How do you check if a variable is an array in JavaScript? [duplicate]

Answer 1

I was using this line of code:

if (variable.push) {
   // variable is array, since AMAIK only arrays have push() method.
}

Answer 2

In Crockford's JavaScript The Good Parts, there is a function to check if the given argument is an array:

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

He explains:

First, we ask if the value is truthy. We do this to reject null and other falsy values. Second, we ask if the typeof value is 'object'. This will be true for objects, arrays, and (weirdly) null. Third, we ask if the value has a length property that is a number. This will always be true for arrays, but usually not for objects. Fourth, we ask if the value contains a splice method. This again will be true for all arrays. Finally, we ask if the length property is enumerable (will length be produced by a for in loop?). That will be false for all arrays. This is the most reliable test for arrayness that I have found. It is unfortunate that it is so complicated.

Answer 3

You could also use:

if (value instanceof Array) {
  alert('value is Array!');
} else {
  alert('Not an array');
}

This seems to me a pretty elegant solution, but to each his own.

Edit:

As of ES5 there is now also:

Array.isArray(value);

But this will break on older browsers, unless you are using polyfills (basically... IE8 or similar).

Answer 4

I personally like Peter's suggestion: https://stackoverflow.com/a/767499/414784 (for ECMAScript 3. For ECMAScript 5, use Array.isArray())

Comments on the post indicate, however, that if toString() is changed at all, that way of checking an array will fail. If you really want to be specific and make sure toString() has not been changed, and there are no problems with the objects class attribute ([object Array] is the class attribute of an object that is an array), then I recommend doing something like this:

//see if toString returns proper class attributes of objects that are arrays
//returns -1 if it fails test
//returns true if it passes test and it's an array
//returns false if it passes test and it's not an array
function is_array(o)
{
    // make sure an array has a class attribute of [object Array]
    var check_class = Object.prototype.toString.call([]);
    if(check_class === '[object Array]')
    {
        // test passed, now check
        return Object.prototype.toString.call(o) === '[object Array]';
    }
    else
    {
        // may want to change return value to something more desirable
        return -1; 
    }
}

Note that in JavaScript The Definitive Guide 6th edition, 7.10, it says Array.isArray() is implemented using Object.prototype.toString.call() in ECMAScript 5. Also note that if you're going to worry about toString()'s implementation changing, you should also worry about every other built in method changing too. Why use push()? Someone can change it! Such an approach is silly. The above check is an offered solution to those worried about toString() changing, but I believe the check is unnecessary.

Answer 5

Since the .length property is special for arrays in javascript you can simply say

obj.length === +obj.length // true if obj is an array

Underscorejs and several other libraries use this short and simple trick.