How to check if an object is an array?

Question

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

Answer 1

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

Answer 2

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]';
        }

Answer 3

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)

Answer 4

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).

Answer 5

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']

Answer 6

Simple function to check this:

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