How does the "this" keyword work, and when should it be used?

javascript, this

Solution

`this` is a keyword in JavaScript that is a property of an execution context. Its main use is in functions and constructors. The rules for `this` are quite simple (if you stick to best practices).

Technical description of `this` in the specification

The ECMAScript standard defines `this` via the abstract operation (abbreviated AO) ResolveThisBinding:

The [AO] ResolveThisBinding […] determines the binding of the keyword `this` using the LexicalEnvironment of the running execution context. [Steps]:

- Let envRec be GetThisEnvironment().

- Return ? envRec.GetThisBinding().

Global Environment Records, module Environment Records, and function Environment Records each have their own GetThisBinding method.

The GetThisEnvironment AO finds the current running execution context’s LexicalEnvironment and finds the closest ascendant Environment Record (by iteratively accessing their [[OuterEnv]] properties) which has a this binding (i.e. HasThisBinding returns true). This process ends in one of the three Environment Record types.

The value of `this` often depends on whether code is in strict mode.

The return value of GetThisBinding reflects the value of `this` of the current execution context, so whenever a new execution context is established, `this` resolves to a distinct value. This can also happen when the current execution context is modified. The following subsections list the five cases where this can happen.

You can put the code samples in the AST explorer to follow along with specification details.

1. Global execution context in scripts

This is script code evaluated at the top level, e.g. directly inside a `<script>`:

<script>
// Global context
console.log(this); // Logs global object.

setTimeout(function(){
  console.log("Not global context");
});
</script>

When in the initial global execution context of a script, evaluating `this` causes GetThisBinding to take the following steps:

The GetThisBinding concrete method of a global Environment Record envRec […] [does this]:

- Return envRec.[[GlobalThisValue]].

The [[GlobalThisValue]] property of a global Environment Record is always set to the host-defined global object, which is reachable via `globalThis` (`window` on Web, `global` on Node.js; Docs on MDN). Follow the steps of InitializeHostDefinedRealm to learn how the [[GlobalThisValue]] property comes to be.

2. Global execution context in modules

Modules have been introduced in ECMAScript 2015.

This applies to modules, e.g. when directly inside a `<script type="module">`, as opposed to a simple `<script>`.

When in the initial global execution context of a module, evaluating `this` causes GetThisBinding to take the following steps:

The GetThisBinding concrete method of a module Environment Record […] [does this]:

- Return undefined.

In modules, the value of `this` is always `undefined` in the global context. Modules are implicitly in strict mode.

3. Entering eval code

There are two kinds of `eval` calls: direct and indirect. This distinction exists since the ECMAScript 5th edition.

- A direct `eval` call usually looks like `eval(`…`);` or `(eval)(`…`);` (or `((eval))(`…`);`, etc.).1 It’s only direct if the call expression fits a narrow pattern.2

- An indirect `eval` call involves calling the function reference `eval` in any other way. It could be `eval?.(`…`)`, `(`…`, eval)(`…`)`, `window.eval(`…`)`, `eval.call(`…`,`…`)`, etc. Given `const aliasEval1 = eval; window.aliasEval2 = eval;`, it would also be `aliasEval1(`…`)`, `aliasEval2(`…`)`. Separately, given `const originalEval = eval; window.eval = (x) => originalEval(x);`, calling `eval(`…`)` would also be indirect.

See chuckj’s answer to “(1, eval)('this') vs eval('this') in JavaScript?” and Dmitry Soshnikov’s ECMA-262-5 in detail – Chapter 2: Strict Mode (archived) for when you might use an indirect `eval()` call.

PerformEval executes the `eval` code. It creates a new declarative Environment Record as its LexicalEnvironment, which is where GetThisEnvironment gets the `this` value from.

Then, if `this` appears in `eval` code, the GetThisBinding method of the Environment Record found by GetThisEnvironment is called and its value returned.

And the created declarative Environment Record depends on whether the `eval` call was direct or indirect:

- In a direct eval, it will be based on the current running execution context’s LexicalEnvironment.

