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

Question

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

if (variable.constructor == Array)

• aleemb: Checking for an object to be an array has some specific caveats... Peter's answer is the only one you should use.
• Peter Smit: @Andy It seems that my answer is not the best. Maybe you should select a different answer as accepted?
• Andy McCluggage: Good point Peter. I hadn't realised your answer was receiving comments like this. I think I have long since begun to use the JQuery.isArray function when checking for arrays, and interestingly that is implemented differently to any other answer given here. I have marked the popular answer as correct.
• Andy McCluggage: Sorry that's wrong. I looked a little deeper and (as of version 1.6.2) JQuery still type checks using comparisons in the form.... toString.call(obj) === "[object Array]"
• Dexygen: "This question has been asked before" ... NO, that question got asked AFTER this one

Answer 1

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 (削除) slowest (削除ここまで) fastest. (削除) Running about 1/5 the speed as the first example. (削除ここまで) 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 (削除) 1/3 the speed (削除ここまで) 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.

• Michael Scott Asato Cuthbert: Note that if you're not sure if the variable is defined or if it could be null, be sure to do those checks first since those are the common values/objects that do not have a constructor.
• GorvGoyl: NOTE: 'variable.constructor === Array' will throw EXCEPTION if variable is null but 'variable instanceof Array' not!
• Hedley Smith: As of Chrome 59 isArray() seems to be significantly faster, so much so I see no reason to not use isArray() in all situations.
• Bash: @jemiloii I disagree about it being clear. Your updates say that instanceof runs 2/3 the speed of the original answer. Does that mean faster? Slower? There's some ambiguity to the wording, although admittedly the context of the paragraph seems to indicate slower. I ran some benchmarks of my own by modifying the code in jsben.ch/QgYAV but those results suggested instanceof was fastest. On a related note, jsben.ch/QgYAV now links to an empty benchmark.
• AtilioA: If you're gonna edit your answer, please consider making it more readable for those who have not read it before.

Answer 2

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).

• BYK: I suggest, actually insist on sticking with this "instanceof" operator if you are not working with multiple frames. This is the right way of checking the object type.
• Pierre: The one case where this would fail is if you were trying to test for an Array or Object since Array instanceof Object == true.
• tuck: If you are using jQuery to pass elements with find('code') or something similar you would want to check the variable with variable instanceof Object since it is not an instance of an Array.
• Nobbynob Littlun: @BrettBender If you're still active, might you update your answer to reflect that as of ES5 we have Array.isArray?
• user3802811: @AndrewK see Fela Winkelmolen's answer, which has the Array.isArray method. As for this answer, it's probably not a great idea to morph an answer into a different answer via editing.

Answer 3

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)

• John: This is probably the best most modern approach. I've seen it along with the polyfill on MDN so that means Mozilla trusts it developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…
• Brock: Aren't you missing prototype there? Seems it should be Object.prototype.toString.call.
• aj go: we can also determine if the objects has the methods that are existing in array like push, splice, etc

Answer 4

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]'; 
}

• BYK: If you read carefully, it says this method is needed when you are working with mult-frame documents which, is not recommended. This method can easly borke on a little change in the "toString" function.
• Peter Smit: Therefor the link is given so that Brett can check them out and see in which case his function has to work
• Brian: See my answer below. I recommend Peter Smit's way.
• Hank: This method is recommended by Mozilla.
• traktor: This method was devised because the array constructor Array is not a constant across all window objects accessible from within JavaScipt. Checks using instanceof and constructor==Array techniques can fail in complex environments. In 2024 and earlier the recommended solution is to use the static method Array.isArray(valueToTest) .

Answer 5

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.

• Nick G.: See this article for a good discussion on the topic. The conclusion is to use this solution.
• polm23: This gives me the error SCRIPT65535 in IE10.

Answer 6

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

• Markus Bruckner: @Vitimtk A prototype acts as a fallback for the actual object, so this should work even if the array in question already existed. It won't work before the source line is processed, of course.
• ErikE: Assuming no one does Object.prototype.isArray = true;! :(
• JaredMcAteer: Note that in ES5 Array.isArray is a method (e.g., Array.isArray([1,2,3]) === true) so @ErikE wasn't being a troll. I would avoid following this answer as it will break code in some modern browsers.
• ErikE: @Ibu and you can do {}.isArray === true with my "solution", which was the whole point...
• BentOnCoding: Modifying the prototype of Data Types is a bad practice in my opinion

Answer 7

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.

Answer 8

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

Answer 9

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.

• styfle: Good call on the ECMAScript 5 standard. Sure you can't guarantee the browser supports it but this should be the first way to check in new code.
• Grant Lindsay: I'll start by saying that this is a bit over my head. However, would a test like this be more robust?: return Object.prototype.toString.call(o) === Object.prototype.toString.call([]);

Answer 10

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]"

Answer 11

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.

• Kris: The underscore js implementation uses the native Array.isArray if available, otherwise it uses the toString method.
• Jundiaius: The same function is present in Lodash

Answer 12

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.

• FrancescoMM: And that was just the good parts. Imagine when "JavaScript The Bad Parts" gets published...

Answer 13

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

• PDA: You can also use non Angular specific, but only IE9+ and all standards browsers: <pre><code> Array.isArray(myArray); //returns true Array.isArray(myObj); //returns false </code> </pre>

Answer 14

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

Answer 15

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]

Answer 16

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);};

• Marco Demaio: Why do you test both concat and unshift, wouldn't suffice to test for unshift?
• Kris: The more methods we check that Array has the move likely it really is an array. Other objects might have concat or unshift but less likely to have both.

Answer 17

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.

Answer 18

From w3schools:

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

Answer 19

I was using this line of code:

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

• teleclimber: This is not a good solution at all. With this "solution" any object with a property push that is truthy will be considered an Array.

Answer 20

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([]);

Answer 21

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.

Answer 22

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){}

Answer 23

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
}

Answer 24

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

Answer 25

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.

• Andy McCluggage: Would you mind explaining how that works? Mainly, what does the '+' do?
• John: This is good, but it is also true when the object is a function or a string as well as any other object with property length of type number. Why? Well, the unary + operator actually casts a variable as a number. So basically they are checking if obj.length is a number. [object Object] has no property length it is undefined so when you cast undefined as a number it becomes NaN the above check comes out as false. So it returns true if the object property length is a number, which in the case of arrays, strings, and functions it would be true. Underscore has to be doing more than just this.
• Advait Junnarkar: This isn't good because you might create your own object that has a length prop, e.g. const myNovel = { length: 150 } It'll create false positives.

Answer 26

Something I just came up with:

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

• kev: var item = 'this_is_not_an_array'
• pmrotule: That's a bad solution! A string has also a length.
• Shingala94: Actually a string is an array of characters so in essence this does work
• wahwahwah: @PatrickNijhuis - string is a primitive type in javascript, whereas array is an object. In common language you're correct - a string is an array of characters - but in javascript, this assertion is wrong. It's an important distinction, and the reason why this is a bad answer.
• Dave Burton: No, a zero-length array is still an array.