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Annie Liao
Annie Liao

Posted on • Updated on

Take 'this' Quiz, Understand How 'this' Works in JavaScript

Among all the abstract ideas JavaScript has to offer, the 'this' keyword can be one of the most challenging concepts to grasp. On the surface, 'this' seems like an intuitive keyword that simply refers to the environment (or context) it resides in.

As you look deeper into the JavaScript runtime, i.e. where the code is being executed, 'this' keyword might end up grabbing things you did not expect.

In this post, I created 4 simple scenarios where 'this' keyword can be interpreted differently, each followed by a multiple-choice section, a long pause (in case you scrolled too fast and accidentally saw the answer), and the answer with explanations.

Feel free to play around with the code on your console or text editor. Remember, the more and varied situations you encounter, the better you'll be at identifying and understanding 'this' keywords.

Ready? Let's do this!

Challenge #1

const call = {
  caller: "mom", 
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

call.says();
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What will the code above log to the console?

(A) Hey, undefined just called.
(B) Hey, mom just called.
(C) Hey, caller just called.

...

...

...

...

...

...

...

...

...

...

The answer is...

(B) Hey, mom just called.

Here's the code block again:

const call = {
  caller: "mom", 
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

call.says();
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Here we have a function declaration inside the call object. As a general rule, 'this' is determined by the object invoking a function. Therefore, when the call object invokes says function (call.says()), the 'this' keyword inside the says function refers to the call object, making this.caller equal to "mom".

Pretty straight forward, right?

Challenge #2

const call = {
  caller: "mom", 
  says: () => {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

call.says();
Enter fullscreen mode Exit fullscreen mode

What will the code above log to the console?

(A) Hey, undefined just called.
(B) Hey, mom just called.
(C) Hey, caller just called.

...

...

...

...

...

...

...

...

...

...

The answer is...

(A) Hey, undefined just called.

Here's the code block again:

const call = {
  caller: "mom", 
  says: () => {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

call.says();
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Wait, isn't this code the same as the first one?

If you look closely, the function declaration from Challenge#1 is now replaced by an arrow function.

Arrow functions, as part of ES6 syntax, do NOT have their own 'this' keyword. Instead, they will use the 'this' keyword of whatever 'this' was outside the function when it was created.

In other words, 'this' inside the arrow function is not bound to our call object, but instead is already bound to where the call object is being created originally, which in this case is the global object. And because the global object does not know anything about say() function, 'this' is undefined. And because the global object does not have a caller property, this.caller is undefined. (shout out to James Nylen for the correction!)

Challenge #3

const call = {
  caller: "mom", 
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

let newCall = call.says;

newCall();
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What will the code above log to the console?

(A) Hey, undefined just called.
(B) Hey, mom just called.

...

...

...

...

...

...

...

...

...

...

The answer is...

(A) Hey, undefined just called.

What happened? Let's look at the code again:

const call = {
  caller: "mom", 
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

let newCall = call.says;

newCall();
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Here, we declare a new variable, newCall, and assign the says function inside the call object to newCall. And then we invoke newCall, which is a now simple function call.

Notice where we invoke the function. Is it inside the call object? No. We are invoking newCall() function globally, which in turn makes the 'this' keyword equal to the global object.

As demonstrated in Challenge#2, since the global object does not have a caller property, you get "undefined" as a result.

By now, you might notice a key pattern:
Regular functions change their behaviors BASED ON the object that is CALLING the function.

Challenge #4

function anotherCaller() {
  console.log(`${this.caller} called, too!`);
}

const call = {
  caller: "mom", 
  anotherCaller: anotherCaller,
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

let newCall = call.anotherCaller;

newCall();
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What will the code above log in the console?

(A) mom called, too!
(B) Hey, mom just called.
(C) undefined called, too!

...

...

...

...

...

...

...

...

...

...

The answer is...

(C) undefined called, too!

Again, pay attention to where the function is being invoked:

function anotherCaller() {
  console.log(`${this.caller} called, too!`);
}

const call = {
  caller: "mom", 
  anotherCaller: anotherCaller,
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};

let newCall = call.anotherCaller;

newCall();
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We are invoking newCall() function globally, which means the 'this' keyword is referring to the global object. It doesn't matter that we are assigning newCall to a function inside the call object. We are calling newCall globally, and globally is where 'this' is assigned.

