How do I check if a variable is an array in JavaScript?

There are several ways of checking if an variable is an array or not. The best solution is the one you have chosen.

variable.constructor === Array

This is the fastest method on Chrome, and most likely all other browsers. All arrays are objects, so checking the constructor property is a fast process for JavaScript engines.

If you are having issues with finding out if an objects property is an array, you must first check if the property is there.

variable.prop && variable.prop.constructor === Array

Some other ways are:

Array.isArray(variable)

Update May 23, 2019 using Chrome 75, shout out to @AnduAndrici for having me revisit this with his question This last one is, in my opinion the ugliest, and it is one of the slowest fastest. Running about 1/5 the speed as the first example. This guy is about 2-5% slower, but it's pretty hard to tell. Solid to use! Quite impressed by the outcome. Array.prototype, is actually an array. you can read more about it here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

variable instanceof Array

This method runs about 1/3 the speed as the first example. Still pretty solid, looks cleaner, if you're all about pretty code and not so much on performance. Note that checking for numbers does not work as variable instanceof Number always returns false. Update: instanceof now goes 2/3 the speed!

So yet another update

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

This guy is the slowest for trying to check for an Array. However, this is a one stop shop for any type you're looking for. However, since you're looking for an array, just use the fastest method above.

Also, I ran some test: http://jsperf.com/instanceof-array-vs-array-isarray/35 So have some fun and check it out.

Note: @EscapeNetscape has created another test as jsperf.com is down. http://jsben.ch/#/QgYAV I wanted to make sure the original link stay for whenever jsperf comes back online.

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).

In modern browsers (and some legacy browsers), you can do

Array.isArray(obj)

(Supported by Chrome 5, Firefox 4.0, IE 9, Opera 10.5 and Safari 5)

If you need to support older versions of IE, you can use es5-shim to polyfill Array.isArray; or add the following

# only implement if no native implementation is available
if (typeof Array.isArray === 'undefined') {
  Array.isArray = function(obj) {
    return Object.prototype.toString.call(obj) === '[object Array]';
  }
};

If you use jQuery you can use jQuery.isArray(obj) or $.isArray(obj). If you use underscore you can use _.isArray(obj)

If you don't need to detect arrays created in different frames you can also just use instanceof

obj instanceof Array

Note: the arguments keyword that can be used to access the argument of a function isn't an Array, even though it (usually) behaves like one:

var func = function() {
  console.log(arguments)        // [1, 2, 3]
  console.log(arguments.length) // 3
  console.log(Array.isArray(arguments)) // false !!!
  console.log(arguments.slice)  // undefined (Array.prototype methods not available)
  console.log([3,4,5].slice)    // function slice() { [native code] } 
}
func(1, 2, 3)

There are multiple solutions with all their own quirks. This page gives a good overview. One possible solution is:

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

I noticed someone mentioned jQuery, but I didn't know there was an isArray() function. It turns out it was added in version 1.3.

jQuery implements it as Peter suggests:

isArray: function( obj ) {
    return toString.call(obj) === "[object Array]";
},

Having put a lot of faith in jQuery already (especially their techniques for cross-browser compatibility) I will either upgrade to version 1.3 and use their function (providing that upgrading doesn’t cause too many problems) or use this suggested method directly in my code.

Many thanks for the suggestions.

This is an old question but having the same problem i found a very elegant solution that i want to share.

Adding a prototype to Array makes it very simple

Array.prototype.isArray = true;

Now once if you have an object you want to test to see if its an array all you need is to check for the new property

var box = doSomething();

if (box.isArray) {
    // do something
}

isArray is only available if its an array

Via Crockford:

function typeOf(value) {
    var s = typeof value;
    if (s === 'object') {
        if (value) {
            if (value instanceof Array) {
                s = 'array';
            }
        } else {
            s = 'null';
        }
    }
    return s;
}

The main failing Crockford mentions is an inability to correctly determine arrays that were created in a different context, e.g., window. That page has a much more sophisticated version if this is insufficient.

