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this (: ). , this . .

JavaScript this (binding) . (obj.method()), this . (func()) ( ) undefined( ) . Function.prototype.bind() this , Function.prototype.apply() Function.prototype.call() this .

this . this . . this . bind(), apply(), call() this , .

function getThis() {
  return this;
}

const obj1 = { name: "obj1" };
const obj2 = { name: "obj2" };

obj1.getThis = getThis;
obj2.getThis = getThis;

console.log(obj1.getThis()); // { name: 'obj1', getThis: [Function: getThis] }
console.log(obj2.getThis()); // { name: 'obj2', getThis: [Function: getThis] }

. this . .

this , . .

const obj3 = {
  __proto__: obj1,
  name: "obj3",
};

console.log(obj3.getThis()); // { name: 'obj3' }

this , .

const obj4 = {
  name: "obj4",
  getThis() {
    return this;
  },
};

const obj5 = { name: "obj5" };

obj5.getThis = obj4.getThis;
console.log(obj5.getThis()); // { name: 'obj5', getThis: [Function: getThis] }

, this . , .

function getThisStrict() {
  "use strict"; //  
  return this;
}

//     ,    .
Number.prototype.getThisStrict = getThisStrict;
console.log(typeof (1).getThisStrict()); // "number"

, this undefined . , .

console.log(typeof getThisStrict()); // "undefined"

this (this substitution) this .

this undefined null , this globalThis .
this , this .

function getThis() {
  return this;
}

//     ,    .
Number.prototype.getThis = getThis;
console.log(typeof (1).getThis()); // "object"
console.log(getThis() === globalThis); // true

this ((.) ) . Function.prototype.call(), Function.prototype.apply(), Reflect.apply() this . Function.prototype.bind() , this . , this .

, this . API . , this undefined . , , this (globalThis) . , Promise() .

function logThis() {
  "use strict";
  console.log(this);
}

[1, 2, 3].forEach(logThis); // undefined, undefined, undefined

API this . , Set.prototype.forEach() thisArg this .

[1, 2, 3].forEach(logThis, { name: "obj" });
// { name: 'obj' }, { name: 'obj' }, { name: 'obj' }

undefined this . , JSON.parse() reviver JSON.stringify() replacer this .

this this . , , this .

, , this globalThis .

const globalObject = this;
const foo = () => this;
console.log(foo() === globalObject); // true

this . , " " . , this . ( ) . this . .

, call(), bind(), apply() , thisArg . .

const obj = { name: "obj" };

// call  this   
console.log(foo.call(obj) === globalObject); // true

// bind  this   
const boundFoo = foo.bind(obj);
console.log(boundFoo() === globalObject); // true

(new ), this . , . this new , .

function C() {
  this.a = 37;
}

let o = new C();
console.log(o.a); // 37

function C2() {
  this.a = 37;
  return { a: 38 };
}

o = new C2();
console.log(o.a); // 38

(C2), , this . (, this.a = 37; . , .)

super

super.method() , method this super.method() this . this super . super.method , . super .

. , , (public private) . , , . this .

new , . this . . this . , this .

this . , this ( ) . this .

. this . this . , .

class C {
  instanceField = this;
  static staticField = this;
}

const c = new C();
console.log(c.instanceField === c); // true
console.log(C.staticField === C); // true

, ( ) this . super() this , . (Base .)

this = new Base();

: super() this .

super() . , this , .

class Base {}
class Good extends Base {}
class AlsoGood extends Base {
  constructor() {
    return { a: 5 };
  }
}
class Bad extends Base {
  constructor() {}
}

new Good();
new AlsoGood();
new Bad(); // ReferenceError:   this       super   .

( , ) this . , this () .

, this globalThis . . HTML <script> this === window .

: globalThis . , globalThis . Node.js . , globalThis .

//   window   
console.log(this === window); // true

this.b = "MDN";
console.log(window.b); // "MDN"
console.log(b); // "MDN"

(HTML <script> type="module" ), this undefined.

eval() , eval this . eval this globalThis ( ).

function test() {
  //  eval
  console.log(eval("this") === this);
  //  eval,  
  console.log(eval?.("this") === globalThis);
  //  eval, 
  console.log(eval?.("'use strict'; this") === globalThis);
}

test.call({ name: "obj" }); // Logs 3 "true"

, . , Node.js CommonJS , this module.exports . this .

this . () . this , .

const obj = {
  a: this,
};

console.log(obj.a === window); // true

this

this .

