this

In non–strict mode, this is always a reference to an object. In strict mode, it can be any value. For more information on how the value is determined, see the description below.

The value of this depends on in which context it appears: function, class, or global.

Inside a function, the value of this depends on how the function is called. Think about this as a hidden parameter of a function — just like the parameters declared in the function definition, this is a binding that the language creates for you when the function body is evaluated.

For a regular function (not an arrow function, bound function, etc.), the value of this is the object that the function is accessed on. In other words, if the function call is in the form obj.f(), then this refers to obj. For example:

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] }

Note how the function is the same, but based on how it's invoked, the value of this is different. This is analogous to how function parameters work.

The value of this is not the object that has the function as an own property, but the object that is used to call the function. You can prove this by calling a method of an object up in the prototype chain.

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

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

The value of this always changes based on how a function is called, even when the function was defined on an object at creation:

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

const obj5 = { name: "obj5" };

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

If the value that the method is accessed on is a primitive, this will be a primitive value as well — but only if the function is in strict mode.

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

// Only for demonstration — you should not mutate built-in prototypes
Number.prototype.getThisStrict = getThisStrict;
console.log(typeof (1).getThisStrict()); // "number"

If the function is called without being accessed on anything, this will be undefined — but only if the function is in strict mode.

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

In non-strict mode, a special process called this substitution ensures that the value of this is always an object. This means:

• If a function is called with this set to undefined or null, this gets substituted with globalThis.
• If the function is called with this set to a primitive value, this gets substituted with the primitive value's wrapper object.

function getThis() {
  return this;
}

// Only for demonstration — you should not mutate built-in prototypes
Number.prototype.getThis = getThis;
console.log(typeof (1).getThis()); // "object"
console.log(getThis() === globalThis); // true

In typical function calls, this is implicitly passed like a parameter through the function's prefix (the part before the dot). You can also explicitly set the value of this using the Function.prototype.call(), Function.prototype.apply(), or Reflect.apply() methods. Using Function.prototype.bind(), you can create a new function with a specific value of this that doesn't change regardless of how the function is called. When using these methods, the this substitution rules above still apply if the function is non-strict.

Callbacks

When a function is passed as a callback, the value of this depends on how the callback is called, which is determined by the implementor of the API. Callbacks are typically called with a this value of undefined (calling it directly without attaching it to any object), which means if the function is non–strict, the value of this is the global object (globalThis). This is the case for iterative array methods, the Promise() constructor, etc.

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

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

Some APIs allow you to set a this value for invocations of the callback. For example, all iterative array methods and related ones like Set.prototype.forEach() accept an optional thisArg parameter.

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

Occasionally, a callback is called with a this value other than undefined. For example, the reviver parameter of JSON.parse() and the replacer parameter of JSON.stringify() are both called with this set to the object that the property being parsed/serialized belongs to.

Arrow functions

In arrow functions, this retains the value of the enclosing lexical context's this. In other words, when evaluating an arrow function's body, the language does not create a new this binding.

For example, in global code, this is always globalThis regardless of strictness, because of the global context binding:

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

Arrow functions create a closure over the this value of its surrounding scope, which means arrow functions behave as if they are "auto-bound" — no matter how it's invoked, this is bound to what it was when the function was created (in the example above, the global object). The same applies to arrow functions created inside other functions: their this remains that of the enclosing lexical context. See example below.

Furthermore, when invoking arrow functions using call(), bind(), or apply(), the thisArg parameter is ignored. You can still pass other arguments using these methods, though.

const obj = { name: "obj" };

// Attempt to set this using call
console.log(foo.call(obj) === globalObject); // true

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

Constructors

When a function is used as a constructor (with the new keyword), its this is bound to the new object being constructed, no matter which object the constructor function is accessed on. The value of this becomes the value of the new expression unless the constructor returns another non–primitive value.

