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Eloquent JavaScript 4th edition (2024) (eloquentjavascript.net)
1274 points by vajdagabor 73 days ago | hide | past | favorite | 231 comments

This is, in my opinion, the book to use to learn JavaScript at more than a surface level. The only other materials I recommend as much (but for a different level of learner) are the “You don’t know JavaScript” in-depth book series.

In 2015, I was consulting for a distance learning program, administered by a major California University, that wanted to replace their current textbook (one of those “Head First” O’Reilly books) with something that had a bit more meat but was very approachable. I immediately recommended this and it was fawned over by both the advisors and instructors. It was also the cheapest option they had in the running (even excluding the fact it can be read for free) as it was competing against traditional text books. One year later, students were polled on it and it was met with a lot of positivity as well.

One hundred percent agree on both this book and the “You don’t know JavaScript”. Both are free (search Kyle Simpson GitHub). I ended up buying the YDKJ first edition paperbacks but see the second edition of all of them are done or a work in progress. EJs is the one I tell my students to start with as IMO is more of a programming book that just happens to us JS. I can tell the ones that read it (it’s optional) and the ones that do not. I do a few random chapters every year and learn something that I either missed or forgot I knew every time. I am mostly using TS these days but also enjoy vanilla JS for side projects and prototypes. Note the YDKJ books can come off as very ‘only my opinion is the right one’ kind of like JS the good parts but just look past that and absorb the content, what you do with it after that is up to you regardless of the author’s opinion.

> Note the YDKJ books can come off as very ‘only my opinion is the right one’ kind of like JS the good parts

I got the same feeling and it's very off-putting. KS seems like a dick. It's ironic that so much of his dickishness seems to be reacting against what he takes as Doug Crockford's dickishness. Ah the irony.

He did a day workshop at my old company about 10 years ago. He did come off as a dick, and very opinionated, but I really liked his no-stone-unturned approach, and ended up buying a few of the books when they came out later, and I got so much good stuff from reading them. I personally disagree with many of his opinions and have no problem with this at all; he covers everything (including parts of the language he clearly hates) in such detail that I can form my own dissenting opinions whenever I want. If anything, it’s a good thing that he is so openly opinionated - it makes it easier for me to pay attention and prompts me to ‘argue back’ and form my own (better) opinions, compared to just reading dry material that always tries to remain neutral and void of feeling. The only problem with opinionated code writers is if they actually fail to cover the stuff they don’t like. KS covers it all, with relish, often bitterly. I love it. Same for Doug Crockford. Always found him hilariously opinionated, often to the point that he was clearly fighting against reality, and yet I learned a ton by reading the cantankerous old git.

These are great points.

I wonder sometimes at how much my whole line of reasoning should matter at all. In other words: if it as you describe (and I agree with your representation of the issue) then KS can be as dickish as he wants, he's providing awesome information that I can then use as I like. His actual personality should be irrelevant.

And yet, to me, it isn't. I suppose it's deep-seated social processing at work that's hard to override.

I know Kyle and he is very opinionated and adheres very tightly to his set of beliefs. I think this can rub some people the wrong way and maybe this leads to the belief that he is a dick.

From my personal experience I think he's actually a really nice guy. He's also been unemployed for quite a while now and seems to be struggling with something. Doesn't seem very kind to kick him while he's down even if it's a virtual kicking.

I'm all for not kicking people while they're down. But if you build your brand in part on throwing haymakers every which way, people are going to have bad feelings about you that will come out in passing discussion, as here.

Went to JSConf India, specially for Kyle Simpson talk. His whole talk was just how his company can be a game changer for web development. Just so disappointing

YDKJ 2nd edition after first two parts the rest seems less actively worked on? hope the 2nd revision will be done soon.

I think Kyle Simpson is out of work and looking for something full-time and also started some other company, so probably not his main priority atm.

I remember reading the first edition back in maybe 2011 when I decided I should sit down and actually learn JavaScript beyond the superficial grasp I had of it from working on web things (mainly via Rails at the time). What a great book, I learned so much from it at the time. For years after, whenever anyone told me they wanted to learn programming, I told them to pick up this book as a first introduction to it. Plus, it was available for free on his website.

I've gotten rid of lots of old programming books over the years, but I've held on to my first edition copy of Eloquent Javascript. Lots of thanks to Marijn for writing this!

I don't think this is a good book for someone who wants to learn programming for the first time.

Why do you feel that way? What books would you recommend instead?

Eloquent Javascript is better suited for someone who already has some experience with basic programming and wants to learn javascript while also exploring more advanced concepts in programming in general.

Javascript was not my first language, but I have refered to javascript.info from time to time and it seems to have a gentle introduction.

Kernighan & Pikes Unix Programming Environment.

It’s *nix specific but a great introduction to all kinds of programming concepts with great working examples in a variety of languages you’ll use every day

I second JavaScriptInfo as another great resource to learn JS: https://javascript.info/

I agree

+1 for "You don't know js", it's a must read for any js programmer IMO.

I haven't read eloquent JS though, you say it's a different level of learner. Can you expand a bit? Is Eloquent for after "you don't know js" or vice-versa?

