in javascript why use " var that = this " [duplicate]

Question

hi i am new with javascript

What is the benefit of using this line

var that = this

An example

function Person( firstname, lastname, age ) {

    this.firstname = firstname;

    this.lastname = lastname;

    this.age = age;

    getfullname = function() {

        return firstname + “ “ + lastname;
    };

    var that = this;


    this.sayHi = function() {

        document.write( “Hi my name is “ + getfullname() + “ and I am “ + that.age + “years old.”);

    };
}

thanks

Answer 1

because in the inner function this will not be the same object as in the outer, so by aliasing it to that you can make sure you are talking to the same object.