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🔥🕺🏼 JavaScript Visualized: Hoisting

lydiahallie profile image Lydia Hallie Updated on ・3 min read

Hoisting is one of those terms that every JS dev has heard of because you googled your annoying error and ended up on StackOverflow, where this person told you that this error was caused because of hoisting 🙃 So, what is hoisting? (FYI - scope will be covered in another post, I like to keep posts small and focused)

If you’re new to JavaScript, you may have experienced “weird” behavior where some variables are randomly undefined, ReferenceErrors get thrown, and so on. Hoisting is often explained as putting variables and functions to the top of the file but nah, that’s not what’s happening, although the behavior might seem like it 😃

When the JS engine gets our script, the first thing it does is setting up memory for the data in our code. No code is executed at this point, it’s simply just preparing everything for execution. The way that function declarations and variables are stored is different. Functions are stored with a reference to the entire function.

With variables, it’s a bit different. ES6 introduced two new keywords to declare variables: let and const. Variables declared with the let or const keyword are stored uninitialized.

Variables declared with the var keyword are stored with the default value of undefined.

Now that the creation phase is done, we can actually execute the code. Let's see what happens if we had 3 console.log statements on top of the file, before we declared the function or any of the variables.

Since functions are stored with a reference to the entire function code, we can invoke them even before the line on which we created them! 🔥

When we reference a variable declared with the var keyword before their declaration, it’ll simply return its default value that it was stored with: undefined! However, this could sometimes lead to "unexpected" behavior. In most cases this means you’re referencing it unintentionally (you probably don’t want it to actually have the value of undefined) 😬

In order to prevent being able to accidentally reference an undefined variable, like we could with the var keyword, a ReferenceError gets thrown whenever we try to access uninitialized variables. The "zone" before their actual declaration, is called the temporal dead zone: you cannot reference the variables (this includes ES6 classes as well!) before their initialization.

When the engine passes the line on which we actually declared the variables, the values in memory are overwritten with the values we actually declared them with.

(Oops I notice now this should be number 7. Will update asap 😬)


All done! 🎉 Quick recap:

  • Functions and variables are stored in memory for an execution context before we execute our code. This is called hoisting.
  • Functions are stored with a reference to the entire functions, variables with the var keyword with the value of undefined, and variables with the let and const keyword are stored uninitialized.

I hope that the term hoisting is a bit less vague now that we've looked at what's happening when we execute our code. As always, don't worry if it still doesn't make a lot of sense yet. You'll get a lot more comfortable with it the more you work with it. Feel free to ask me for help, I'd love to help you! 😃

Discussion

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aminnairi profile image
Amin

Your GIFs are on point! Very good article. I think most of us know or have heard about hoisting but this makes it totally clear in my head now. Keep up the good work!

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lvidakovic profile image
Luka Vidaković

It's a nice refreshment to see gifs that break up the text to digestible chunks while also helping with the visualization. Keep it up!

Hoisting is often explained as putting variables and functions to the top of the file

I can see why it's described that way to some extent. Parser needs to go over the whole file/script, and it only does something meaningful with functions, and vars at first. So at the top of the file, very first line, you already have the access to those identifiers.

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lydiahallie profile image
Lydia Hallie Author

It’s just important to understand that the parser is not actually physically moving them to the top - as some people like to explain it

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lvidakovic profile image
Luka Vidaković

Yup I agree. Semantically it only makes difference with vars, but you descriptions are to the point 👌

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Shafkathullah Ihsan

Testing comment UI...

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Shafkathullah Ihsan

2Testing comment UI...

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Matija X

That's reason I love this site. I just learn JS and term hoisting was unknown for me until this moment. I read text above and resolve my doubts. Thx!

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Jimi Friis

It's such a simple thing to explain when looking at yours.. I wonder why so many teach the "Hoisting is often explained as putting variables and functions to the top of the file" - perhaps that's what they learned; perhaps it is the fallacy of thinking "people won't understand because it's complex".
If someone would have asked me to explain hoisting before reading this article I would fall back on that explanation but not really knowing why; now I know better! I have quite good understanding of javascript (for being a mostly C# developer - learning react) and like to know what is going on, one of my favorites in the JS world to read and listen to is Kyle Simpson ('You don't know JS' , + 'Yet') - he is awesome , but you know what.. you, Lydia, are fantastic teacher and a ⭐ on the same sky as Kyle.

I hope you take this advantage to a next level and produce courses on Udemy and/or Pluralsight - I think you could make a fortune (or at least some extra cash) on it 😄

Keep it up 💚 !
//Jimi

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lfkwtz profile image
Michael Lefkowitz

Love this entire collection, Lydia. Shared them all in my react native newsletter

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Saurabh Daware 🌻

Thank you for explaining how it works internally "var brings variables to top" totally makes sense now!

Thank you for sharing! deserves a lot of 🦄🦄🦄

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namkwon profile image
Nam Kwon

Great article! What about const function? ex. const sum = (x,y) => x+y

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beginwithjs profile image
beginWithJS

You may consider it as the "hoisting variable const" situation.

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islamhanafi94 profile image
Islam Hanafi Mahmoud

First things first, thank you Lydia for this amazing topic. and I've a question regarding using let.

I think let variables is initialized to with undefined. but due to being in temporal zone it's value is inaccessible.

I tried

let x;
console.log(x); // resulting >> undefind

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inrsaurabh profile image
Saurabh Ranjan

Very informative article with amazing GIFs.

How you created these GIFs. Is there any tool or created those animations first than converted to GIFs.

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savokiss profile image
savokiss

Hi, Lydia~ Very nice articles~ Can I translate this series articles into Chinese, I'll keep the original links~

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priyank88544513 profile image
Priyanka Singh

Very helpful and to the point!

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kingisaac95 profile image
Kingdom Orjiewuru

Hi Lydia, please what tool did you use for the visualizations?

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kinglouisxvii profile image
Daniel Jakobian

love all of your articles, keep up the great work!

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deepanshu188 profile image
Zexu Knub

Thank You so much that actually cleared what hoisting really is

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edwinwong90 profile image
Edwin Wong

Well Explained! Keep up for the work you have done!!

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kkroeger93 profile image
Kevin-Kelly Kröger

Lydia you cleared everything what was gibberish before. Really enjoying reading your articles. Keep it up :).

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tiagoha profile image
Tiago A

JS Developers that use only arrow functions do not understand hoisting.

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Matheus Barone

Awesome content, and really clean explanation.

Thanks for that, and keep up the good work!

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gopinathaa profile image
Gopinatha

Thanks, Lydia for a simplified & awesome GIF representation.

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Kashyap Patel

How did you make these visualizations?

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cyberburi profile image
Buri

Thanks for visualization, perfect! :)

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qinlu20 profile image
从此萧郎是路人

Wow, that's a great article.I want to translate your article into Chinese so that more people can see it.Do you agree with me to do this?

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Saleem Malik

Would you like to share how you prepared this gif?