in javascript whay use " var that = this " -

Hello I am new to javascript

What is the benefit of using this line

var key = an

one example

  function person (first name, last name, age) {this.firstname = Firstname; This .lastname = lastname; This.age = age; Getfullname = function () {return firstname + â ???? a???? + Last name; } Var that = this; This psi = function () {document.write (â ???? Hi my name is ???? getfullname () + a ???? and i am a ??? + that.age + a ?? ?? years old; ??????); }}   
 thanks   
 
 
  this  The reference is sensitive using the   ensures that when  is called  is called, then this  this  will return the value  getfullname < / Code> was called <. / P>