Finally Understanding the Advanced uses of "This" in Javascript

In this short tutorial we are going to learn how to use "this" in Javascript with 7 different examples …

If the function is defined as an arrow function: {: #arrow-functions }

const arrowFunction = () => {
  console.log(this);
};

In this case, the value of this is always the same as this in the parent scope:

const outerThis = this;

const arrowFunction = () => {
  // Always logs `true`:
  console.log(this === outerThis);
};

Arrow functions are great because the inner value of this can't be changed, it's always the same
as the outer this.

Other examples

With arrow functions, the value of this can't be changed with bind:

// Logs `true` - bound `this` value is ignored:
arrowFunction.bind({foo: 'bar'})();

With arrow functions, the value of this can't be changed with call or apply:

// Logs `true` - called `this` value is ignored:
arrowFunction.call({foo: 'bar'});
// Logs `true` - applied `this` value is ignored:
arrowFunction.apply({foo: 'bar'});

With arrow functions, the value of this can't be changed by calling the function as a member of
another object:

const obj = {arrowFunction};
// Logs `true` - parent object is ignored:
obj.arrowFunction();

With arrow functions, the value of this can't be changed by calling the function as a
constructor:

// TypeError: arrowFunction is not a constructor
new arrowFunction();

'Bound' instance methods

With instance methods, if you want to ensure this always refers to the class instance, the best
way is to use arrow functions and class
fields:

class Whatever {
  someMethod = () => {
    // Always the instance of Whatever:
    console.log(this);
  };
}

This pattern is really useful when using instance methods as event listeners in components (such as
React components, or web components).

The above might feel like it's breaking the "this will be the same as this in the parent scope"
rule, but it starts to make sense if you think of class fields as syntactic sugar for setting things
in the constructor:

class Whatever {
  someMethod = (() => {
    const outerThis = this;
    return () => {
      // Always logs `true`:
      console.log(this === outerThis);
    };
  })();
}

// …is roughly equivalent to:

class Whatever {
  constructor() {
    const outerThis = this;
    this.someMethod = () => {
      // Always logs `true`:
      console.log(this === outerThis);
    };
  }
}

Alternative pattens involve binding an existing function in the constructor, or assigning the
function in the constructor. If you can't use class fields for some reason, assigning functions in
the constructor is a reasonable alternative:

class Whatever {
  constructor() {
    this.someMethod = () => {
      // …
    };
  }
}

Otherwise, if the function/class is called with new: {: #new }

new Whatever();

The above will call Whatever (or its constructor function if it's a class) with this set to the
result of Object.create(Whatever.prototype).

class MyClass {
  constructor() {
    console.log(
      this.constructor === Object.create(MyClass.prototype).constructor,
    );
  }
}

// Logs `true`:
new MyClass();

The same is true for older-style constructors:

function MyClass() {
  console.log(
    this.constructor === Object.create(MyClass.prototype).constructor,
  );
}

// Logs `true`:
new MyClass();

Other examples

When called with new, the value of this can't be changed with bind:

const BoundMyClass = MyClass.bind({foo: 'bar'});
// Logs `true` - bound `this` value is ignored:
new BoundMyClass();

When called with new, the value of this can't be changed by calling the function as a member
of another object:

const obj = {MyClass};
// Logs `true` - parent object is ignored:
new obj.MyClass();

Otherwise, if the function has a 'bound' this value: {: #bound }

function someFunction() {
  return this;
}

const boundObject = {hello: 'world'};
const boundFunction = someFunction.bind(boundObject);

Whenever boundFunction is called, its this value will be the object passed to bind
(boundObject).

// Logs `false`:
console.log(someFunction() === boundObject);
// Logs `true`:
console.log(boundFunction() === boundObject);

Avoid using bind to bind a function to its outer this. Instead, use arrow functions, as they make this clear from the function declaration, rather than
something that happens later in the code.

Don't use bind to set this to some value unrelated to the parent object; it's usually unexpected and it's why this gets such a bad reputation. Consider passing the value as an argument instead; it's more explicit, and works with arrow functions.