If you're feeling adventurous, try moving anotherCaller() function inside of the call object, like so:

const call = {
  caller: "mom", 
  anotherCaller: function() {
        console.log(`${this.caller} called, too!`)
      },
  says: function() {
    console.log(`Hey, ${this.caller} just called.`);
  }
};


let newCall = call.anotherCaller;
newCall();
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Based on what we just discussed, what do you think the output will be?

Try running the code mentally before checking the answer in your browser. If you got it, you got this (the basics, at least)!


I hope these examples give you a better picture of how 'this' keyword works. If you still find it confusing, worry not. As with everything in programming, practice is key.

For more examples, check out the official MDN documentation on 'this'. I also highly recommend this awesome article. The author provided clear explanations and actually gave me additional insight into some tricky parts in my last challenge.

Oldest comments (39)

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pclundaahl profile image
Patrick Charles-Lundaahl

Nice! A quick, well-done test of this knowledge.

My goto whenever anyone asks about this is always You Don't Know JS (github.com/getify/You-Dont-Know-JS...).

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aleksandrhovhannisyan profile image
Aleksandr Hovhannisyan

On the surface, 'this' seems like an intuitive keyword that simply refers to the environment (or scope) it resides in.

Note: It's actually context, not scope.

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liaowow profile image
Annie Liao

Ah, I see. So context is object-based, and scope is function-based (reference here: ryanmorr.com/understanding-scope-a...). Just corrected it. Thanks so much for clarifying!

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yimammichael profile image
Michael

Thank You.

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jmvallejo profile image
Juan Manuel Vallejo

Very well illustrated πŸ‘

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adusoft profile image
Geofrey Aduda

Very well put , I always confuse the two! Using arrow and using none when dealing with 'this'

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karataev profile image
Eugene Karataev

Thanks for the fun quiz.
I was able to answer every question correctly thankfully to this post here on dev.


It helps to figure out this value for every possible case in 5 simple steps.
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liaowow profile image
Annie Liao

Loved the flow chart in that post. Thanks, Eugene!

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nylen profile image
James Nylen

Hi Annie, a couple of small corrections...

In the explanation for examples 2 and 3:

And because the global object does not know anything about say() function, 'this' is undefined.

Should be something like this instead...

And because the global object does not have a caller property, this.caller is undefined.

In the explanation for example 2:

(if you're using a browser console, you might see "null" (Firefox) or "[url referring to the window object]"(Chrome) instead of "undefined")

I tested in Chrome and Firefox and I get undefined in both browsers. This makes sense, because in these examples this.caller is equivalent to window.caller, which is undefined (unless some code on the current page has set this property).

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liaowow profile image
Annie Liao

Thank you, James! Yes. That makes much more sense. I was googling around trying to find the simplest explainer, only to complicate my own thought process. Really appreciate your input :)

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sambarnes90 profile image
Sam

I actually got 100% on this but I am constantly getting 'this' wrong.

Having it in this format really made me think about where the invocations and definitions were happening.

Really helpful exercise for a relative novice like me - thanks Annie!

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liaowow profile image
Annie Liao

Just gotta keep practicing. Thanks for following along, Sam! Made my day.

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justinwozniak profile image
Justin Wozniak

Nice quiz. Nailed it β™₯

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keltroth profile image
Django Janny • Edited on

Nice !! You should add other tricky ones with something like :

newCall.call(call);

and :

newCall.bind(call)();
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liaowow profile image
Annie Liao

Thanks, Django! I actually thought about that, but then I'd have to introduce call, apply and bind, which might turn this little quiz into a full-fledged code challenge lol.

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wanmaoor profile image
wanmaoor

4 of 4, all right.

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alin11 profile image
Ali Nazari

Very nice way of teaching. (Although I answered all correctly 😜😜)

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anshulnegitc profile image
Anshul Negi

It is good to see the efforts you have put.

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hoffmann profile image
Peter Hoffmann

Just to be nitpicking the correct solution for challenge #4 is not "Undefined called, too!" but "undefined called, too!" (lowercase).
Besides, thanks for the refresh.
The hardest part for me was to remember that an object doesn't have to be new'd to create it's context.

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liaowow profile image
Annie Liao

Haha, good call. Thanks, Peter! That's a good reminder for me too, especially when working on more complex projects.

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marinesm profile image
Maria Ines Montenegro

Nice quiz! Just had challenge # 2 wrong...helped me realize I didn't have a full understanding of arrow functions

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liaowow profile image
Annie Liao

Thanks Maria! Yeah, that one messed me up too. That’s why I put them side by side. Glad it helps :)

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