Edit: nevermind, reading the TOC of eloquent JS gave me a good enough idea

I get so jealous that people can absorb information via books as an adult. I can read the same chapter a hundred times and nothing sinks in.

I know this is off topic but do you, or anyone passing, have a system or tips for how y'all do this? I've got so many programming books but they only collect dust after I read through them without benefit.

Are you reading programming books the same way you would read a fiction book? If so, stop doing that.

Programming or any technical learning is a hands on experience. Take notes and apply techniques, using pen-and-paper or the keyboard, as they are presented.

If you really have to just "read" a technical book, IME a less-is-more approach works best. 5-10 minutes at a time, not even a chapter at once. Maybe a few paragraphs if its something really information dense. Funny aside, I find my morning "business" is the perfect time to this sort of reading using the Kindle app on my phone.

I read about fifty books a year for a decade now. Only about four of them per year, are programming books. They are indeed to be read very differently. Here's how I read them, ymmv.

- never in bed. never as audiobook. But sitting. At a table or desk.

- no distractions. At most "focus music".

- read a chapter through. Then read it again and do all excercises (on a computer without wifi)

- make copious notes, highlight quotes, summarize. Most important for me is to write down why I made that note.

- a time (years sometimes) later, do it again. E.g. when having worked on the concepts from a book in real prod projects.

- at most an hour. I have ADHD and my mind often flies everywhere suddenly; time to give up and grab a beer or coffee.

Very few of these work as ebook (Kobo) for me. The formatting of code is poor and diagrams unreadable. Prefer paper or PDF (but read on a computer or tablet without network).

That's all good advice. You should definitely not read programming books like normal books. But I'll extend it somewhat:

~ Yes, always do all the exercises. It's important how you do them. No copypasting. Enter all the code examples in your text editor of choice and run them if possible. Experiment liberally and don't be afraid of errors; instead, adjust the program in response to the errors. Create a directory and save all your files; don't just keep overwriting the same exercise. This is so you can go back and review if necessary.

~ If possible, always use the PDF formatted version of the book. EPUB and other formats too often don't look good and aren't formatted as the author(s) intended, and PDFs tend to be easier to follow because the page is formatted the same as the print version.

~ I use the Pomodoro technique of working intensely on the book for 25-30 minutes and then taking a short break before continuing. This tends to help me focus and retain more of the book.

Back when I was learning Python, I used two books in sequence, "Python Crash Course" by Matthes and "Think Python" by Downey. This turned out to be fortuitous, because until I started working through them I didn't know they are two completely different approaches: PCC teaches you how to program in Python and TP teaches you computer science using Python. Working through both books consecutively gave me a much better scope and understanding of the language to build on than using one book alone and stopping there.

That's some great advice.

When you say "don't overwrite", di you mean to start from scratch every chapter? I can imagine this would work for me: repetition is key. But also to get frustratingly boring after a few chapters.

I'll just treat is as any trunk based git repo. Commit significant progress, several times per hour. Then rebase to "summarize" my learnings into a history. I'll commit them to a public repo and treat as if that annoying colleague is going to review. Not that anyone will ever read them. I probably won't myself. But the art of rebasing, amending and pulling apart helps me with what would have been the perfect eLearning history.

No, I just meant save every exercise as a file, like chpt1ex1.js, chpt1ex2.js, &c. A lot of consecutive exercises are just variations on the previous exercise, helping you build your knowledge, but that makes it tempting to overwrite the previous exercise or never save the file. Then you don't have anything to review with after working hard on the book.

I agree with all your bulleted points. Especially the "at most an hour". Maybe its age or outside responsibilities, but mental fatigue is a real thing for me, and spending more time than that on technically-challenging materially brings rapidly diminishing returns.

> Very few of these work as ebook (Kobo) for me. The formatting of code is poor and diagrams unreadable. Prefer paper or PDF (but read on a computer or tablet without network).

My ancient (2011) Kindle is borderline useless for technical material but the iPhone app renders diagrams and equations acceptably well (my experience, of course). The small form factor of the phone is helpful too - more desk or table space for notebooks. Mind you, I make heavy use of app limits and downtime so its not the distracting experience of typical smartphone use.

I find my ReMarkable 2 e-ink tablet provides fantastic UX for technical PDFs. Zero distractions, handwritten (or typed!) annotations, etc.

It's not large enough, in my opinion. For PDFs, personally I think that an iPad 12.9 inch is the best choice.

Adding this just in case someone is tempted to get a Remarkable for reading PDFs.

Thanks, I thought whether I want ReMarkable or iPad 12.9 for reading PDFs.

I had this the biggest iPad back in the days it was introduced, but gave it to a friend as it felt too big for me, and way too expensive for what I wanted from an iPad. (Basically just a YouTube streamer, and my long obsolete iPad 3 from 2012 still does the job, surprisingly.) I checked iPad Pro 12.9 and there are justifiable prices for used ones, so I think I’ll replace my iPad 3 with this big one, which will allow me to read PDFs more comfortably as well.

Btw, I tried printing some PDFs and I don’t like it that way. I know to each their own, someone likes the books printed, but I’m the opposite of that. So much that I had donated all the physical library of my dad’s books I had in the house after his passing out, as I realized I would never read them in paper.

