javascript toString.call()

The

toString()

method returns a string representing
object.

Syntax

obj.toString()

Description

Every object has a

toString()

method that is automatically called when the object is to be represented
as a text value or when an object is referred to in a manner in which a string is expected. By default, the

toString()

method
is inherited by every object descended from

Object

. If this method is not overridden in a custom object,

toString()

returns
"[object type]", where

type

is
the object type. The following code illustrates this:

var o = new Object();
o.toString(); // returns [object Object]

Note: Starting in JavaScript 1.8.5

toString()

called
on

null

returns

[object Null]

,
and

undefined

returns

[object Undefined]

,
as defined in the 5th Edition of ECMAScript and a subsequent Errata. See Using
toString to detect object type.

Examples

Overriding the default

toString

method

You can create a function to be called in place of the default

toString()

method. The

toString()

method
takes no arguments and should return a string. The

toString()

method you create can be any value you
want, but it will be most useful if it carries information about the object.

The following code defines the

Dog

object type and creates

theDog

,
an object of type

Dog

:

function Dog(name, breed, color, sex) {
this.name = name;
this.breed = breed;
this.color = color;
this.sex = sex;
}

theDog = new Dog('Gabby', 'Lab', 'chocolate', 'female');

If you call the

toString()

method on this custom object, it returns the default value inherited from

Object

:

theDog.toString(); // returns [object Object]

The following code creates and assigns

dogToString()

to override the default

toString()

method.
This function generates a string containing the name, breed, color, and sex of the object, in the form "

property
= value;

".

Dog.prototype.toString = function dogToString() {
var ret = 'Dog ' + this.name + ' is a ' + this.sex + ' ' + this.color + ' ' + this.breed;
return ret;
}

With the preceding code in place, any time

theDog

is used in a string context, JavaScript automatically
calls the

dogToString()

function, which returns the following string:

"Dog Gabby is a female chocolate Lab"

Using

toString()

to detect object class

toString()

can be used with every object and allows you to get its class. To use the

Object.prototype.toString()

with
every object, you need to call

Function.prototype.call()

or

Function.prototype.apply()

on
it, passing the object you want to inspect as the first parameter called

thisArg

.

var toString = Object.prototype.toString;

toString.call(new Date);    // [object Date]
toString.call(new String);  // [object String]
toString.call(Math);        // [object Math]

// Since JavaScript 1.8.5
toString.call(undefined);   // [object Undefined]
toString.call(null);        // [object Null]