Function.prototype.call()

Summary

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

Syntax

fun.call(thisArg[, arg1[, arg2[, ...]]])

Parameters

thisArg

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

arg1, arg2, ...

Arguments for the object.

Description

You can assign a different this object when calling an existing function. this refers to the current object, the calling object.

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;

  if (price < 0)
    throw RangeError('Cannot create product 
                     "' + name + '" with a negative price');
  return this;
}

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

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

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

Using call to invoke an anonymous function

In this purely constructed example, we create 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);
}

Specifications

Browser compatibility

See also

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