> do all excercises (on a computer without wifi)

And make sure you are typing the code, and not just copy pasta. Writing it out helps most people process it.

yes! Very much.

And turn off copilot. The code from the book is very likely in its training set. It will produce it almost verbatim. And you learn nothing.

I don’t mind distractions when learning via books (I wouldn’t be able to finish my degree otherwise), but the other advices still apply. Reading a chapter without doing the exercices is like listening to a lecture without taking notes. You may understand a few things (or everything), but you’ll find that doing practice, drawing diagrams, or summarizing it lead to a deeper understanding. More often than not, you have to dedicate a few days or weeks depending on how dense it is. You find yourself rereading a page from a previous chapter or consulting another book. You don’t have to read it end to end unless you view it as taking a course.

As mentioned, I have ADHD. Which means I cannot handle distractions. So I try to uses processes and tools to manage distractions as much as possible for me.

But I guess a neurotypical zoomer can code fine with a TV in the background, a podcast in one ear, insta and DMs ploinging into their notifications and slack nagging in the status bar. I cannot.

Do you keep some notes on the books you read? (I mean online notes here; but if you don’t keep them online, while maintaining the notes, the question is still valid. Maybe you have some system that works for you and you want to share.)

I use Joplin. And that has no way to share a part of my notes. And I'm really not ready to share my woes with my wife with the world. These are all mixed with my Kobo notes.

Also. Notes have always been a mess. I note a lot with pen and paper. Most even. I have piles of random paper. Pen drives with markdown. And large gaps.

If anything, adhd is terrible for consistency in this kind of stuff. Which is why I go full on plain text formats. Any binary, SQLite, cloud whatevs will rot within weeks after me loosing interst. Yet my diary.md and my bookkeeping.ledger, albeit gapped with years, still goes strong.

> Most important for me is to write down why I made that note.

My ears perked at this. Great advice!

I, for one, was a really bad student, until I discovered learning in bed. A desk is still to distracting for me.

In bed, phone out of reach and reading one hour, sleeping half an hour was super efficient for me. You only process material while sleeping anyway.

> I find my morning "business" is the perfect time to this sort of reading using the Kindle app on my phone.

Beware of hemorrhoids later in life if you’re spending too much time sitting on the throne.

Yup. The key to learning is doing. You need exercises at the end of every chapter if you want to learn. Math books, programming books etc. And they need to be challenging. That's what helps retention and understanding.

What I like to do, when not under time pressure, is to read such a book cover-to-cover, as a first pass. I get an idea which parts will be difficult for me, and which are probably not.

In this stage, I don't sweat the parts I don't get, I just note for later, that, if these turn out to be important, will require some care, patience and time. I also build some sort of "map" in my mind about the elements that exist, to get an overview.

After doing so, I feel a lot more confident to tackle the topic of the book "properly", doing exercises, etc.

Take notes as you go or by chapter. If a chapter has a summary at the end, read that first before going through the chapter. If there are code examples, write them out and play around with it. Get the important bits out that way.

Also, realistically you probably won't remember most of what you read. I suck at that as well, but you do build up a lot of peripheral knowledge. You may not remember how to do that one thing, but maybe you do remember that it exists, or that it was in a particular book. Just that type of knowledge has worked well for me.

1. Take notes. If you are reading an ebook or a webpage, handwritten notes may be better because you will not be tempted to copy and paste.

2. If feasible for you, consider getting a neuropsychological evaluation to rule out any learning disabilities like adult attention deficit disorder.

Not the original poster, but I learn best reading textbooks cover-to-cover. A couple things that help me:

1) I buy physical textbooks and absolutely destroy them with notes in the margins, highlighting, etc. It helps me to interact with the material instead of letting it wash over me, which means I'm both thinking about it more in-depth and as side effect I'm less bored. Otherwise I'll fall asleep and won't learn anything. 2) I accept that I'm not going to remember the entire book. A lot of books are most useful as references anyway. But if you ever find yourself going, "Oh that's really handy to know," then you can make a special note of it or even put it into flashcards. I've been using Anki. The trick is to recognize what is actually worth doing this for. 3) If something is especially worth knowing, (see point 2), see if you can either do problems from the book or try out the concept in some way if there are no problems available.

If you're reading something just because you feel like you should, you won't get anything out of it (or a least I don't).

I was a bit like you, read everything cover to cover, until i read Natsume Soseki’s Kusamakura, in which he says:

“Reading a book from the beginning to the end is like insisting on marrying every woman (or man) you fall in love with.”

And that opened my eyes and now i no longer feel in debt to the book or the author :)

When reading a book, I don't do anything special, but I frequently stop and think about what I've just read. It's not something I do by a command, either: it's just that a good book engages my attention, and then I kind of "chew" on it.

Unlike others here, I never take notes, and rarely do suggested exercises. But I read and think through examples; and as to exercises, I do think of "how I would approach it" and "what is that the author wants me to learn from this exercise".

Spaced-repetition and active recall are underutilized tools that can increase your retention of material you deem worth remembering.

