Dmitri Pavlutin
I help developers understand JavaScript and React

A Helpful Algorithm to Determine "this" value in JavaScript

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!

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:

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:

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:

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.

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

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

// 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.

// 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.

function sum(number1, number2) {
  return number1 + number2;
}
sum(1, 3);           // Invocationsum.call({}, 3, 4);  // Invocationsum.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.

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.

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().

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(...).

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

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

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

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

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

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

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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About Dmitri Pavlutin

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