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

In the second example (C2), because an object was returned during construction, the new object that this was bound to gets discarded. (This essentially makes the statement this.a = 37; dead code. It's not exactly dead because it gets executed, but it can be eliminated with no outside effects.)

super

When a function is invoked in the super.method() form, the this inside the method function is the same value as the this value around the super.method() call, and is generally not equal to the object that super refers to. This is because super.method is not an object member access like the ones above — it's a special syntax with different binding rules. For examples, see the super reference.

A class can be split into two contexts: static and instance. Constructors, methods, and instance field initializers (public or private) belong to the instance context. Static methods, static field initializers, and static initialization blocks belong to the static context. The this value is different in each context.

Class constructors are always called with new, so their behavior is the same as function constructors: the this value is the new instance being created. Class methods behave like methods in object literals — the this value is the object that the method was accessed on. If the method is not transferred to another object, this is generally an instance of the class.

Static methods are not properties of this. They are properties of the class itself. Therefore, they are generally accessed on the class, and this is the value of the class (or a subclass). Static initialization blocks are also evaluated with this set to the current class.

Field initializers are also evaluated in the context of the class. Instance fields are evaluated with this set to the instance being constructed. Static fields are evaluated with this set to the current class. This is why arrow functions in field initializers are bound to the instance for instance fields and to the class for static fields.

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

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

Derived class constructors

Unlike base class constructors, derived constructors have no initial this binding. Calling super() creates a this binding within the constructor and essentially has the effect of evaluating the following line of code, where Base is the base class:

this = new Base();

Derived classes must not return before calling super(), unless the constructor returns an object (so the this value is overridden) or the class has no constructor at all.

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: Must call super constructor in derived class before accessing 'this' or returning from derived constructor

In the global execution context (outside of any functions or classes; may be inside blocks or arrow functions defined in the global scope), the this value depends on what execution context the script runs in. Like callbacks, the this value is determined by the runtime environment (the caller).

At the top level of a script, this refers to globalThis whether in strict mode or not. This is generally the same as the global object — for example, if the source is put inside an HTML <script> element and executed as a script, this === window.

// In web browsers, the window object is also the global object:
console.log(this === window); // true

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

If the source is loaded as a module (for HTML, this means adding type="module" to the <script> tag), this is always undefined at the top level.

If the source is executed with eval(), this is the same as the enclosing context for direct eval, or globalThis (as if it's run in a separate global script) for indirect eval.

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

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

Note that some source code, while looking like the global scope, is actually wrapped in a function when executed. For example, Node.js CommonJS modules are wrapped in a function and executed with the this value set to module.exports. Event handler attributes are executed with this set to the element they are attached to.

Object literals don't create a this scope — only functions (methods) defined within the object do. Using this in an object literal inherits the value from the surrounding scope.

const obj = {
  a: this,
};

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

The value of the this parameter depends on how the function is called, not on how it's defined.

// An object can be passed as the first argument to 'call'
// or 'apply' and 'this' will be bound to it.
const obj = { a: "Custom" };

// Variables declared with var become properties of 'globalThis'.
var a = "Global";

function whatsThis() {
  return this.a; // 'this' depends on how the function is called
}

whatsThis(); // 'Global'; the 'this' parameter defaults to 'globalThis' in non–strict mode
obj.whatsThis = whatsThis;
obj.whatsThis(); // 'Custom'; the 'this' parameter is bound to obj

Using call() and apply(), you can pass the value of this as if it's an explicit parameter.

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

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

// The first argument is bound to the implicit 'this' parameter; the remaining
// arguments are bound to the named parameters.
add.call(o, 5, 7); // 16

// The first argument is bound to the implicit 'this' parameter; the second
// argument is an array whose members are bound to the named parameters.
add.apply(o, [10, 20]); // 34

In non–strict mode, if a function is called with a this value that's not an object, the this value is substituted with an object. null and undefined become globalThis. Primitives like 7 or 'foo' are converted to an object using the related constructor, so the primitive number 7 is converted to a Number wrapper class and the string 'foo' to a String wrapper class.