Other examples

When calling a bound function, the value of this can't be changed with call or
apply:

// Logs `true` - called `this` value is ignored:
console.log(boundFunction.call({foo: 'bar'}) === boundObject);
// Logs `true` - applied `this` value is ignored:
console.log(boundFunction.apply({foo: 'bar'}) === boundObject);

When calling a bound function, the value of this can't be changed by calling the function as a
member of another object:

const obj = {boundFunction};
// Logs `true` - parent object is ignored:
console.log(obj.boundFunction() === boundObject);

Otherwise, if this is set at call-time: {: #call-apply }

function someFunction() {
  return this;
}

const someObject = {hello: 'world'};

// Logs `true`:
console.log(someFunction.call(someObject) === someObject);
// Logs `true`:
console.log(someFunction.apply(someObject) === someObject);

The value of this is the object passed to call/apply.

Don't use call/apply to set this to some value unrelated to the parent object; it's usually unexpected and it's why this gets such a bad reputation. Consider passing the value as an argument instead; it's more explicit, and works with arrow functions.

Unfortunately this is set to some other value by things like DOM event listeners, and using it can
result in difficult-to-understand code:

element.addEventListener('click', function (event) {
  // Logs `element`, since the DOM spec sets `this` to
  // the element the handler is attached to.
  console.log(this);
});

I avoid using this in cases like above, and instead:

element.addEventListener('click', (event) => {
  // Ideally, grab it from a parent scope:
  console.log(element);
  // But if you can't do that, get it from the event object:
  console.log(event.currentTarget);
});

Otherwise, if the function is called via a parent object (parent.func()): {: #object-member }

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

// Logs `true`:
console.log(obj.someMethod() === obj);

In this case the function is called as a member of obj, so this will be obj. This happens at
call-time, so the link is broken if the function is called without its parent object, or with a
different parent object:

const {someMethod} = obj;
// Logs `false`:
console.log(someMethod() === obj);

const anotherObj = {someMethod};
// Logs `false`:
console.log(anotherObj.someMethod() === obj);
// Logs `true`:
console.log(anotherObj.someMethod() === anotherObj);

someMethod() === obj is false because someMethod isn't called as a member of obj. You might
have encountered this gotcha when trying something like this:

const $ = document.querySelector;
// TypeError: Illegal invocation
const el = $('.some-element');

This breaks because the implementation of querySelector looks at its own this value and expects
it to be a DOM node of sorts, and the above breaks that connection. To achieve the above correctly:

const $ = document.querySelector.bind(document);
// Or:
const $ = (...args) => document.querySelector(...args);

Fun fact: Not all APIs use this internally. Console methods like console.log were changed to
avoid this references, so log doesn't need to be bound to console.

Don't transplant a function onto an object just to set this to some value unrelated to the parent object; it's usually unexpected and it's why this gets such a bad reputation. Consider passing the value as an argument instead; it's more explicit, and works with arrow functions.

Otherwise, if the function or parent scope is in strict mode: {: #strict }

function someFunction() {
  'use strict';
  return this;
}

// Logs `true`:
console.log(someFunction() === undefined);

In this case, the value of this is undefined. 'use strict' isn't needed in the function if the parent scope is in strict
mode (and all modules are in strict mode).

Don't rely on this. I mean, there are easier ways to get an undefined value 😀.

Otherwise: {: #otherwise }

function someFunction() {
  return this;
}

// Logs `true`:
console.log(someFunction() === globalThis);

In this case, the value of this is the same as globalThis.

Most folks (including me) call globalThis the global object, but this isn't 100% technically correct. Here's [Mathias Bynens with the details (https://mathiasbynens.be/notes/globalthis#terminology), including why it's called globalThis rather than simply global.

Avoid using this to reference the global object (yes, I'm still calling it that). Instead, use globalThis,
which is much more explicit.

Additional resources to learn Javascript:

Get the book: Javascript Challenges

JavaScript: Understanding the Weird Parts

Monster JavaScript Course - 50+ projects and applications

reference sites: https://web.dev/javascript-this/