user.ts

"use strict";

class User {

	constructor (private _name : string,
	             private _email : string) {}
	
	get name () : string  { return this._name; }
	
	get email () : string { return this._email; }
	
	// Return function via getter
	get speak () : (() => void) {
	  return function _speak () {
	    console.log(`I am ${this.name}!`);
	  }
	}
	
}

const peleke = new User ('Peleke', 'resil.design@gmail.com');
peleke.speak(); // 'I am Peleke!'

try {
  User.prototype.speak = null; // 
} catc (err) {
  // TypeError
}