context to use call and apply in Javascript?

Question

Guys can any one explain context to use call and apply methods in Javascript?

Why to use call and apply instead of calling a function directly ?

Answer 1

You use call or apply when you want to pass a different this value to the function. In essence, this means that you want to execute a function as if it were a method of a particular object. The only difference between the two is that call expects parameters separated by commas, while apply expects parameters in an array.

An example from Mozilla's apply page, where constructors are chained:

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.apply(this, arguments);     this.category = 'food'; } Food.prototype = new Product();  function Toy(name, price) {     Product.apply(this, arguments);     this.category = 'toy'; } Toy.prototype = new Product();  var cheese = new Food('feta', 5); var fun = new Toy('robot', 40); 

What Product.apply(this, arguments) does is the following: The Product constructor is applied as a function within each of the Food and Toy constructors, and each of these object instances are being passed as this. Thus, each of Food and Toy now have this.name and this.category properties.

Answer 2

Only if you use call or apply you can modify the this context inside the function.

Unlike other languages - in JavaScript this does not refer to the current object - rather to the execution context and can be set by the caller.

If you call a function using the new keyword this will correctly refer to the new object (inside the constructor function)..

But in all other cases - this will refer to the global object unless set explicitly through call