- In an indirect eval, it will be based on the [[GlobalEnv]] property (a global Environment Record) of the Realm Record which executed the indirect eval.

Which means:

- In a direct eval, the `this` value doesn’t change; it’s taken from the lexical scope that called `eval`.

- In an indirect eval, the `this` value is the global object (`globalThis`).

What about `new Function`? — `new Function` is similar to `eval`, but it doesn’t call the code immediately; it creates a function. A this binding doesn’t apply anywhere here, except when the function is called, which works normally, as explained in the next subsection.

4. Entering function code

Entering function code occurs when calling a function.

There are four categories of syntax to invoke a function.

- The EvaluateCall AO is performed for these three:3

- Normal function calls

- Optional chaining calls

- Tagged templates

- And EvaluateNew is performed for this one:3

- Constructor invocations

The actual function call happens at the Call AO, which is called with a thisValue determined from context; this argument is passed along in a long chain of call-related calls. Call calls the [[Call]] internal slot of the function. This calls PrepareForOrdinaryCall where a new function Environment Record is created:

A function Environment Record is a declarative Environment Record that is used to represent the top-level scope of a function and, if the function is not an ArrowFunction, provides a `this` binding. If a function is not an ArrowFunction function and references `super`, its function Environment Record also contains the state that is used to perform `super` method invocations from within the function.

In addition, there is the [[ThisValue]] field in a function Environment Record:

This is the `this` value used for this invocation of the function.

The NewFunctionEnvironment call also sets the function environment’s [[ThisBindingStatus]] property.

[[Call]] also calls OrdinaryCallBindThis, where the appropriate thisArgument is determined based on:

- the original reference,

- the kind of the function, and

- whether or not the code is in strict mode.

Once determined, a final call to the BindThisValue method of the newly created function Environment Record actually sets the [[ThisValue]] field to the thisArgument.

Finally, this very field is where a function Environment Record’s GetThisBinding AO gets the value for `this` from:

The GetThisBinding concrete method of a function Environment Record envRec […] [does this]:

[…] 3. Return envRec.[[ThisValue]].

Again, how exactly the this value is determined depends on many factors; this was just a general overview. With this technical background, let’s examine all the concrete examples.

Arrow functions

When an arrow function is evaluated, the [[ThisMode]] internal slot of the function object is set to “lexical” in OrdinaryFunctionCreate.

At OrdinaryCallBindThis, which takes a function F:

- Let thisMode be F.[[ThisMode]].

- If thisMode is lexical, return NormalCompletion(`undefined`). […]

which just means that the rest of the algorithm which binds this is skipped. An arrow function does not bind its own this value.

So, what is `this` inside an arrow function, then? Looking back at ResolveThisBinding and GetThisEnvironment, the HasThisBinding method explicitly returns false.

The HasThisBinding concrete method of a function Environment Record envRec […] [does this]:

- If envRec.[[ThisBindingStatus]] is lexical, return false; otherwise, return true.

So the outer environment is looked up instead, iteratively. The process will end in one of the three environments that have a this binding.

This just means that, in arrow function bodies, `this` comes from the lexical scope of the arrow function, or in other words (from Arrow function vs function declaration / expressions: Are they equivalent / exchangeable?):

Arrow functions don’t have their own `this` […] binding. Instead, [this identifier is] resolved in the lexical scope like any other variable. That means that inside an arrow function, `this` [refers] to the [value of `this`] in the environment the arrow function is defined in (i.e. “outside” the arrow function).

Function properties

In normal functions (`function`, methods), `this` is determined by how the function is called.

This is where these “syntax variants” come in handy.

Consider this object containing a function:

const refObj = {
    func: function(){
      console.log(this);
    }
  };

Alternatively:

const refObj = {
    func(){
      console.log(this);
    }
  };

In any of the following function calls, the `this` value inside `func` will be `refObj`.1

- `refObj.func()`

- `refObj["func"]()`

- `refObj?.func()`

- `refObj.func?.()`

- `refObj.func


If the called function is syntactically a property of a base object, then this base will be the “reference” of the call, which, in usual cases, will be the value of `this`. This is explained by the evaluation steps linked above; for example, in `refObj.func()` (or `refObj["func"]()`), the CallMemberExpression is the entire expression `refObj.func()`, which consists of the MemberExpression `refObj.func` and the Arguments `()`.

