3065
votes

I'm trying to write a function that either accepts a list of strings, or a single string. If it's a string, then I want to convert it to an array with just the one item so I can loop over it without fear of an error.

So how do I check if the variable is an array?


I've rounded up the various solutions below and created a jsperf test. They're all fast, so just use Array.isArray -- it's well-supported now and works across frames.

30
I thought you meant to 'check if object is an array', but you want to check if 'object is an array of strings or a single string' specifically. Not sure if you see it? Or is it just me? I was thinking of something more like this... am I the one missing something here?rr1g0
TL;DR - arr.constructor === Array is fastest.Neta
jsben.ch/#/QgYAV - a benchmark for the most common waysEscapeNetscape
TL;DR - Array.isArray(arr) since ES5; and $.isArray(arr) in jQuery.Ondra Žižka
Just bear in mind that if you by any reason overwrite your constructor via prototype that arr.constructor === Array test will return false. Array.isArray(arr) still returns true though.ghaschel

30 Answers

1347
votes

In modern browsers you can do:

Array.isArray(obj)

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

For backward compatibility you can 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
1972
votes

The method given in the ECMAScript standard to find the class of Object is to use the toString method from Object.prototype.

if( Object.prototype.toString.call( someVar ) === '[object Array]' ) {
    alert( 'Array!' );
}

Or you could use typeof to test if it is a String:

if( typeof someVar === 'string' ) {
    someVar = [ someVar ];
}

Or if you're not concerned about performance, you could just do a concat to a new empty Array.

someVar = [].concat( someVar );

There's also the constructor which you can query directly:

if (somevar.constructor.name == "Array") {
    // do something
}

Check out a thorough treatment from @T.J. Crowder's blog, as posted in his comment below.

Check out this benchmark to get an idea which method performs better: http://jsben.ch/#/QgYAV

From @Bharath convert string to array using Es6 for the question asked:

const convertStringToArray = (object) => {
   return (typeof object === 'string') ? Array(object) : object 
}

suppose:

let m = 'bla'
let n = ['bla','Meow']
let y = convertStringToArray(m)
let z = convertStringToArray(n)
console.log('check y: '+JSON.stringify(y)) . // check y: ['bla']
console.log('check y: '+JSON.stringify(z)) . // check y: ['bla','Meow']
1302
votes

I would first check if your implementation supports isArray:

if (Array.isArray)
    return Array.isArray(v);

You could also try using the instanceof operator

v instanceof Array
304
votes

jQuery also offers an $.isArray() method:

var a = ["A", "AA", "AAA"];

if($.isArray(a)) {
  alert("a is an array!");
} else {
  alert("a is not an array!");
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
108
votes

This is the fastest among all methods (all browsers supported):

function isArray(obj){
    return !!obj && obj.constructor === Array;
}
50
votes

Imagine you have this array below:

var arr = [1,2,3,4,5];

Javascript (new and older browsers):

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

or

function isArray(arr) {
  return arr instanceof Array;
}

or

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

then call it like this:

isArray(arr);

Javascript (IE9+, Ch5+, FF4+, Saf5+, Opera10.5+)

Array.isArray(arr);

jQuery:

$.isArray(arr);

Angular:

angular.isArray(arr);

Underscore and Lodash:

_.isArray(arr);
35
votes

Array.isArray works fast, but it isn't supported by all versions of browsers. So you could make an exception for others and use universal method:

    Utils = {};    
    Utils.isArray = ('isArray' in Array) ? 
        Array.isArray : 
        function (value) {
            return Object.prototype.toString.call(value) === '[object Array]';
        }
32
votes

Simple function to check this:

function isArray(object)
{
    return object.constructor === Array;
}
19
votes

There's just one line solution for this question

x instanceof Array

where x is the variable it will return true if x is an array and false if it is not.

18
votes

As MDN says in here:

use Array.isArray or Object.prototype.toString.call to differentiate regular objects from arrays

Like this:

  • Object.prototype.toString.call(arr) === '[object Array]', or

  • Array.isArray(arr)

16
votes

You can check the type of your variable whether it is an array with;

var myArray=[];

if(myArray instanceof Array)
{
....
}
16
votes

I would make a function to test the type of object you are dealing with...

function whatAmI(me){ return Object.prototype.toString.call(me).split(/\W/)[2]; }

// tests
console.log(
  whatAmI(["aiming","@"]),
  whatAmI({living:4,breathing:4}),
  whatAmI(function(ing){ return ing+" to the global window" }),
  whatAmI("going to do with you?")
);

// output: Array Object Function String

then you can write a simple if statement...

if(whatAmI(myVar) === "Array"){
    // do array stuff
} else { // could also check `if(whatAmI(myVar) === "String")` here to be sure
    // do string stuff
}
14
votes

I do this in a very simple way. Works for me. Any drawbacks?

Array.prototype.isArray = true;

a=[]; b={};
a.isArray  // true
b.isArray  // (undefined -> false)
13
votes

This is my attempt to improve on this answer taking into account the comments:

var isArray = myArray && myArray.constructor === Array;

It gets rid of the if/else, and accounts for the possibility of the array being null or undefined

12
votes

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array/isArray

Array.isArray = Array.isArray || function (vArg) {
    return Object.prototype.toString.call(vArg) === "[object Array]";
};
12
votes

I have updated the jsperf fiddle with two alternative methods as well as error checking.

It turns out that the method defining a constant value in the 'Object' and 'Array' prototypes is faster than any of the other methods. It is a somewhat surprising result.

