluded manually.

In most cases, what you actually want to know is whether you can iterate over the object via numeric indices. Therefore, it might be a good idea to check if the object has a property named 0 instead, which can be done via one of these checks:

typeof obj[0] !== 'undefined' // false negative for `obj[0] = undefined`
obj.hasOwnProperty('0') // exclude array-likes with inherited entries
'0' in Object(obj) // include array-likes with inherited entries

The cast to object is necessary to work correctly for array-like primitives (ie strings).

Here's the code for robust checks for JS arrays:

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

and iterable (ie non-empty) array-like objects:

function isNonEmptyArrayLike(obj) {
    try { // don't bother with `typeof` - just access `length` and `catch`
        return obj.length > 0 && '0' in Object(obj);
    }
    catch(e) {
        return false;
    }
}

The arrival of ECMAScript 5th Edition gives us the most sure-fire method of testing if a variable is an array, Array.isArray():

Array.isArray([]); // true

While the accepted answer here will work across frames and windows for most browsers, it doesn't for Internet Explorer 7 and lower, because Object.prototype.toString called on an array from a different window will return [object Object], not [object Array]. IE 9 appears to have regressed to this behaviour also (see updated fix below).

If you want a solution that works across all browsers, you can use:

(function () {
    var toString = Object.prototype.toString,
        strArray = Array.toString(),
        jscript  = /*@cc_on @_jscript_version @*/ +0;

    // jscript will be 0 for browsers other than IE
    if (!jscript) {
        Array.isArray = Array.isArray || function (obj) {
            return toString.call(obj) == "[object Array]";
        }
    }
    else {
        Array.isArray = function (obj) {
            return "constructor" in obj && String(obj.constructor) == strArray;
        }
    }
})();

It's not entirely unbreakable, but it would only be broken by someone trying hard to break it. It works around the problems in IE7 and lower and IE9. The bug still exists in IE 10 PP2, but it might be fixed before release.

PS, if you're unsure about the solution then I recommend you test it to your hearts content and/or read the blog post. There are other potential solutions there if you're uncomfortable using conditional compilation.

Crockford has two answers on page 106 of "The Good Parts." The first one checks the constructor, but will give false negatives across different frames or windows. Here's the second:

if (my_value && typeof my_value === 'object' &&
        typeof my_value.length === 'number' &&
        !(my_value.propertyIsEnumerable('length')) {
    // my_value is truly an array!
}

Crockford points out that this version will identify the arguments array as an array, even though it doesn't have any of the array methods.

His interesting discussion of the problem begins on page 105.

There is further interesting discussion (post-Good Parts) here which includes this proposal:

var isArray = function (o) {
    return (o instanceof Array) ||
        (Object.prototype.toString.apply(o) === '[object Array]');
};

All the discussion makes me never want to know whether or not something is an array.