But also, `refObj.func` and `refObj` play three roles, each:

- they’re both expressions,

- they’re both references, and

- they’re both values.

`refObj.func` as a value is the callable function object; the corresponding reference is used to determine the `this` binding.

The optional chaining and tagged template examples work very similarly: basically, the reference is everything before the `?.()`, before the

`, or before the `()`.

EvaluateCall uses IsPropertyReference of that reference to determine if it is a property of an object, syntactically. It’s trying to get the [[Base]] property of the reference (which is e.g. `refObj`, when applied to `refObj.func`; or `foo.bar` when applied to `foo.bar.baz`). If it is written as a property, then GetThisValue will get this [[Base]] property and use it as the this value.

Note: Getters / Setters work the same way as methods, regarding `this`. Simple properties don’t affect the execution context, e.g. here, `this` is in global scope:

const o = {
    a: 1,
    b: this.a, // Is `globalThis.a`.
    [this.a]: 2 // Refers to `globalThis.a`.
  };

Calls without base reference, strict mode, and `with`

A call without a base reference is usually a function that isn’t called as a property. For example:

func(); // As opposed to `refObj.func();`.

This also happens when passing or assigning methods, or using the comma operator. This is where the difference between Reference Record and Value is relevant.

Note function `j`: following the specification, you will notice that `j` can only return the function object (Value) itself, but not a Reference Record. Therefore the base reference `refObj` is lost.

const g = (f) => f(); // No base ref.
const h = refObj.func;
const j = () => refObj.func;

g(refObj.func);
h(); // No base ref.
j()(); // No base ref.
(0, refObj.func)(); // Another common pattern to remove the base ref.

EvaluateCall calls Call with a thisValue of undefined here. This makes a difference in OrdinaryCallBindThis (F: the function object; thisArgument: the thisValue passed to Call):

- Let thisMode be F.[[ThisMode]].

[…]

- If thisMode is strict, let thisValue be thisArgument.

- Else,

- If thisArgument is undefined or null, then

- Let globalEnv be calleeRealm.[[GlobalEnv]].

- […]

- Let thisValue be globalEnv.[[GlobalThisValue]].

- Else,

- Let thisValue be ! ToObject(thisArgument).

- NOTE: ToObject produces wrapper objects […].

[…]

Note: step 5 sets the actual value of `this` to the supplied thisArgument in strict mode — `undefined` in this case. In “sloppy mode”, an undefined or null thisArgument results in `this` being the global this value.

If IsPropertyReference returns false, then EvaluateCall takes these steps:

- Let refEnv be ref.[[Base]].

- Assert: refEnv is an Environment Record.

- Let thisValue be refEnv.WithBaseObject().

This is where an undefined thisValue may come from: refEnv.WithBaseObject() is always undefined, except in `with` statements. In this case, thisValue will be the binding object.

There’s also `Symbol.unscopables` (Docs on MDN) to control the `with` binding behavior.

To summarize, so far:

function f1(){
  console.log(this);
}

function f2(){
  console.log(this);
}

function f3(){
  console.log(this);
}

const o = {
    f1,
    f2,
    [Symbol.unscopables]: {
      f2: true
    }
  };

f1(); // Logs `globalThis`.

with(o){
  f1(); // Logs `o`.
  f2(); // `f2` is unscopable, so this logs `globalThis`.
  f3(); // `f3` is not on `o`, so this logs `globalThis`.
}

and:

"use strict";

function f(){
  console.log(this);
}

f(); // Logs `undefined`.

// `with` statements are not allowed in strict-mode code.

Note that when evaluating `this`, it doesn’t matter where a normal function is defined.

`.call`, `.apply`, `.bind`, thisArg, and primitives

Another consequence of step 5 of OrdinaryCallBindThis, in conjunction with step 6.2 (6.b in the spec), is that a primitive this value is coerced to an object only in “sloppy” mode.

