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A Helpful Algorithm to Determine "this" value in JavaScript

Updated April 19, 2021

Every JavaScript developer, including myself, has been struggling in understanding how this keyword works.

I've created a universal algorithm to help you determine the value of this keyword in any situation.

While I made the algorithm as accessible as possible, I recommend reading it multiple times and understand the related terms.

Also, I'll show you a step-by-step evaluation of the algorithm for example situations. Finally, try the homework exercises by yourself!

Note: If you don't understand the algorithm from the first time: that's expected! Get back to the post later and try again until you crack it.

Ready? Let's begin!

Before I go on, let me recommend something to you.

If you want to significantly improve your JavaScript knowledge, take the amazingly useful course "Modern JavaScript From The Beginning 2.0" by Brad Traversy. Use the coupon code "DMITRI" and get 20% discount!

1. this algorithm

The formal definition of ThisValueOfFunction(func, invocationType) that returns this value a function func invoked in a certain way invocationType.

ThisValueOfFunction(func, invocationType):

  1. If func is a regular function, then

    1. If invocationType is as a constructor, then

      1. let newObject be the newly constructed object newObject = new func()
      2. return newObject
    2. Else if invocationType is indirectly, then

      1. let thisArg be the argument of func.call(thisArg) or func.apply(thisArg)
      2. return thisArg
    3. Else if invocationType is as a method, then

      1. let object be the object upon which func is invoked on object.func()
      2. return object
    4. Else if invocationType is regular, then

      1. If strict mode is enabled, then return undefined
      2. Else return globalObject
  2. Else if func is an arrow function, then

    1. If func is defined in the outermost scope, then return globalObject
    2. Else
      1. let outerFunc be the outer function of func
      2. return ThisValueOfFunction(outerFunc, outerInvocationType)
  3. Else if func is a bound function of an originFunc function, then

    1. let thisArg be the argument of func = originFunc.bind(thisArg)
    2. return thisArg
  4. Else if func is a constructor() method inside of a class SomeClass, then

    1. let instance be the instance of the class instance = new SomeClass()
    2. return instance

1.1 The terms used in the algorithm

The algorithm uses plenty of JavaScript terms. If you aren't familiar with something, expand and look at the explanation.

Arrow function

An arrow function is a function defined using the fat arrow syntax =>. Example of an arrow function:

javascript
const sum = (number1, number2) => {
return number1 + number2;
}
Bound function

A bound function is a function created from invoking the method myFunc.bind(thisArg, arg1, ..., argN) upon a function. Example of a bound function:

javascript
function originalFunction() {
// ...
}
const boundFunction = originalFunction.bind({ prop: 'Value' });
Regular function

A regular function is a simple JavaScript function defined using function keyword or using a shorthand definition on an object. Examples of regular functions:

javascript
function regularFunction(who) {
return `Hello, ${who}!`;
}
const object = {
anotherRegularFunction(who) {
return `Good bye, ${who}!`
}
};
constructor()

constructor() is a special method inside of a class that initializes the class instance.

javascript
class SomeClass() {
constructor(prop) {
this.prop = prop;
}
}
Outermost scope

The outermost scope is the top scope that doesn't have an outer scope.

javascript
// The outermost scope
let a = 1;
function someFunction() {
// someFunction() scope
// Not the outermost scope
let b = 1;
}
Outer function

The outer function contains another function within its scope.

javascript
// outerFunction() is the outer function of myFunction()
function outerFunction() {
function myFunction() {
}
}
Global object

The global object is the object that always exists in the global scope. window is the global object in a browser environment, global in Node environment.

Invocation

Invocation of a function is just calling the function with some arguments.

javascript
function sum(number1, number2) {
return number1 + number2;
}
sum(1, 3); // Invocation
sum.call({}, 3, 4); // Invocation
sum.apply({}, [5, 9]); // Invocation
const obj = {
method() {
return 'Some method';
}
};
obj.method(); // Invocation
class SomeClass {
constructor(prop) {
this.prop = prop;
}
}
const instance = new SomeClass('Value'); // Invocation
Constructor invocation

Constructor invocation happens when a function or class is invoked using new keyword.

javascript
function MyCat(name) {
this.name = name;
}
const fluffy = new MyCat('Fluffy'); // Constructor invocation
class MyDog {
constructor(name) {
this.name = name;
}
}
const rex = new MyDog('Rex'); // Constructor invocation
Indirect invocation

An indirect invocation of happens when a function is called using func.call(thisArg, ...) or func.apply(thisArg, ...) methods.

