why this keyword is requied in class based compoennets in reactJS code example

Example: how to use a js class in react

class Developer {  constructor(firstname, lastname) {    this.firstname = firstname;    this.lastname = lastname;  }   getName() {    return this.firstname + ' ' + this.lastname;  }} class ReactDeveloper extends Developer {  getJob() {    return 'React Developer';  }} var me = new ReactDeveloper('Robin', 'Wieruch'); console.log(me.getName());console.log(me.getJob());