If you're only dealing with EcmaScript 5 and above then you can use the built in Array.isArray function

e.g.,

Array.isArray([])    // true
Array.isArray("foo") // false
Array.isArray({})    // false

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.

When I posted this question the version of JQuery that I was using didn't include an isArray function. If it had have I would have probably just used it trusting that implementation to be the best browser independant way to perform this particular type check.

Since JQuery now does offer this function, I would always use it...

$.isArray(obj);

(as of version 1.6.2) It is still implemented using comparisons on strings in the form

toString.call(obj) === "[object Array]"

Thought I would add another option for those who might already be using the Underscore.js library in their script. Underscore.js has an isArray() function (see http://underscorejs.org/#isArray).

_.isArray(object) 

Returns true if object is an Array.

If you are using Angular, you can use the angular.isArray() function

var myArray = [];
angular.isArray(myArray); // returns true

var myObj = {};
angular.isArray(myObj); //returns false

http://docs.angularjs.org/api/ng/function/angular.isArray

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.

The universal solution is below:

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

Starting from ECMAScript 5, a formal solution is :

Array.isArray(arr)

Also, for old JavaScript libs, you can find below solution although it's not accurate enough:

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

The solutions are from http://www.pixelstech.net/topic/85-How-to-check-whether-an-object-is-an-array-or-not-in-JavaScript

For those who code-golf, an unreliable test with fewest characters:

function isArray(a) {
  return a.map;
}

This is commonly used when traversing/flattening a hierarchy:

function golf(a) {
  return a.map?[].concat.apply([],a.map(golf)):a;
}

input: [1,2,[3,4,[5],6],[7,[8,[9]]]]
output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

code referred from https://github.com/miksago/Evan.js/blob/master/src/evan.js

  var isArray = Array.isArray || function(obj) {
    return !!(obj && obj.concat && obj.unshift && !obj.callee);};

I was using this line of code:

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

I have created this little bit of code, which can return true types.

I am not sure about performance yet, but it's an attempt to properly identify the typeof.

https://github.com/valtido/better-typeOf also blogged a little about it here http://www.jqui.net/jquery/better-typeof-than-the-javascript-native-typeof/

it works, similar to the current typeof.

var user = [1,2,3]
typeOf(user); //[object Array]

It think it may need a bit of fine tuning, and take into account things, I have not come across or test it properly. so further improvements are welcomed, whether it's performance wise, or incorrectly re-porting of typeOf.

I think using myObj.constructor==Object and myArray.constructor==Array is the best way. Its almost 20x faster than using toString(). If you extend objects with your own constructors and want those creations to be considered "objects" as well than this doesn't work, but otherwise its way faster. typeof is just as fast as the constructor method but typeof []=='object' returns true which will often be undesirable. http://jsperf.com/constructor-vs-tostring

one thing to note is that null.constructor will throw an error so if you might be checking for null values you will have to first do if(testThing!==null){}

From w3schools:

function isArray(myArray) {
    return myArray.constructor.toString().indexOf("Array") > -1;
}

I liked the Brian answer:

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; 
    }
}

but you could just do like this:

return Object.prototype.toString.call(o) === Object.prototype.toString.call([]);

I tried most of the solutions here. But none of them worked. Then I came up with a simple solution. Hope it will help someone & save their time.

if(variable.constructor != undefined && variable.constructor.length > 0) {
        /// IT IS AN ARRAY
} else {
       /// IT IS NOT AN ARRAY
}

Here is an answer that works without fail both in old browsers and across frames. It takes the recommended Array.isArray() static method of EcmaScript 5+ and the old recommended way before ES5 and combines them so that you have a working function in both new and old JS versions:

isArray = Array.isArray || function(value) {
    return Object.prototype.toString.call(value)=="[object Array]";
}

isArray([]);//true

Of course Array.isArray() is more than 10 years old now. So you might not need to support browsers older than that. However you shouldn't underestimate the number of old browsers that are still out there.

 if(elem.length == undefined){
    // is not an array
 }else{
    // is an array
 } 

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.

Something I just came up with:

if (item.length) //This is an array else //not an array