Here's an example of how to use it to learn quantum mechanics, but you can imagine how it would translate to technical books for software developers: https://www.youtube.com/watch?v=OFuu4pesKf0

That's how I learnt programming (self study) as a high schooler more than 2 decades ago: by studying books (affordable fast internet connection wasn't a thing at that time).

Start from chapter 1, study the concepts and play with the code examples a few times till you understand. Usually there are exercises at the end of each chapter. Try to work on those.

Everyone has different learning styles. I retain maybe 90% of what I read, but only 10% of what I hear. So videos and any audio are the worst formats for me to learn anything.

The first step is figuring out what your learning style is.

This is debunked to some extent. Veritasium made a good video about it.


Personally I find a multi-pronged approach is necessary to really learn anything. Read it. Read it again. Visual guides are helpful. Work some examples. Make some mistakes, debug them, find the corner cases, write tests. Eventually when I've poked around the material for a while it starts to bed in.

Three months later it's forgotten, but when I learn it the next time I move a lot faster!

For me the essential part of comprehending new information is my own thinking on what I'm getting. When reading a book, I stop frequently to think, it's quite natural for me. Often go back a few paragraphs or pages, re-read them with the new understanding, think again, go ahead...

All of this is possible with video and audio in principle, but much less natural and much less convenient; also video somehow "hypnotize" me and I don't feel the urge to think about what I see and hear at the moment; perhaps only afterwards if at all. I have a feeling "oh I get it", but not much remains afterwards.

So I absolutely prefer text to audio or video when learning.

Are you saying that I can learn equally well from reading as well as audio sources and, despite 50+ years of personal experience, I am wrong?

Could be. After 40+ years of thinking I learn better from books I changed my mind after seeing one of Freya Holmer's videos.

I can definitely learn faster from a video as long as there are no talk heads in it, and only content.

Trouble is, all the instructional videos I had tried up to that point were crap. I can't think of even one highly recommended programming instruction video that is any good for me.

Disclaimer: this is my own, metaphor-filled hypothesis.

It’s easy to fully internalize generalizations that someone has presented to you, usually because it’s wrapped up in a way that is mostly compatible with how you see the world. Because someone has done the work of distilling this information, you are not able to share in most of the intellectual benefit. It’s the process of having a question, discovering an answer, internalization of the concept, and synthesizing a summary that brings you closer to understanding. It feels like a series of “lightbulb moments” when we consume this content, but it is often shallow and fleeting because the genesis of the idea was not your own. Airport/self-help books are a good example of this mental candy that makes us feel good when reading it but, unless you are able to fully internalize it, are just empty mental calories.

Comprehension and understanding are derived from amassing knowledge a little bit at a time as your mind is ready to advance your understanding. In other words, you really have to be “primed” for knowledge found in books. This is the reason that information you obtain after an adequately-informed struggle stays so salient: your mental model had all the scaffolding around a concept but there was a clear “void” where this answer fits, and if you find that fits super cleanly, it’s extra satisfying.

When consuming information-dense material, you end up with this knowledge back-pressure where it’s floating around your short term memory but won’t be committed to long term memory because there is just no place for it yet. When recalling information in order to create new content with it, and assuming you are challenging yourself, you will end up in situations where there is information starvation (the opposite of back-pressure) where you must either find answers (to fit in your mental model) or find a workaround (ignore the gap).

Some people can just read books and create efficient mental models. Others (myself included) have a limit of how much they can learn in one sitting because our brains need to be fully ready to accept this information.

The last piece of this is the dopamine response cycle that is a positive feedback system (positive as in “more creates more”). Dopamine makes us feel good by rewarding behavior that evolution deemed necessary for species survival (over simplification). Dopamine indirectly triggers long term memory retention because we need to remember why we felt good in that moment so we can replicate it. This used to be, “this berry tastes good, I should remember where this bush was and what the berry looked like.” In the modern context, it ultimately delivers motivation to us because we have adapted to using it to learn new information that is less relevant to survival. Achieving goals and solving problems at hand causes a huge amount of dopamine release. The problem is that we’ve found ways to hijack these systems and short-circuit this feedback cycle so everything had to become way more stimulating.

This got long, but my advice is to work on a project of your choosing that you are intrinsically motivated to do. Begin work on that project and read some of the book that relates to the problem at hand. Repeat this cycle of reading and working, trying to incorporate patterns and concepts you find while reading into your project. Make little goals for yourself every day before your start, and really hold yourself accountable in finishing them.

When I was a kid my dad taught me to write everything out three times. Applied as a grown up I just write down a lot of notes for things I am learnind

Note sure if you have gone very far into the Eloquent JS book but if you do it online it’s interactive. You read a little bit and then write code directly on the page. You then run it to see if it works or go back and read what you missed to correct it. Don’t get me wrong you cannot do the ‘projects’ and other parts are meant to run local, but for learning something quick the online version is all you need. Read, program, read, program…..

heres how I learn programming languages.

read a page, keep a notebook next to you and rewrite each paragraph in your own words. it forces you to engage more deeply with the material

Thank you for mentioning YDKJS. I hadn’t heard of it before and am in the beginning of my first proper JS project for about 10 years; man has the JS world changed. I have already skimmed the first few chapters and am already understanding it all very concisely. I will be reading this properly via GitHub for sure.

