call

Summary

Calls a function with a given this value and arguments provided individually.

NOTE: While the syntax of this function is almost identical to that of apply(), the fundamental difference is that call() accepts an argument list, while apply() accepts a single array of arguments.
Method of Function
Implemented in JavaScript 1.3
ECMAScript Edition ECMAScript 3rd Edition

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 value. Two other functions Food and Toy invoke Product passing this and name and value. 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 = new Product();

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

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); 
    } 
  }).call(animals[i], i);
}

See Also

apply

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