Checking types in JavaScript

12 Jan 2009

Checking types in JavaScript can be a little tricky, especially with browser (and language) quirks. In light of this recent article I’ve made a small helper function using the recommended method:

The Code:

function type(o){
    return !!o && Object.prototype.toString.call(o).match(/(w+)]/)[1];
}

Usage:

type(101);          // returns 'Number'
type('hello');      // returns 'String'
type({});           // returns 'Object'
type([]);           // returns 'Array'
type(function(){}); // returns 'Function'
type(new Date());   // returns 'Date'
type(document);     // returns 'HTMLDocument'
 
// For example, to test for an Array:
if( type([1,2,3,4,5]) === 'Array' ) {
    doStuff();
}

This new function makes a few improvements on the shameful typeof operator. For one thing you can now successfully test for an array!

Here’s an alternative implementation:

 
(function(){
    var types = ['Array','Function','Object','String','Number'],
        typesLength = types.length;
    while (typesLength--) {
        window['is' + types[typesLength]] = (function(type){
            return function(o) {
                return !!o && ( Object.prototype.toString.call(o) === '[object ' + type + ']' );
            }
        })(types[typesLength]);
    }
})();
 
// Usage:
isFunction( function(){} ); // true
isArray( [1,2,3,4,5] );     // true
isNumber( 3334322 );        // true
isObject( {a:1, b:2} );     // true
isString( 'Hello!' );       // true

Thanks for reading! Please share your thoughts with me on Twitter. Have a great day!

So far there's been 9 Responses to
“Checking types in JavaScript”

Gman January 12th, 2009 at 6:27 pm

what does “===” mean?

James January 12th, 2009 at 6:36 pm

Hi Gman!

‘===’ is known as the strict equality operator. It is normally used instead of the well known general equality operator (‘==’). The only difference is that the former checks for type and the latter doesn’t. It’s best practice to use the strict version for that very reason.

An example:

var realNumber = 999;
var numberInString = '999';
 
realNumber == numberInString; // TRUE
realNumber === numberInString; // FALSE

Paul Irish January 12th, 2009 at 7:48 pm

just for reference, here are the official recommendations for doing these in jQuery: http://docs.jquery.com/JQuery_Core_Style_Guidelines

Pete January 12th, 2009 at 8:35 pm

nice idea

Ryan Townsend January 12th, 2009 at 8:37 pm

Paul, those guidelines do prevent use of a switch() condition though, for better code-readability James’ method works nicely.

Gman January 14th, 2009 at 3:58 am

@James
ya, now I got it. Thank you

GeekFG January 14th, 2009 at 10:11 am

Why did you not simply use typeOf (http://docs.jquery.com/JQuery_Core_Style_Guidelines) ?

James January 14th, 2009 at 10:17 am

@GeekFG, did you read my post (or any of the other comments)? Typeof is okay for most cases but it reports certain types incorrectly. For example, this won’t work:

alert( typeof [1,2,3,4,5] ); // This should alert 'Array', not 'Object'

GeekFG January 14th, 2009 at 11:38 am

That’s why a added this link “http://docs.jquery.com/JQuery_Core_Style_Guidelines”.
When I want to check for an array I use jQuery.isArray(object).

Moreover Object.prototype is a better way for browsers compatibility than window[key] = function(){}

Checking types in JavaScript

The Code:

Usage:

So far there's been 9 Responses to “Checking types in JavaScript”

So far there's been 9 Responses to
“Checking types in JavaScript”