If you want a video series that accomplishes a lot of the content in these books it's Will Sentance's Javascript: The Hard Parts.

Cheers Josh :)

The Hard Parts of UI Development was amazing. Your teaching style really has a way of making things stick. Truly one of my favorite instructors.

yes, also javascript the good parts is really good(hehe) because it's more theoretical and there is not html or web stuff in it just the language

How recently have you read it? I thought it was a worthwhile read when I first read it 12 years ago, but I picked it up to skim a couple years ago and was struck with how much of it was irrelevant for modern JS due to changes in the language spec.

In that sense an annotated "this is what JS used to be when this book first came out" version could be of historical interest.

Last time was 2017 I think, don't know if there is a big difference I use both new and old things

If I put "You don’t know JavaScript" into Amazon I get approx 10 books at £20 each, is there a specific one / author I should look for, of even better and ISBN. Thanks!

There are 6 books, the author recommends reading them in an order:


If the second edition is not available, you can read the first edition, just be aware some small things may be slightly out of date.

For those that don't know the author, Marijn Haverbeke is the creator of CodeMirror (code editor) and later ProseMirror (text editor).



I'll always know him as the author of Postmodern, an impressive CL library for PostgreSQL: https://marijnhaverbeke.nl/postmodern/

Prosemirror user, it is fantastic. Didn't know Marijn was so prolific, all with free/open source projects. Wonder how can make a living...

Fancy seeing this here, some days after finishing the third version :)

I'm also glad to see the asynchronous programming chapter significantly reworked - it was materially weaker than the rest of the book because of some weird analogies involving crows and their nests that didn't seem to make any sort of sense to me.

The third edition also gave me the impression that it was a reasonable book to learn JS and the DOM (and a sprinkle of Node.js, for good measure), but that it was a book aimed primarily at experienced people who were transitioning to JS and the web - not beginners (despite the book's efforts at claiming suitability for beginner programmers).

>book because of some weird analogies involving crows and their nests that didn't seem to make any sort of sense to me.

I am glad I am not the only one. I believe he over-abstracted it to it's own detriment.

I went to purchase a paperback earlier this week. Now I will wait for this one to hit print.

Ah! I vaguely remember rage quitting over the crows but didn't remember which book was it. I should give this another try then.

I don't consider myself a good programmer. I struggled throughout my youth to grasp even the basics. This book pointed me in the right direction. Can't recommend it enough.

And how are you doing today?

My go-to JavaScript book will always remain “JavaScript for impatient programmers”[1], a 639-page book written by Dr. Axel, PhD.

It's quite complete and detailed. But as if it wasn't enough the author wrote a second (smaller book) named “Deep JavaScript: Theory and techniques”[2].

Both are free to read online!

[1] https://exploringjs.com/impatient-js/

[2] https://exploringjs.com/deep-js/index.html

Part of the force of this book comes from its explanation of fundamentals of computing, and how it relates to javascript. Another part is due to how interesting are the projects that it proposes that the reader build. I don't even like programming in javascript but was drawn to read the book.

I love this book, even since its first edition. It's very clear even on elementary stuff, e.g. see the section on bindings/variables: https://eloquentjavascript.net/02_program_structure.html#h-l... — avoids the pitfall of thinking of variables as “boxes”.

I was trying to find what's new in the 4th edition, and following links from the author's website https://marijnhaverbeke.nl/ found this on Mastodon (https://mastodon.social/@marijn/112020092273623390):

> The FOURTH EDITION of Eloquent JavaScript is now online, adjusted to the realities of 2024 and generally touched up.

The book is actually maintained on GitHub too, https://github.com/marijnh/Eloquent-JavaScript.

When I was just starting out I read this book and noticed a small mistake, I was really proud that one of my first contributions to any open source project was a PR to this repo.

Thanks, that gives a way of seeing the diff between the third edition and the current (last commit 45 minutes ago): https://github.com/marijnh/Eloquent-JavaScript/compare/f8f00...

Looks as if there will still be a paperback version released in the future as well for anyone that prefers that format.

I don't work in JS at all professionally but this book has intrigued me for a while at this point after seeing it recommended so often. I think I'll pick up a copy once the paperback is released.

It's a great explanation, but I've never heard the term 'binding' used to describe variables. It's usually reserved to function binding or bridge APIs like the DOM.

The tricky thing is that "boxes" are the right abstraction for primitive values. After that you need to explain how references work, and that's pretty much the same 'tentacle' concept. This method spares the reader one step, but might cause confusion once they face problems that require that understanding.

"Binding" is used thousands of times in the language standard, including for variables: https://tc39.es/ecma262/#sec-variable-statement -- that's my point, that this book is precise while being approachable to beginners.

And I dispute the claim that "boxes" are the right abstraction for anything in JS. (Boxes may work for primitive values, but nothing further.) Directly seeing names as bindings ("tentacles") not only skips the incorrect "boxes" step, but also causes no confusion or problems whatsoever at any point. (If you have an example, I'd be curious to see it.) (There are some differences in the language between primitive values and Object, but none of them are particularly helped treating variables as boxes AFAICT.)

I meant in the context of writing code - you’ll never hear anyone refer to their “bindings”.

