What is importance of ‘this’ keyword in javascript?

‘this’ always point to an object.

when a function executes inside an object , it gets the this property and its value.

alert(this);  // will return object window, because it is executing in global object.

var person = {
firstName   :”john”,
lastName    :”johny”,
myname:function () {
alert(this);  // will return person object beacuse myname function defined inside “person” object
console.log (this.firstName + ” ” + this.lastName);
person.myname ();
alert(this);  // will also return object window, beacuse of “this” has value only inside the function its is defined inside a object.

“this” is not assigned a value until an object invokes the function.

So if we declare a function in global scope this variable will point to the window object(global object)

function myname() {
alert(this);  // will return global object beacuse myname function defined inside global scope.
console.log (this.firstName + ” ” + this.lastName);

“window” is the object that all global variables and functions are defined on.​

Reference: http://javascriptissexy.com/understand-javascripts-this-with-clarity-and-master-it/