Function.prototype.call()

The call() method calls a function with a given this value and arguments provided individually.

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Syntax

function.call(thisArg, arg1, arg2, ...)

Parameters

thisArg

Optional. The value of this provided for the call to a function. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode, null and undefined will be replaced with the global object and primitive values will be converted to objects.

arg1, arg2, ...

Optional. Arguments for the function.

Return value

The result of calling the function with the specified this value and arguments.

Description

The call() allows for a function/method belonging to one object to be assigned and called for a different object.

call() provides a new value of this to the function/method. With call, you can write a method once and then inherit it in another object, without having to rewrite the method for the new object.

Examples

Using call to chain constructors for an object

You can use call to chain constructors for an object, similar to Java. In the following example, the constructor for the Product object is defined with two parameters, name and price. Two other functions Food and Toy invoke Product passing this and name and price. Product initializes the properties name and price, both specialized functions define the category.

function Product(name, price) {
  this.name = name;
  this.price = price;
}

function Food(name, price) {
  Product.call(this, name, price);
  this.category = 'food';
}

function Toy(name, price) {
  Product.call(this, name, price);
  this.category = 'toy';
}

var cheese = new Food('feta', 5);
var fun = new Toy('robot', 40);

Using call to invoke an anonymous function

In this example, we create an anonymous function and use call to invoke it on every object in an array. The main purpose of the anonymous function here is to add a print function to every object, which is able to print the right index of the object in the array. Passing the object as this value was not strictly necessary, but is done for explanatory purpose.

var animals = [
  { species: 'Lion', name: 'King' },
  { species: 'Whale', name: 'Fail' }
];

for (var i = 0; i < animals.length; i++) {
  (function(i) {
    this.print = function() {
      console.log('#' + i + ' ' + this.species
                  + ': ' + this.name);
    }
    this.print();
  }).call(animals[i], i);
}

Using call to invoke a function and specifying the context for 'this'

In the example below, when we call greet, the value of this will be bound to object obj.

function greet() {
  var reply = [this.animal, 'typically sleep between', this.sleepDuration].join(' ');
  console.log(reply);
}

var obj = {
  animal: 'cats', sleepDuration: '12 and 16 hours'
};

greet.call(obj);  // cats typically sleep between 12 and 16 hours

Using call to invoke a function and without specifying the first argument

In the example below, we invoke the display function without passing the first argument. If the first argument is not passed, the value of this is bound to the global object.

var sData = 'Wisen';

function display() {
  console.log('sData value is %s ', this.sData);
}

display.call();  // sData value is Wisen

'use strict';

var sData = 'Wisen';

function display() {
  console.log('sData value is %s ', this.sData);
}

display.call(); // Cannot read the property of 'sData' of undefined

Specifications

Browser compatibility

See also

• Function.prototype.bind()
• Function.prototype.apply()

• Introduction to Object-Oriented JavaScript