To examine this, let’s introduce another source for the this value: the three methods that override the this binding:4

- `Function.prototype.apply(thisArg, argArray)`

- `Function.prototype.` {`call`, `bind`} `(thisArg, ...args)`

`.bind` creates a bound function, whose this binding is set to thisArg and cannot change again. `.call` and `.apply` call the function immediately, with the this binding set to thisArg.

`.call` and `.apply` map directly to Call, using the specified thisArg. `.bind` creates a bound function with BoundFunctionCreate. These have their own [[Call]] method which looks up the function object’s [[BoundThis]] internal slot.

Examples of setting a custom this value:

function f(){
  console.log(this);
}

const myObj = {},
  g = f.bind(myObj),
  h = (m) => m();

// All of these log `myObj`.
g();
f.bind(myObj)();
f.call(myObj);
h(g);

For objects, this is the same in strict and non-strict mode.

Now, try to supply a primitive value:

function f(){
  console.log(this);
}

const myString = "s",
  g = f.bind(myString);

g();              // Logs `String { "s" }`.
f.call(myString); // Logs `String { "s" }`.

In non-strict mode, primitives are coerced to their object-wrapped form. It’s the same kind of object you get when calling `Object("s")` or `new String("s")`. In strict mode, you can use primitives:

"use strict";

function f(){
  console.log(this);
}

const myString = "s",
  g = f.bind(myString);

g();              // Logs `"s"`.
f.call(myString); // Logs `"s"`.

Libraries make use of these methods, e.g. jQuery sets the `this` to the DOM element selected here:

$("button").click(function(){
  console.log(this); // Logs the clicked button.
});

Constructors, classes, and `new`

When calling a function as a constructor using the `new` operator, EvaluateNew calls Construct, which calls the [[Construct]] method. If the function is a base constructor (i.e. not a `class extends`…`{`…`}`), it sets thisArgument to a new object created from the constructor’s prototype. Properties set on `this` in the constructor will end up on the resulting instance object. `this` is implicitly returned, unless you explicitly return your own non-primitive value.

A `class` is a new way of creating constructor functions, introduced in ECMAScript 2015.

function Old(a){
  this.p = a;
}

const o = new Old(1);

console.log(o);  // Logs `Old { p: 1 }`.

class New{
  constructor(a){
    this.p = a;
  }
}

const n = new New(1);

console.log(n); // Logs `New { p: 1 }`.

Class definitions are implicitly in strict mode:

class A{
  m1(){
    return this;
  }
  m2(){
    const m1 = this.m1;
    
    console.log(m1());
  }
}

new A().m2(); // Logs `undefined`.

`super`

The exception to the behavior with `new` is `class extends`…`{`…`}`, as mentioned above. Derived classes do not immediately set their this value upon invocation; they only do so once the base class is reached through a series of `super` calls (happens implicitly without an own `constructor`). Using `this` before calling `super` is not allowed.

Calling `super` calls the super constructor with the this value of the lexical scope (the function Environment Record) of the call. GetThisValue has a special rule for `super` calls. It uses BindThisValue to set `this` to that Environment Record.

class DerivedNew extends New{
  constructor(a, a2){
    // Using `this` before `super` results in a ReferenceError.
    super(a);
    this.p2 = a2;
  }
}

const n2 = new DerivedNew(1, 2);

console.log(n2); // Logs `DerivedNew { p: 1, p2: 2 }`.

5. Evaluating class fields

Instance fields and static fields were introduced in ECMAScript 2022.

When a `class` is evaluated, ClassDefinitionEvaluation is performed, modifying the running execution context. For each ClassElement:

- if a field is static, then `this` refers to the class itself,

- if a field is not static, then `this` refers to the instance.

Private fields (e.g. `#x`) and methods are added to a PrivateEnvironment.

Static blocks are currently a TC39 stage 3 proposal. Static blocks work the same as static fields and methods: `this` inside them refers to the class itself.