It breaks down with the simple `let a = 22; let b = a` example where the tentacle/binding metaphor can lead to wrong intuition of why a change to the value of a is not reflected in b.

In what way does it break down? Maybe if one is thinking of boxes, going by "a change to the value of a"? The book says:

> When a binding points at a value, that does not mean it is tied to that value forever. The = operator can be used at any time on existing bindings to disconnect them from their current value and have them point to a new one

In this case:

    let a = 22;
The binding a points to the value 22.

    let b = a;
The binding b points to the same value that a points to, namely 22.

    a = 23;
The binding a now points to a different value 23. The binding b is not affected.

I've linked it a few times in this thread, but see the equivalent article for Python, which explains it clearly: https://nedbatchelder.com/text/names1.html

(It cannot break down because it's how the language works.)

Have you given any kind of training or lessons in programming (honest question)? Everything can break down!

This is exactly what I’m talking about:

> The binding b points to the same value that a points to

It may look obvious to you, but someone will interpret “the same value” as literally the same. So they might expect a change in a to reflect on b. Whereas if you tell them the “a box contains 22” and “the b box also contains 22” you prevent that misunderstanding. They might not have any concept of references/pointers yet and are just trying to make sense out of everything. These are the kind of silly mistakes people make when learning programming, or a new language.

It’s not about being right or wrong but providing a safe path to understanding that can be built upon. I did not mean the binding/tentacle abstraction is wrong, just that there might be a simpler one to introduce the concept. The book is still great regardless.

> It may look obvious to you, but someone will interpret “the same value” as literally the same.

But it is literally the same? Numbers are immutable, so there is a performance optimization where you can avoid using pointers internally, but the fact that they are immutable also means there is no way to distinguish between them being the same value and them being "different instances".

If you do `let a = []; let b = a; a = [1]` would your students expect that b equals [1] or would they understand that a and b now contain different arrays? If the latter, then why would think that after `let a = 22; let b = a; a = 50;` b also equals 50?

> Have you given any kind of training or lessons in programming (honest question)? Everything can break down!

I have actually, so I understand what you mean, that all sorts of confusions are possible no matter what. I agree with you on that part; I just disagree that the “boxes” metaphor is something that is necessary or “needed later” (what I understood from the wording “This method spares the reader one step, but might cause confusion once they face problems that require that understanding” or “breaks down”).

Yes, the student needs to understand that “let b = a” assigns to b the value of a, and does not make b a permanent alias for a. To some students that misconception may never arise, but to others it might and that is something to watch out for. (This is the part explained here for instance: https://nedbatchelder.com/text/names1.html#:~:text=I%E2%80%9... — sorry if you cannot see the highlighted part e.g. if you're using Firefox (https://caniuse.com/url-scroll-to-text-fragment), look before and after "Reassigning one of them".) But if you explain this as “the a box contains 22” and “the b box also contains 22”, this is an understanding that only applies to primitive values and therefore will break down pretty soon and cause confusion (given how widely non-primitive values are used in JS/Python/etc), while if you say that “let b = a” makes the “b tentacle” point to the value that the “a tentacle” points to, this is a uniform understanding that bypasses the incomplete “boxes” understanding. With the understanding that names are one kind of thing and values are another, and names can only point to values (not to other names), there is no problem with “the same value” being interpreted as literally the same: it is literally the same value (a Platonic ideal “22” that lives out there and that both a and b point to), and a re-assignment like a = 23 does not change the value (the notion of changing a value would not even arise without the boxes metaphor, as value 22 and value 23 are simply different things on the values side).

But I guess ultimately this is an empirical matter: we can try the different paths on different sets of students and over time see which one takes better. My intuition is that the “tentacles” metaphor is just as easy to understand as “boxes” without the latter's problems (there are risks common to both, e.g. "let b = a" neither makes the b tentacle point to a itself, nor does it put the a box inside the b box), but until we actually try it out (I have not tried to compare), I guess we just have different intuitions for now. :-)

You're struggling due to a conflation of two concepts.

Binding refers to associating a name with a value. Assignment is a case of binding, but not the only one; two other examples are positional arguments in a function signature, and ESM imports. A binding can be mutable (let assignment, function arguments) or immutable (const assignment, ESM import).

Value mutability is orthogonal. You can mutably bind a primitive value as "let a = 22" and then mutate the binding via reassignment, but you can't mutate the value itself; you can immutably bind a reference value as "const b = {}" and mutate the referenced object via property access, but you can't mutate the binding.

I refer to bindings all the time, where it's useful to be clear in meaning. I also make sure to introduce the concept to mentees who aren't already familiar with it, and by all reports thus far it's proven as valuable an abstraction for them as it did for me when I first learned of it.

That’s funny. This is about didactics for newcomers to the language, I shared an example of what might trip them up. Thanks for taking the time though.

"binding" is a PLT term, denoting the association between a name and a value.

It's a higher level concept than the variable - a mutable binding is what people usually refer to as a variable, and an immutable binding is the correct term for what people refer to an "immutable variable" (an oxymoron, if you think about it).

Immutable variable isn't a oxymoron. It can still vary between instantiations. If you have (a)=>{const b = a}, b can have different values even though it can't be reassigned.