/* Initialisation */
Object.prototype.isArray = function() {
  return false;
};
Array.prototype.isArray = function() {
  return true;
};
Object.prototype._isArray = false;
Array.prototype._isArray = true;

var arr = ["1", "2"];
var noarr = "1";

/* Method 1 (function) */
if (arr.isArray()) document.write("arr is an array according to function<br/>");
if (!noarr.isArray()) document.write("noarr is not an array according to function<br/>");
/* Method 2 (value) - **** FASTEST ***** */
if (arr._isArray) document.write("arr is an array according to member value<br/>");
if (!noarr._isArray) document.write("noarr is not an array according to member value<br/>");

These two methods do not work if the variable takes the undefined value, but they do work if you are certain that they have a value. With regards to checking with performance in mind if a value is an array or a single value, the second method looks like a valid fast method. It is slightly faster than 'instanceof' on Chrome, twice as fast as the second best method in Internet Explorer, Opera and Safari (on my machine).

12
votes

I know, that people are looking for some kind of raw javascript approach. But if you want think less about, take a look here: http://underscorejs.org/#isArray

_.isArray(object) 

Returns true if object is an Array.

(function(){ return _.isArray(arguments); })();
=> false
_.isArray([1,2,3]);
=> true
7
votes

The best practice is to compare it using constructor, something like this

if(some_variable.constructor === Array){
  // do something
}

You can use other methods too like typeOf, converting it to a string and then comparing but comparing it with dataType is always a better approach.

5
votes

If the only two kinds of values that could be passed to this function are a string or an array of strings, keep it simple and use a typeof check for the string possibility:

function someFunc(arg) {
    var arr = (typeof arg == "string") ? [arg] : arg;
}
5
votes

The best solution I've seen is a cross-browser replacement for typeof. Check Angus Croll's solution here.

The TL;DR version is below, but the article is a great discussion of the issue so you should read it if you have time.

Object.toType = function(obj) {
    return ({}).toString.call(obj).match(/\s([a-z|A-Z]+)/)[1].toLowerCase();
}
// ... and usage:
Object.toType([1,2,3]); //"array" (all browsers)

// or to test...
var shouldBeAnArray = [1,2,3];
if(Object.toType(shouldBeAnArray) === 'array'){/* do stuff */};
5
votes

Here's my lazy approach:

if (Array.prototype.array_ === undefined) {
  Array.prototype.array_ = true;
}

// ...

var test = [],
    wat = {};

console.log(test.array_ === true); // true
console.log(wat.array_ === true);  // false

I know it's sacrilege to "mess with" the prototype, but it appears to perform significantly better than the recommended toString method.

Note: A pitfall of this approach is that it wont work across iframe boundaries, but for my use case this is not an issue.

5
votes

There is a nice example in Stoyan Stefanov's book JavaScript Patterns which suppose to handle all possible problems as well as utilize ECMAScript 5 method Array.isArray().

So here it is:

if (typeof Array.isArray === "undefined") {
    Array.isArray = function (arg) {
        return Object.prototype.toString.call(arg) === "[object Array]";
    };
}

By the way, if you are using jQuery, you can use it's method $.isArray()

5
votes

This function will turn almost anything into an array:

function arr(x) {
    if(x === null || x === undefined) {
        return [];
    }
    if(Array.isArray(x)) {
        return x;
    }
    if(isString(x) || isNumber(x)) {
        return [x];
    }
    if(x[Symbol.iterator] !== undefined || x.length !== undefined) {
        return Array.from(x);
    }
    return [x];
}

function isString(x) {
    return Object.prototype.toString.call(x) === "[object String]"
}

function isNumber(x) {
    return Object.prototype.toString.call(x) === "[object Number]"
}

It uses some newer browser features so you may want to polyfill this for maximum support.

Examples:

> arr(null);
[]
> arr(undefined)
[]
> arr(3.14)
[ 3.14 ]
> arr(1/0)
[ Infinity ]
> gen = function*() { yield 1; yield 2; yield 3; }
[Function: gen]
> arr(gen())
[ 1, 2, 3 ]
> arr([4,5,6])
[ 4, 5, 6 ]
> arr("foo")
[ 'foo' ]

N.B. strings will be converted into an array with a single element instead of an array of chars. Delete the isString check if you would prefer it the other way around.

I've used Array.isArray here because it's the most robust and also simplest.

5
votes

The following could be used if you know that your object doesn't have a concat method.

var arr = [];
if (typeof arr.concat === 'function') {
    console.log("It's an array");
}
5
votes

You could is isArray method but I would prefer to check with

Object.getPrototypeOf(yourvariable) === Array.prototype

5
votes
var a = [], b = {};

console.log(a.constructor.name == "Array");
console.log(b.constructor.name == "Object");
4
votes
A = [1,2,3]
console.log(A.map==[].map)

In search for shortest version here is what I got so far.

Note, there is no perfect function that will always detect all possible combinations. It is better to know all abilities and limitations of your tools than expect a magic tool.

4
votes
function isArray(value) {
    if (value) {
        if (typeof value === 'object') {
            return (Object.prototype.toString.call(value) == '[object Array]')
        }
    }
    return false;
}

var ar = ["ff","tt"]
alert(isArray(ar))
4
votes

A simple function for testing if an input value is an array is the following:

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

This works cross browser, and with older browsers. This is pulled from T.J. Crowders' blog post

4
votes

You can try this:

var arr = []; (or) arr = new Array();
var obj = {}; (or) arr = new Object();

arr.constructor.prototype.hasOwnProperty('push') //true

obj.constructor.prototype.hasOwnProperty('push') // false