JavaScript - The "this" key word in depth

Originally posted on my personal blog debuggr.io

In this article we will learn how to identify and recognize what this refers to in a given context and we will explore what rules and conditions are taken under consideration by the engine to determine the reference of the this key word.

You can also read this and other articles at my blog debuggr.io

The challenge

One of the most challenging concepts in JavaScript is the this key word, maybe because it is so different than other languages or maybe because the rules to determine it's value are not that clear.

Lets quote a paragraph from MDN:

In most cases, the value of this is determined by how a function is called (runtime binding). It can't be set by assignment during execution, and it may be different each time the function is called...

Challenging indeed, on one hand it says that this is determined at run-time - i.e, a dynamic binding, but on the other hand it says In most cases..., meaning it can be statically bound. How doe's something can be both static and dynamic and how can we be sure which one it is at a given context? This is exactly what we are going to find out now!

What is static?

Let's look at an example of something static in JavaScript, like the "Local variable environment" - often refers to as scope.

Every time a function is invoked, a new execution context is created and pushed to the top of the call-stack (when our application starts, there is already a default execution context which is often referred to as the global-context).
Each execution context contains a "Local variable environment" which usually referred to as the local-scope (or global-scope in the global execution context).

Given this code snippet:

function foo(){
  var message = 'Hello!';
  console.log(message);
}
foo()

Just by looking at foo's declaration, we know what scope message belongs to - the local scope of the foo function execution-context. Because var statement declares a function-scoped variable.

Another example:

function foo(){
  var message = 'Hello';
  {
    let message = 'there!'
    console.log(message) // there!
  }
  console.log(message) // Hello
}

foo()

Notice how inside the block we get a different result than outside of it, that's because let statement declares a block scope local variable.

We know what to expect just by looking at the deceleration of the function because scope in JavaScript is statically determined (lexical), or at "Design time" if you will.
No matter where and how we will run the function, it's local scope won't change.
In other words, we can say that the scope of a variable is depended on where the variable was declared.

What is dynamic?

If static means "Where something WAS declared", we might say dynamic means "How something WILL run".

Lets imagine for a moment that scope was dynamic in JavaScript:
note, this is not a real syntax ⚠️

function foo(){
  // not a real syntax!!! ⚠️
  let message = if(foo in myObj) "Hello" else "There"
  console.log(message)
}

let myObj = {
  foo
}; 

myObj.foo() // Hello
foo() // There

As you can see, in contrast to the static scope example we now can't determine the final value of message just by looking at the declaration of foo, we will need to see where and how its being invoked. That's because the value of the message variable is determined upon the execution of foo with a set of conditions.
It may look strange but this is not that far away from the truth when we are dealing with the this context, every time we run a function the JavaScript engine is doing some checks and conditionally set the reference of this.

There are some rules, and order matters.
You know what, lets just write them out as if we are writing the engine ourselves:
note, this is not a real syntax ⚠️

function foo(){
  // not real syntax!!! ⚠️
  if(foo is ArrowFunction) doNothing;
  else if(foo called with new) this = {};
  else if(
    foo called with apply || 
    foo called with call  ||
    foo called with bind  ||
  ) this = thisArg
  else if(foo called within an object) this = thatObject
  else if(strictMode){
    this = undefined
  } else{
    // default binding, last resort
    this = window;
    // or global in node
  }

  console.log(this); // who knows? we need to see where and how it runs
}

Seems a bit cumbersome and complex, maybe this flow chart will provide a better visualization:

As you can see we can split the flow into two parts:

• Static binding - The arrow function
• Dynamic binding - The rest of the conditions

Lets walk them through:

1. Is it an arrow function? - If the relevant execution context is created by an arrow function then do nothing, meaning this will be whatever it was set by the wrapping execution context.

2. Was the function called with new? -
   When invoking a function with the new key word the engine will do some things for us:

   • Create a new object and set this to reference it.
   • Reference that object's __proto__ (called [[Prototype]] in the spec) to the function's prototype object.
   • Return the newly created object (this).

   So for our purpose to determine what this is, we know it will be a new object that was created automatically just by invoking the function with the new key word.

3. Was the function called with call / apply or bind? -
   Then set this to whatever passed as the first argument.

4. Was the function called as an object method -
   Then set this to the object left to the dot or square brackets.

5. Is strict mode on? -
   Then this is undefined

6. default case -
   this will reference the global / window.

The Quiz

The best way to measure our understanding is to test ourselves, so lets do a quiz. open the flowchart on a new tab and walk through it from top to bottom for each question (answers are listed below):

Try to answer what will be printed to the console.