// 'call' 'apply'     
//   this .
const obj = { a: "Custom" };

// var   globalThis  .
var a = "Global";

function whatsThis() {
  return this.a; // this     .
}

whatsThis(); // 'Global';   this  globalThis .
obj.whatsThis = whatsThis;
obj.whatsThis(); // 'Custom'; this obj  .

call() apply() this .

function add(c, d) {
  return this.a + this.b + c + d;
}

const o = { a: 1, b: 3 };

//     this  .
//     .
add.call(o, 5, 7); // 16

//     this  .
//    ,       .
add.apply(o, [10, 20]); // 34

this

, this , this . null undefined globalThis . , (7, 'foo') . 7 Number 'foo' String .

function bar() {
  console.log(Object.prototype.toString.call(this));
}

bar.call(7); // [object Number]
bar.call("foo"); // [object String]
bar.call(undefined); // [object Window]

bind()

f.bind(someObject) , f . , this bind .

function f() {
  return this.a;
}

const g = f.bind({ a: "azerty" });
console.log(g()); // azerty

const h = g.bind({ a: "yoo" }); // bind   !
console.log(h()); // azerty

const o = { a: 37, f, g, h };
console.log(o.a, o.f(), o.g(), o.h()); // 37 37 azerty azerty

this

this . obj , getThisGetter . this . , this this . getThisGetter this , this . getThisGetter . , , .

const obj = {
  getThisGetter() {
    const getter = () => this;
    return getter;
  },
};

getThisGetter obj , this obj . fn . fn , this getThisGetter obj . , this globalThis . getThisGetter .

const fn = obj.getThisGetter();
console.log(fn() === obj); // true

obj . getThisGetter this . fn2()() globalThis . fn2() , this globalThis .

const fn2 = obj.getThisGetter;
console.log(fn2()() === globalThis); // true in non-strict mode

. this , this . , this , this (: ) . setTimeout() .

getter setter this

getter setter this , . , getter setter this .

function sum() {
  return this.a + this.b + this.c;
}

var o = {
  a: 1,
  b: 2,
  c: 3,
  get average() {
    return (this.a + this.b + this.c) / 3;
  },
};

Object.defineProperty(o, "sum", {
  get: sum,
  enumerable: true,
  configurable: true,
});

console.log(o.average, o.sum); // 2, 6

DOM this

, this DOM . (, addEventListener() .)

//      
function bluify(e) {
  //  true
  console.log(this === e.currentTarget);
  // currentTarget target   true
  console.log(this === e.target);
  this.style.backgroundColor = "#A5D9F3";
}

//     
var elements = document.getElementsByTagName("*");

//     
// bluify   
for (var i = 0; i < elements.length; i++) {
  elements[i].addEventListener("click", bluify, false);
}

this

, this DOM .

html

<button >this </button>

button . this .

html

<button >
   this 
</button>

, this globalThis . (, this .)

, this . , this . bind .

class Car {
  constructor() {
    //    sayHi  , sayBye bind .
    this.sayBye = this.sayBye.bind(this);
  }

  sayHi() {
    console.log(`Hello from ${this.name}`);
  }

  sayBye() {
    console.log(`Bye from ${this.name}`);
  }

  get name() {
    return "Ferrari";
  }
}

class Bird {
  get name() {
    return "Tweety";
  }
}

const car = new Car();
const bird = new Bird();

//   'this'    (caller)  .
car.sayHi(); // Hello from Ferrari
bird.sayHi = car.sayHi;
bird.sayHi(); // Hello from Tweety

//  bind   'this'     .
bird.sayBye = car.sayBye;
bird.sayBye(); // Bye from Ferrari

: . undefined this , this .

const carSayHi = car.sayHi;
carSayHi(); // TypeError .  this undefined, sayHi  this.name   .

, . , . . Intl.NumberFormat.prototype.format() . , getter , , .

with this

with ( ), this . with , this . obj. .

const obj = {
  foo() {
    return this;
  },
};

with (obj) {
  console.log(foo() === obj); // true
}

Specification
ECMAScript 2027 LanguageSpecification # sec-this-keyword

globalThis

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