JavaScript is cool (don't @ me), but how can a machine actually understand the code you've written? As JavaScript devs, we usually don't have to deal with compilers ourselves. However, it's definitely good to know the basics of the JavaScript engine and see how it handles our human-friendly JS code, and turns it into something machines understand! π₯³
| Note: This post is mainly based on the V8 engine used by Node.js and Chromium-based browsers.
The HTML parser encounters a script tag with a source. Code from this source gets loaded from either the network, cache, or an installed service worker. The response is the requested script as a stream of bytes, which the byte stream decoder takes care of! The byte stream decoder decodes the stream of bytes as itβs being downloaded.
The byte stream decoder creates tokens from the decoded stream of bytes. For example, 0066 decodes to f, 0075 to u, 006e to n, 0063 to c, 0074 to t, 0069 to i, 006f to o, and 006e to n followed by a white space. Seems like you wrote function! This is a reserved keyword in JavaScript, a token gets created, and sent to the parser (and pre-parser, which I didn't cover in the gifs but will explain later). The same happens for the rest of the byte stream.
The engine uses two parsers: the pre-parser, and the parser. In order to reduce the time it takes to load up a website, the engine tries to avoid parsing code that's not necessary right away. The preparser handles code that may be used later on, while the parser handles the code thatβs needed immediately! If a certain function will only get invoked after a user clicks a button, it's not necessary that this code is compiled immediately just to load up a website. If the user eventually ends up clicking the button and requiring that piece of code, it gets sent to the parser.
The parser creates nodes based on the tokens it receives from the byte stream decoder. With these nodes, it creates an Abstract Syntax Tree, or AST. π³
Next, it's time for the interpreter! The interpreter which walks through the AST, and generates byte code based on the information that the AST contains. Once the byte code has been generated fully, the AST is deleted, clearing up memory space. Finally, we have something that a machine can work with! π
Although byte code is fast, it can be faster. As this bytecode runs, information is being generated. It can detect whether certain behavior happens often, and the types of the data thatβs been used. Maybe you've been invoking a function dozens of times: it's time to optimize this so it'll run even faster! ππ½ββοΈ
The byte code, together with the generated type feedback, is sent to an optimizing compiler. The optimizing compiler takes the byte code and type feedback, and generates highly optimized machine code from these. π
JavaScript is a dynamically typed language, meaning that the types of data can change constantly. It would be extremely slow if the JavaScript engine had to check each time which data type a certain value has.
In order to reduce the time it takes to interpret the code, optimized machine code only handles the cases the engine has seen before while running the bytecode. If we repeatedly used a certain piece of code that returned the same data type over and over, the optimized machine code can simply be re-used in order to speed things up. However, since JavaScript is dynamically typed, it can happen that the same piece of code suddenly returns a different type of data. If that happens, the machine code gets de-optimized, and the engine falls back to interpreting the generated byte code.
Say a certain function is invoked a 100 times and has always returned the same value so far. It will assume that it will also return this value the 101st time you invoke it.
Letβs say that we have the following function sum, thatβs (so far) always been called with numerical values as arguments each time:
This returns the number 3! The next time we invoke it, it will assume that weβre invoking it again with two numerical values.
If thatβs true, no dynamic lookup is required, and it can just re-use the optimized machine code. Else, if the assumption was incorrect, it will revert back to the original byte code instead of the optimized machine code.
For example, the next time we invoke it, we pass a string instead of a number. Since JavaScript is dynamically typed, we can do this without any errors!
This means that the number 2 will get coerced into a string, and the function will return the string "12" instead. It goes back to executing the interpreted bytecode and updates the type feedback.
I hope this post was useful to you! π Of course, there are many parts to the engine that I haven't covered in this post (JS heap, call stack, etc.) which I might cover later! I definitely encourage you to start to doing some research yourself if you're interested in the internals of JavaScript, V8 is open source and has some great documentation on how it works under the hood! π€
V8 Docs || V8 Github || Chrome University 2018: Life Of A Script
Feel free to reach out to me! Twitter || Instagram || GitHub || LinkedIn
FAQ: I use Keynote to make the animations and screen record it lol. Feel free to translate this blog to your language, and thanks so much for doing so! Just keep a reference to the original article and let me know if you've translated it please! π
Oldest comments (146)
This is so beautiful.
Although you didn't cover the whole story of it, this is a decent explanation of how JavaScript engine works. All those gifs are amazing. They help my brain to solidify the theories. Thank you so much. I'm looking forward to your upcoming posts.
Thanks!
Agreed with the GIFs thing. They help heaps to understand what is going on. GIFs for the win!
Lovely animations! Thanks for a really clear article
The Gifs really pull the article together. Great stuff.
This is so detailed and beautifully written with those illustrations!
Stunning visualizations!
Thanks!
Hello Lydia, do you mind to share what tools you've used to create such visualizations?
I was thinking about such animations for some meetups I conduct from time to time.
I would really appreciate it, thank you!
Great stuff!
Amazing!
Nice post. Very comprehensive. I shared it on our JS group
Thanks you for all explanation. I've been looking for this information for a long time ^^.
But with what tool do you make these wonderful gifs ? :)
I use Keynote!
Thank you very much ! :D
Apple Keynote? really? I did not know it was good to do that :-)
Haha yeah you can export it to animated gifs :)
It would be fantastic to know your animation creation process.
Congratulations
Hero!
Thanks for the informative post. Your animations are superb! I am interested to know which software did you use to create your animations?
It's very interesting!
Loved the animations, thanks for the overview :D!
Amazing, thanks!
Wow! This is really cool. I did a talk on this subject before, but your gifs do such a great job (wish I had seen them when I was learning) !
This is amazing!! I wish I'd had this at my coding bootcamp last year. I don't think it would have all sunk in at first but seeing it laid out visually would have helped things click at certain points as we learned more. I'm sharing it with them right now so future cohorts will be able to use this resource πππ
Thanks!! π Hope itβll help them!
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