Question #1

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

const myObj = {
  logThis
}

myObj.logThis()

Question #2

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

const myObj = {
  foo: function(){
    logThis();
  }
}

myObj.foo()

Question #3

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

const myObj = {
  foo: logThis
}

myObj.foo()

Question #4

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

const myObj = { name: "sag1v" }

logThis.apply(myObj)

Question #5

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

const myObj = { name: "sag1v" }

logThis.apply(myObj)

Question #6

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

const someObj = new logThis()

Question #7

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

function myFunc(){
  logThis();
}

const someObj = new myFunc()

Question #8

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

class myClass {
  logThat(){
    logThis()
  }
}

const myClassInstance = new myClass()
myClassInstance.logThat()

Question #9

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

class myClass {
  logThat(){
    logThis.call(this)
  }
}

const myClassInstance = new myClass()
myClassInstance.logThat()

Question #10

class myClass {
  logThis = () => {
    console.log(this);
  }
}

const myObj = { name: 'sagiv' };

const myClassInstance = new myClass()
myClassInstance.logThis.call(myObj)

Bonus questions

Question #11

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

const btn = document.getElementById('btn');
btn.addEventListener('click', logThis);

Question #12

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

const btn = document.getElementById('btn');
btn.addEventListener('click', logThis);

Answers

Answer #1

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

const myObj = {
  logThis
}

myObj.logThis()

Result - myObj.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - No.
• Was logThis called as an object method? - Yes, myObj is left to the dot.

Answer #2

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

const myObj = {
  foo: function(){
    logThis();
  }
}

myObj.foo()

Result - window.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - No.
• Was logThis called as an object method? - No.
• Is strict mode on? - No.
• default case - window (or global).

Answer #3

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

const myObj = {
  foo: logThis
}

myObj.foo()

Result - window.
Explanation:

• Is logThis an arrow function? - Yes, whatever this set in the wrapping context. In this case the wrapping context is the "Global execution context" which inside it this refers to the window / global object.

Answer #4

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

const myObj = { name: "sag1v" }

logThis.apply(myObj)

Result - myObj.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - Yeas, whatever passed in as the first argument - myObj in this case.

Answer #5

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

const myObj = { name: "sag1v" }

logThis.apply(myObj)

Result - window.
Explanation:

• Is logThis an arrow function? - Yes, whatever this set in the wrapping context. In this case the wrapping context is the "Global execution context" which inside it this refers to the window / global object.

Answer #6

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

const someObj = new logThis()

Result - The object created by logThis.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - Yes, then this is an auto created object inside the function.

Answer #7

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

function myFunc(){
  logThis();
}

const someObj = new myFunc()

Result - undefined.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - No.
• Was logThis called as an object method? - No.
• Is strict mode on? - Yes, this is undefined.

Answer #8

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

class myClass {
  logThat(){
    logThis()
  }
}

const myClassInstance = new myClass()
myClassInstance.logThat()

Result - window.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - No.
• Was logThis called as an object method? - No.
• Is strict mode on? - No.
• default case - window (or global).

Answer #9

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

class myClass {
  logThat(){
    logThis.call(this)
  }
}

const myClassInstance = new myClass()
myClassInstance.logThat()

Result - The object created by myClass.
Explanation:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - Yes, whatever passed in as first argument. OK, but we are passing this! what is this refers to inside the logThat execution context? Lets check:
  • Is logThat an arrow function? - No.
  • Was logThat called with new? - No.
  • Was logThat called with call / apply / bind? - No.
  • Was logThat called as an object method? - Yes, this is the object left to the dot - The auto created object inside myClass in this case.

Answer #10

class myClass {
  logThis = () => {
    console.log(this);
  }
}

const myObj = { name: 'sagiv' };

const myClassInstance = new myClass()
myClassInstance.logThis.call(myObj)

Result - The object created by myClass.
Explanation:

• Is logThis an arrow function? - Yes, this refers to whatever the wrapping context set it, myClass in this case. Lets check what this refers to in the wrapping context:
  • Is myClass an arrow function? - No.
  • Was myClass called with new? - Yes, this refers to the newly created object (the instance).

note that we are using class fields which is a proposal currently in stage 3

Answer #11

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

const btn = document.getElementById('btn');
btn.addEventListener('click', logThis);

Result - The btn element.
Explanation
This is a tricky question because we never talked about event handlers attached to DOM elements. You can look at event handlers that are attached to DOM elements as if the function is a method inside the element's object, In our case the btn object. We can look at it as if we did btn.click() or even btn.logThis(). Note that this is not exactly whats going on under the hood, but this visualization of the invocation of the handler can help us with the formation of our "mental model" regarding the setting of this.
You can read more about it on the MDN

Now lets walk through the flow:

• Is logThis an arrow function? - No.
• Was logThis called with new? - No.
• Was logThis called with call / apply / bind? - No.
• Was logThis called as an object method? - Yes (sort of), in our case btn is left to the dot.

Answer #12

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

const btn = document.getElementById('btn');
btn.addEventListener('click', logThis);

Result - window.
Explanation

• Is logThis an arrow function? - Yes, whatever this set in the wrapping context. In this case the wrapping context is the "Global execution context" which inside it this refers to the window / global object.

Wrapping up

We now understand that the assignment of this can be both dynamic and static (lexical).

• Arrow functions will make it static and won't even bother to mutate this at all. which means we will need to understand what this was set to in the wrapping execution context.
• Plain Functions will make it dynamically, meaning it depends on how the function was invoked.

It may look intimidating and complex now, you probably thinking how would you remember the flow chart. Well you don't need to, you can save or print this flow-chart or maybe even make your own. Every time you need to know what this refers to in your code just look at it and start going through the conditions. Rest assure, you will need to look at this flow-chart less and less as time goes by.

I hope it was informative and helpful, if you have any further clarifications or corrections, feel free to comment or DM me on twitter (@sag1v).

You can read more of my articles at my blog debuggr.io