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

This question already has answers here: What does 'var that = this;' mean in JavaScript? (6 answers) Closed 8 years ago.

hi i am new with javascript

What is the b开发者_开发问答enefit 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

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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.