javascript
function sum(number1, number2) {
return number1 + number2;
}
sum.call({}, 1, 2); // Indirect invocation
sum.apply({}, 3, 5); // Indirect invocation
Method invocation

Method invocation happens when a function is invoked in a property accessor expression object.method().

javascript
const object = {
greeting(who) {
return `Hello, ${who}!`
}
};
object.greeting('World'); // Method invocation
object['greeting']('World'); // Method invocation
Regular invocation

Regular invocation happens when the sole variable containing the function is used for invocation func(...).

javascript
function sum(number1, number2) {
return number1 + number2;
}
sum(1, 4); // Regular invocation
Strict mode

Strict mode is a special mode imposed upon running JavaScript code having some special restrictions. The strict mode is enabled by adding 'use strict' directive at the start of the script or the top of the function scope.

2. Examples

Example 1

javascript
const myFunc = () => {
console.log(this); // logs `window`
};
myFunc();

Try the demo.

ThisValueOfFunction(myFunc, "regular")

myFunc is an arrow function: thus matching the point 2 in the algorithm. Also myFunc is defined in the outermost scope, matching the point 2.1.

The point 2.1 of the algorithm says return globalObject: meaning that this value inside myFunc is the global object — window (in a browser environment).

Example 2

javascript
const object = {
method() {
console.log(this); // logs { method() {...} }
}
};
object.method();

Try the demo.

ThisValueOfFunction(object.method, "as a method")

method(), while being a property of the object, is a regular function. The point 1 of the algorithm is matched.

object.method() is a method invocation because of the property accessor usage: thus the point 1.3 is matched.

Then, according to point 1.3, this value inside method() equals the owning object of the method invocation (object.method()) — object.

Example 3

javascript
function MyCat(name) {
this.name = name;
const getName = () => {
console.log(this); // logs { name: 'Fluffy', getName() {...} }
return this.name;
}
this.getName = getName;
}
const fluffy = new MyCat('Fluffy');
fluffy.getName();

Try the demo.

ThisValueOfFunction(getName, "as a method")

getName() is an arrow function, thus the point 2 of the algorithm is applied. Then the point 2.2 matches, because MyCat is the outer function of getName().

The point 2.2.2 says that this value inside getName() arrow function equals this value of the outer function: MyCat.

So, let's run the algorithm recursively again on MyCat function — ThisValueOfFunction(MyCat, "as a constructor").

ThisValueOfFunction(MyCat, "as a constructor")

MyCat is a regular function, thus the point 1 of the algorithm is applied.

Because MyCat was invoked as a constructor new MyCat('Fluffy'), the point 1.1 is applied. Finally, according to points 1.1.1 and 1.1.2, this value inside MyCat equals to the constructed object: fluffy.

And, returning back to the arrow function's point 2.2.2, this inside of the getName() equals this of the MyCat — which is finally fluffy.

3. Homework

The best way to understand the algorithm is by trying it yourself. Follow the 3 exercises in determining this value.

Exercise 1

javascript
const myRegularFunc = function() {
console.log(this); // logs ???
};
myRegularFunc();

How would the algorithm determine this value inside myRegularFunc()? Write the step-by-step evaluation.

Exercise 2

javascript
class MyCat {
constructor(name) {
this.name = name;
console.log(this); // logs ???
}
}
const myCat = new MyCat('Lucy');

How would the algorithm determine this value inside new MyCat('Lucy')? Write the step-by-step evaluation.

Exercise 3

javascript
const object = {
name: 'Batman',
getName() {
const arrow = () => {
console.log(this); // logs ???
return this.name;
};
return arrow();
};
}
object.getName();

How would the algorithm determine this value inside arrow() function? Write the step-by-step evaluation.

4. Summary

In this post, I presented a universal algorithm to determine the value of this inside of an invoked function.

While the algorithm might be challenging at first, if you understand the step-by-step examples, you will realize how easy is to apply the algorithm.

Struggle applying the algorithm for a certain situation? Describe your case in a comment below!

Like the post? Please share!

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Dmitri Pavlutin

About Dmitri Pavlutin

Tech writer and coach. My daily routine consists of (but not limited to) drinking coffee, coding, writing, coaching, overcoming boredom 😉.

Quality posts into your inbox

I regularly publish posts containing:

  • Important JavaScript concepts explained in simple words
  • Overview of new JavaScript features
  • How to use TypeScript and typing
  • Software design and good coding practices

Subscribe to my newsletter to get them right into your inbox.

Join 7094 other subscribers.
Dmitri Pavlutin

About Dmitri Pavlutin

Tech writer and coach. My daily routine consists of (but not limited to) drinking coffee, coding, writing, coaching, overcoming boredom 😉.