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

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

Calling f.bind(someObject) creates a new function with the same body and scope as f, but the value of this is permanently bound to the first argument of bind, regardless of how the function is being called.

function f() {
  return this.a;
}

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

const h = g.bind({ a: "yoo" }); // bind only works once!
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

Arrow functions create closures over the this value of the enclosing execution context. In the following example, we create obj with a method getThisGetter that returns a function that returns the value of this. The returned function is created as an arrow function, so its this is permanently bound to the this of its enclosing function. The value of this inside getThisGetter can be set in the call, which in turn sets the return value of the returned function. We will assume that getThisGetter is a non-strict function, which means it's contained in a non-strict script and not further nested in a class or strict function.

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

We can call getThisGetter as a method of obj, which binds this to obj inside its body. The returned function is assigned to a variable fn. Now, when calling fn, the value of this returned is still the one set by the call to getThisGetter, which is obj. If the returned function was not an arrow function, such calls would cause the this value to be globalThis, because getThisGetter is non-strict.

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

But be careful if you unbind the method of obj without calling it, because getThisGetter is still a method that has a varying this value. Calling fn2()() in the following example returns globalThis, because it follows the this from fn2(), which is globalThis since it's called without being attached to any object.

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

This behavior is very useful when defining callbacks. Usually, each function expression creates its own this binding, which shadows the this value of the upper scope. Now, you can define functions as arrow functions if you don't care about the this value, and only create this bindings where you do (e.g. in class methods). See example with setTimeout().

this in getters and setters is based on which object the property is accessed on, not which object the property is defined on. A function used as getter or setter has its this bound to the object from which the property is being set or gotten.

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

const 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

When a function is used as an event handler, its this parameter is bound to the DOM element on which the listener is placed (some browsers do not follow this convention for listeners added dynamically with methods other than addEventListener()).

// When called as a listener, turns the related element blue
function bluify(e) {
  // Always true
  console.log(this === e.currentTarget);
  // true when currentTarget and target are the same object
  console.log(this === e.target);
  this.style.backgroundColor = "#A5D9F3";
}

// Get a list of every element in the document
const elements = document.getElementsByTagName("*");

// Add bluify as a click listener so when the
// element is clicked on, it turns blue
for (const element of elements) {
  element.addEventListener("click", bluify, false);
}

When the code is called from an inline event handler attribute, its this is bound to the DOM element on which the listener is placed:

<button onclick="alert(this.tagName.toLowerCase());">Show this</button>

The above alert shows button. Note, however, that only the outer scope has its this bound this way:

<button onclick="alert((function () { return this; })());">
  Show inner this
</button>

In this case, the this parameter of the inner function is bound to globalThis (i.e. the default object in non–strict mode where this isn't passed in the call).

Just like with regular functions, the value of this within methods depends on how they are called. Sometimes it is useful to override this behavior so that this within classes always refers to the class instance. To achieve this, bind the class methods in the constructor:

class Car {
  constructor() {
    // Bind sayBye but not sayHi to show the difference
    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();

// The value of 'this' in methods depends on their caller
car.sayHi(); // Hello from Ferrari
bird.sayHi = car.sayHi;
bird.sayHi(); // Hello from Tweety

// For bound methods, 'this' doesn't depend on the caller
bird.sayBye = car.sayBye;
bird.sayBye(); // Bye from Ferrari

const carSayHi = car.sayHi;
carSayHi(); // TypeError because the 'sayHi' method tries to access 'this.name', but 'this' is undefined in strict mode.

Note, however, that auto-bound methods suffer from the same problem as using arrow functions for class properties: each instance of the class will have its own copy of the method, which increases memory usage. Only use it where absolutely necessary. You can also mimic the implementation of Intl.NumberFormat.prototype.format(): define the property as a getter that returns a bound function when accessed and saves it, so that the function is only created once and only created when necessary.

Although with statements are deprecated and not available in strict mode, they still serve as an exception to the normal this binding rules. If a function is called within a with statement and that function is a property of the scope object, the this value is bound to the scope object, as if the obj1. prefix exists.

const obj1 = {
  foo() {
    return this;
  },
};

with (obj1) {
  console.log(foo() === obj1); // true
}

• Strict mode
• globalThis