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 fromObject:

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 theObject.prototype.toString() with every object, you need to call Function.prototype.call() orFunction.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]