JavaScript: Check if object is array?

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

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!");
}

You can try this approach: https://waybackassets.bk21.net/20100424091244/http://www.ajaxdr.com/code/javascript-version-of-phps-is_array-function/

EDIT: also, if you are already using JQuery in your project, you can use its function $.isArray().

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]';
        }

Simple function to check this:

function isArray(object)
{
    if (object.constructor === Array) return true;
    else return false;
}

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)

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

var myArray=[];

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

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
}

Here's my lazy-man 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.

Since I don't like any Object.prototype-calls, I searched for another solution. Especially because the solutions of ChaosPandion won't always work, and the solution of MidnightTortoise with isArray() doesn't work with arrays coming from the DOM (like getElementsByTagName). And finally I found an easy and cross-browser solution, which probably also would have worked with Netscape 4. ;)

It's just these 4 lines (checking any object h):

function isArray(h){
    if((h.length!=undefined&&h[0]!=undefined)||(h.length===0&&h[0]===undefined)){
        return true;
    }
    else{ return false; }
}

I already tested these arrays (all return true):

1) array=d.getElementsByName('some_element'); //'some_element' can be a real or unreal element
2) array=[];
3) array=[10];
4) array=new Array();
5) array=new Array();
   array.push("whatever");

Can anybody confirm that this works for all cases? Or does anybody find a case where my solution don't work?

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.

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;
}

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 */};

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://documentcloud.github.com/underscore/#isArray

isArray_.isArray(object) 

Returns true if object is an Array.

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

Dojo Toolkit has deprecated its isArray() function and now recommends using simply:

val instanceof Array

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]";
};

You could try checking if the object in question has an array method as one of its methods such as push or slice. This doesn't guarantee absolutely that it is an array, but objects that have methods with those names are probably going to be arrays.

var a = [];
var b = {};

if ((a.slice) && (a.slice === function))
{
    alert ('A is an array');
}
else
{
    alert ('A is not an array');
}

if ((b.slice) && (b.slice === function))
{
    alert ('B is an array');
}
else
{
    alert ('B is not an array');
}

Beware, however, this method can be defeated if you have a non-array object that happens to implement a slice methods.

b.slice = function (){}

if ((b.slice) && (b.slice === function))
{
    alert ('B is an array');
}
else
{
    alert ('B is not an array');
}