JS interview in 2 minutes / this

// browserconsole.log(window.a) // undefinedthis.a = 1console.log(window.a) // 1// nodeconsole.log(global.a) // undefinedthis.a = 1console.log(global.a) // 1

function f1() {  return this // default to global context}function f2() {  'use strict';  return this // undefined}

let f1 = function() {  return this}let f2 = () => thisconsole.log(f1(), f2()) // Window, Windowlet obj = { f1, f2 }console.log(obj.f1(), obj.f2()) // obj reference, Window// ^^^ f1 changed reference, but f2 didn't

class Tmp {  a = 10  method() {    console.log(this)  }}let tmp = new Tmp()tmp.method() // Tmp{a: 10}

class User {  _say(text) {    console.log(text)  }  sayHello() {    this._say('Hello world')  }  sayHi = () => this._say('Hi!')}let user = new User()user.sayHi() // Worksuser.sayHello() // WorkssetTimeout(user.sayHi, 1000) // Works// callback will show an error, because `this` reference will change// Uncaught TypeError: this._say is not a function at sayHellosetTimeout(user.sayHello, 1000)