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();
What will the code above log to the console?
(A) Hey, undefined just called.
(B) Hey, mom just called.
(C) Hey, caller just called.
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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();
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();
What will the code above log to the console?
(A) Hey, undefined just called.
(B) Hey, mom just called.
(C) Hey, caller just called.
...
...
...
...
...
...
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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();
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();
What will the code above log to the console?
(A) Hey, undefined just called.
(B) Hey, mom just called.
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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, ${this.caller} just called.`);
}
};
let newCall = call.says;
newCall();
Here, we declare a new variable, newCall
, and assign the says function inside the call object to newCall
. And then we invoke newCall
, which is a now simple function call.
Notice where we invoke the function. Is it inside the call object? No. We are invoking newCall() function globally, which in turn makes the 'this' keyword equal to the global object.
As demonstrated in Challenge#2, since the global object does not have a caller property, you get "undefined" as a result.
By now, you might notice a key pattern:
Regular functions change their behaviors BASED ON the object that is CALLING the function.
Challenge #4
function anotherCaller() {
console.log(`${this.caller} called, too!`);
}
const call = {
caller: "mom",
anotherCaller: anotherCaller,
says: function() {
console.log(`Hey, ${this.caller} just called.`);
}
};
let newCall = call.anotherCaller;
newCall();
What will the code above log in the console?
(A) mom called, too!
(B) Hey, mom just called.
(C) undefined called, too!
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The answer is...
(C) undefined called, too!
Again, pay attention to where the function is being invoked:
function anotherCaller() {
console.log(`${this.caller} called, too!`);
}
const call = {
caller: "mom",
anotherCaller: anotherCaller,
says: function() {
console.log(`Hey, ${this.caller} just called.`);
}
};
let newCall = call.anotherCaller;
newCall();
We are invoking newCall() function globally, which means the 'this' keyword is referring to the global object. It doesn't matter that we are assigning newCall to a function inside the call object. We are calling newCall globally, and globally is where 'this' is assigned.
If you're feeling adventurous, try moving anotherCaller() function inside of the call object, like so:
const call = {
caller: "mom",
anotherCaller: function() {
console.log(`${this.caller} called, too!`)
},
says: function() {
console.log(`Hey, ${this.caller} just called.`);
}
};
let newCall = call.anotherCaller;
newCall();
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.
Top comments (36)
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...).As someone who learned about the
self
keyword (Smalltalk) and learning aboutthis
in JS, the article helps a lot! Especially that arrow function! So does this mean that it is better to create a Class for call instead of just a block variable? That way you can instantiate the Class call.Glad it helps, Chi! And thanks for mentioning Smalltalk -- I had to look it up haha. I don't think there's a preferred way, it all depends on your use case. Also, JavaScript's
class
keyword is not really a true class as in other object-oriented languages like Smalltalk. I found this deep-dive article, hope it helps as well: scotch.io/tutorials/better-javascr...Thanks for the fun quiz.
I was able to answer every question correctly thankfully to this post here on dev.
JavaScript - The "this" key word in depth
Sagiv ben giat ・ Aug 21 '19 ・ 9 min read
It helps to figure out
this
value for every possible case in 5 simple steps.Loved the flow chart in that post. Thanks, Eugene!
Hi Annie, a couple of small corrections...
In the explanation for examples 2 and 3:
Should be something like this instead...
In the explanation for example 2:
I tested in Chrome and Firefox and I get
undefined
in both browsers. This makes sense, because in these examplesthis.caller
is equivalent towindow.caller
, which isundefined
(unless some code on the current page has set this property).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 :)
It's good to practice 'this' like this but there's always a whole bunch of other scenarios that might still trip people up. The best thing to do is to have some theory to solidify the understanding:
1) 'this' is an object created for every Execution Context that is created (on program startup and on every function invocation).
I think it's also important to mention that arrow functions don't get own
this
keyword - they can usethis
keyword of their closest regular function parentCouldn't agree more. Thanks so much for providing an overarching concept around 'this'. That could've been a better opening for 'this' post (pun intended).
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!
Just gotta keep practicing. Thanks for following along, Sam! Made my day.
Nice !! You should add other tricky ones with something like :
and :
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.
Very nice! I've been reading other's attempts at this and have just been getting more confused but this lays out some straight forward rules.
FWIW, This sentence is confusing to me, "Here, we assign a new variable to the says function inside the call object." Unless I'm misinterpreting, I would describe it more as, "Here we assign the says function inside the call object to a new variable."
Thank you for writing this up!
Glad you find this quiz helpful! And thanks for the feedback – I have rephrased it accordingly:
"Here, we declare a new variable,
newCall
, and assign the says function inside the call object tonewCall
."Hope this clarifies.
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.
Haha, good call. Thanks, Peter! That's a good reminder for me too, especially when working on more complex projects.
Hey, there are nice tasks=)
If you have a plan to prepare next episode of this keyword challenges then have a look at my post about this in setTimeout callback:
dev.to/smlka/easyspec-how-does-set...