Note that in methods and getters / setters, `this` works just like in normal function properties.

class Demo{
  a = this;
  b(){
    return this;
  }
  static c = this;
  static d(){
    return this;
  }
  // Getters, setters, private modifiers are also possible.
}

const demo = new Demo;

console.log(demo.a, demo.b()); // Both log `demo`.
console.log(Demo.c, Demo.d()); // Both log `Demo`.

1: `(o.f)()` is equivalent to `o.f()`; `(f)()` is equivalent to `f()`. This is explained in this 2ality article (archived). Particularly see how a ParenthesizedExpression is evaluated.

2: It must be a MemberExpression, must not be a property, must have a [[ReferencedName]] of exactly "eval", and must be the %eval% intrinsic object.

3: Whenever the specification says “Let ref be the result of evaluating X.”, then X is some expression that you need to find the evaluation steps for. For example, evaluating a MemberExpression or CallExpression is the result of one of these algorithms. Some of them result in a Reference Record.

4: There are also several other native and host methods that allow providing a this value, notably `Array.prototype.map`, `Array.prototype.forEach`, etc. that accept a thisArg as their second argument. Anyone can make their own methods to alter `this` like `(func, thisArg) => func.bind(thisArg)`, `(func, thisArg) => func.call(thisArg)`, etc. As always, MDN offers great documentation.

Just for fun, test your understanding with some examples

For each code snippet, answer the question: “What is the value of `this` at the marked line? Why?”.

To reveal the answers, click the gray boxes.

if(true){
  console.log(this); // What is `this` here?
}

`globalThis`. The marked line is evaluated in the initial global execution context.

const obj = {};

function myFun(){
  return { // What is `this` here?
    "is obj": this === obj,
    "is globalThis": this === globalThis
  };
}

obj.method = myFun;

console.log(obj.method());

`obj`. When calling a function as a property of an object, it is called with the this binding set to the base of the reference `obj.method`, i.e. `obj`.

const obj = {
    myMethod: function(){
      return { // What is `this` here?
        "is obj": this === obj,
        "is globalThis": this === globalThis
      };
    }
  },
  myFun = obj.myMethod;

console.log(myFun());

`globalThis`. Since the function value `myFun` / `obj.myMethod` is not called off of an object, as a property, the this binding will be `globalThis`. This is different from Python, in which accessing a method (`obj.myMethod`) creates a bound method object.

const obj = {
    myFun: () => ({ // What is `this` here?
      "is obj": this === obj,
      "is globalThis": this === globalThis
    })
  };

console.log(obj.myFun());

`globalThis`. Arrow functions don’t create their own this binding. The lexical scope is the same as the initial global scope, so `this` is `globalThis`.

function myFun(){
  console.log(this); // What is `this` here?
}

const obj = {
    myMethod: function(){
      eval("myFun()");
    }
  };

obj.myMethod();

`globalThis`. When evaluating the direct eval call, `this` is `obj`. However, in the eval code, `myFun` is not called off of an object, so the this binding is set to the global object.

function myFun() {
  // What is `this` here?
  return {
    "is obj": this === obj,
    "is globalThis": this === globalThis
  };
}

const obj = {};

console.log(myFun.call(obj));

`obj`. The line `myFun.call(obj);` is invoking the special built-in function `Function.prototype.call`, which accepts `thisArg` as the first argument.

class MyCls{
  arrow = () => ({ // What is `this` here?
    "is MyCls": this === MyCls,
    "is globalThis": this === globalThis,
    "is instance": this instanceof MyCls
  });
}

console.log(new MyCls().arrow());

It’s the instance of `MyCls`. Arrow functions don’t change the this binding, so it comes from lexical scope. Therefore, this is exactly the same as with the class fields mentioned above, like `a = this;`. Try changing it to `static arrow`. Do you get the result you expect?

Problem

I am looking to find a clear explanation of what the "this" keyword does, and how to use it correctly. It seems to behave strangely, and I don't fully understand why. How does `this` work and when should it be used?

Original source

How does the "this" keyword work, and when should it be used?

Solution

Problem

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