trjhtr

I'm trying out some beginner level typescript. As you notice in the typescript file, I have used arrow functions and plain vanilla javascript functions. The plain functions are added to the Object.prototype, but the arrow functions are added to the "this" object. Can someone help me figure out, why the differences?

// .ts file
interface IPerson 
 getFirstName(): string;
 getLastName(): string;
 setFirstName(firstName: string): void;
 setLastName(lastName: string): void;


interface IStudent 
 getDepartment(): string;
 setDepartment(dept: string): void;


class Person 
 private _firstName: string;
 private _lastName: string;

 constructor()
 this._firstName = 'John';
 this._lastName = 'Smith';
 
 getFirstName = () => 
 return this._firstName; 
 
 getLastName ()
 return this._lastName; 
 
 setFirstName = (firstName: string): void => 
 this._firstName = firstName; 
 
 setLastName(lastName: string): void 
 this._lastName = lastName; 
 


class TA extends Person implements IPerson, IStudent
 private _dept: string;

 constructor()
 super();
 this._dept = 'NA';
 

 getDepartment ()
 return this._dept; 
 
 setDepartment (dept: string)
 this._dept = dept; 
 


let ta1 = new TA();
ta1.setFirstName('F1');
ta1.setLastName('L1');
ta1.setDepartment('D1');

// transpiled js file
"use strict";
var __extends = (this && this.__extends) || (function () 
 var extendStatics = function (d, b) 
 function (d, b) for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; ;
 return extendStatics(d, b);
 
 return function (d, b) 
 extendStatics(d, b);
 function __() this.constructor = d; 
 d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
 ;
)();
var Person = /** @class */ (function () 
 function Person() 
 var _this = this;
 this.getFirstName = function () 
 return _this._firstName;
 ;
 this.setFirstName = function (firstName) 
 _this._firstName = firstName;
 ;
 this._firstName = 'John';
 this._lastName = 'Smith';
 
 Person.prototype.getLastName = function () 
 return this._lastName;
 ;
 Person.prototype.setLastName = function (lastName) 
 this._lastName = lastName;
 ;
 return Person;
());
var TA = /** @class */ (function (_super) 
 __extends(TA, _super);
 function TA() 
 var _this = _super.call(this) 
 TA.prototype.getDepartment = function () 
 return this._dept;
 ;
 TA.prototype.setDepartment = function (dept) 
 this._dept = dept;
 ;
 return TA;
(Person));
var ta1 = new TA();
ta1.setFirstName('F1');
ta1.setLastName('L1');
ta1.setDepartment('D1');