Question

What is the best de-facto standard cross-browser method to determine if a variable in JavaScript is an array or not?

Searching the web there are a number of different suggestions, some good and quite a few invalid.

For example, the following is a basic approach:

function isArray(obj) {
    return (obj && obj.length);
}

However, note what happens if the array is empty, or obj actually is not an array but implements a length property, etc.

So which implementation is the best in terms of actually working, being cross-browser and still perform efficiently?

Answer 1

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;
    }
}