in javascript why use " var that = this "
<p>hi i am new with javascript</p>

<p>What is the benefit of using this line</p>

<p><code>var that = this</code></p>

<p>An example</p>

<pre><code>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.”);

    };
}
</code></pre>

<p>thanks</p>