In the case of the code that you have cited, these are all different elaborations of the binding at the invocation of the lambda, due to the interplay between activation records and scope rules.

It's not really an "immutable variable" - it's a local binding getting bound to different values on each scope entry.

EDIT: By the way, the `b` binding in your code can be modified. Did you mean `const b = a;` ?

> it's a local binding getting bound to different values on each scope entry.

It is, I just wanted to point out that the term "immutable variable" is sensible. I think a good way to put it is that b is a variable, and when the statement runs a value is bound to b. So the value bound to b varies yet b can be immutable, in contrast to a constant which is a binding to always the same value.

> Did you mean `const b = a;` ?

Fixed, thanks :)

1) the most common binding used in well written modern JavaScript is a const binding. It is by definition not a variable.

2) the ‘binding/tentacle’ metaphor works just fine for primitives and the ‘boxes’ model adds more complexity.

"binding" is pretty standard terminology in some oldtimey languages; lisp, ML, etc.

Bash documentation says that "set -u" traps variables that are "unset".

The actual diagnostic itself is better educated:

  $ set -u
  $ asdasdf
  $ echo $asdasdf
  bash: asdasdf: unbound variable

"binding" seems like a more casual term for memory pointer. I guess if people are just getting started with programming it make sense to simplify things a bit.

It's not a simplification. It is abstraction. Binding doesn't imply a particular implementation.

A variable binding can disappear entirely. If you bind var x = 42 and never use it, the variable need not exist. If it doesn't exist, then there is no pointer.

If you do use it, constant propagation/folding can again make the variable disappear. If the variable is never assigned, all uses of it can be replaced with 2.

Variables that care captured by lexical closures can be treated differently from ones that are not. Variables captured by closures but not shared among different closures, or not mutated, can be treated differently from mutated variables shared among closures.

The abstraction of binding is more complicated than "every variable is a memory pointer" because it allows more possibilities, which can coexist.

My point is that it's not a simplification, it's precisely how the language works: bindings between values and names (JavaScript has no separate notion of memory pointer; everything is a "pointer"). (Similarly for Python: https://nedbatchelder.com/text/names1.html) Describing variables in this way gives readers the correct understanding, and the analogy of tentacles is no harder than that of boxes. Such things are what I most appreciated, that the author manages to be approachable without sacrificing accuracy.

If the explanation doesn't mention memory positions then it is a simplification.

JavaScript does not provide a way for the programmer to access memory positions (and e.g. does not guarantee that the the language runtime will maintain unvarying memory positions), so it doesn't make sense to talk about memory positions: there's nothing to be gained at that level of abstraction. (Unless you'd call it a "simplification" to not talk of electrons and transistors too, in which case fine, yes it's a simplification in that sense.)

I'm curious - could you expand on why it's a pitfall to think of variables as boxes?

Because that's not how variables work in JavaScript/Python etc (though it may be fine for say C++ in the case of value types and copy constructors). For example:

I'm typing on phone so for a quick example:

    let a = [];
    let b = a;
and consider the value of b now (or the fact that we could write "const" above because the binding is constant, even though we mutate the value). Or see the equivalent for Python by Ned Batchelder, which has more examples and elaboration: https://nedbatchelder.com/text/names1.html

I knew that in Python all variables are really references to objects (even when we're using a number) - is JavaScript the same way?

Also, does anyone have a link/reference to the place in the spec where it specifies this? I briefly skimmed through parts of [1] but couldn't find anything that says that JavaScript treats numbers this way.

[1] https://tc39.es/ecma262/multipage/#sec-intro

I don't think there's an explicit reference in the JavaScript spec to numbers being treated this way, because this is how all variables are treated in JavaScript - the relevant part of the specification is probably the definition of the "PutValue" abstract operation[1], which doesn't include any special cases for numbers (or other primitive types) vs. objects.

[1] https://tc39.es/ecma262/multipage/ecmascript-data-types-and-...

As I understand it, JavaScript does have a distinction between the primitive types and Object, but this does not show up in any significant way to the programmer / in any way relevant to how to view variables. To put it differently, the "boxes" view only applies to primitive values, while the "tentacles" view applies to all values, so the former is unnecessary and need not even be considered/taught.

Chapter 11 on async is particularly good, I still get confused by async/promises sometimes in Javascript - https://eloquentjavascript.net/11_async.html

I'm currently going through a hard copy of the book's third edition. But I'm wondering whether the description of the language in the book is detailed enough. Could you share some opinions on whether it will be good to go through some other JavaScript books after it? I'm considering going through "JavaScript: The Definitive Guide"[1] or "The Modern JavaScript Tutorial"[2] after it.

[1] https://www.amazon.com/JavaScript-Definitive-Most-Used-Progr...

[2] https://javascript.info/

"JavaScript: The Definitive Guide" does go deeper, with thorough examples in all the topics mentioned in the index. It also provides examples of static types using Flow instead of TypeScript.

>The field of programming is young and still developing rapidly, and it is varied enough to have room for wildly different approaches.

This was an interesting line of thought to digest. He's right, of course. Programming is probably still in it's infancy.

Studying and learning about programming however, can make you believe it's some ancient art mastered by the giants of our recent past, the perfection of which is never to be surpassed again.

The one odd thing in it is that it still claims SVG markup needs a namespace. Which it doesn't, SVG became part of the HTML5 spec and emphatically should _not_ use namespaces when used as "just another element" inside an HTML document.

(even though you do need a namespace when creating svg elements through createElementNS, both "of course" and "unfortunately", and of course you need namespaces if you're creating an actual stand-alone SVG document)

Like that this now includes a chapter on Node. I think it would be nice to see a follow-up book in a similar style that covers a bit more with Node, Deno and Bun.

I really like the Deno approach so far. I prefer TS mostly these days as well as the esm stroke modules. I think node just made usage harder in their approach. I understand why, I still disagree on the solution.

Seconding the sentiment on this thread, I used this book to learn JS 5 years ago, and it's awesome. I've never seen another resource as good. YDKJS is more of an advanced treatment. If you're a beginner it feels academic, while Eloquent JS is very practical and approachable.

> Every now and then, someone comes up with a new way to circumvent the limitations of a browser and do something harmful, ranging from leaking minor private information to taking over the whole machine that the browser runs on. The browser developers respond by fixing the hole, and all is well again—until the next problem is discovered, and hopefully publicized, rather than secretly exploited by some government agency or criminal organisation.

This is from the chapter on HTML and JS (emphasis mine). It is funny to see how govt agencies and criminal organisations are mentioned in the same breath. How did we end up here?

this book taught me javascript! great book, highly recommend it.

I agree. Have you also read "JavaScript: The Good Parts" ?

"JavaScript: The Good Parts" was a great and a very important book back in the day. I am sure it inspired many of the improvements JavaScript has seen since then. But as it has seen these improvements, and as they were many indeed, I am not sure the book is still as relevant as it once was. Or to put it differently: There are many more good parts to JS these days compared to when the book was released :)

As others have stated, The Good Parts is no longer something I'd reccommend. It's more or less the K&R of JavaScript at this point. An interesting historical primer, but it would only serve to confuse a newcomer today, as all of the warts in the language discussed are ancient obsolete history.

nope, looks like a deep cut and will fill in some knowledge gaps i have. thanks for the rec.

another good one for my learning was "secrets of the javascript ninja"

Secrets of the Javascript Ninja is my favorite JS book by far.

I have Zakas' Professional JavaScript for Web Developers and ECMAS 6 update. Was very happy with them (easy to read) but both are getting long in the tooth and the author seems to have lost interest in updates.

How does this one compare as an all-in-one? Being up to date is a win, but I'm wondering about the quality of the writing.

I don't mean to throw shade on the whole book, but I don't think the section on errors takes things in the right direction.

A distinction should be made between errors and exceptions. In JavaScript and many languages, we conflate the two and use exception handling as logic flow control. In my experience, this can end up being a headache and encourage unnecessarily weird structuring of code.

Look at this example from the page on errors:


function getAccount() {

  let accountName = prompt("Enter an account name");

  if (!Object.hasOwn(accounts, accountName)) {

    throw new Error(`No such account: ${accountName}`);


  return accountName;


The possibility that a user will enter a account name that doesn't exist is not an exception, but we are treating it like one in this case. In order to handle this exception when getAccount is called, we have to wrap it or some higher level scope in a try-block and then regex-match the error message if we want to handle it differently from other errors.

You might be saying "it's just an example", but there's plenty of production code in the wild that is written this way. Maybe this could be improved by subclassing Error, but now you're having to manage a bunch of clutter and using object-oriented features as a way to reliably determine what kind of exception you're dealing with.

I find this pattern to be preferable:



function getAccount() {

  let accountName = prompt("Enter an account name");

  if (!Object.hasOwn(accounts, accountName)) {

    return {

      accountName: null,

      error: {


        message: `No such account: ${accountName}`,




  return { accountName, error: null };


Then we can call the function like this:


const { accountName, error } = getAccount();

if (error) {

  if (error.code === ACCOUNT_NOT_FOUND_ERROR_CODE) {


} else {

  // do something with the account name


No doubt, you may still want to catch exceptions at a higher level scope, but at the nice thing here is that exceptions (almost) always represent actual unexpected conditions that aren't being handled properly while return values with error codes represent expected conditions and can be handled like any other logic in your code. It also reduces any ambiguity of how an error should be handled but without subclassing. An error can even contain more information than just a message if you want it to.

Also, if you really want to ignore an error for some reason, then you can just pretend that the error doesn't exist. No need to use a try-catch where the catch-block is a no-op.

I wish we'd encourage this sort of pattern, but maybe that's one of many pipe dreams of mine.

In other languages like Java this can work because of checked exceptions (can work, I know there are a lot of mediocre devs who don't know what to do with checked exceptions but that's another issue). But in JS this is a terrible way to deal with any problem that can be handled.

I'd agree, but incidentally I've also been extracting string labels into some higher level, such as a constant or message creator function that looks up the message by it's code in some dictionary. It helps with readability, particularly in modern view libraries, to just have all your labels in one place and not have to scan the JSX or HTML for the string literals, and likewise all modifications will produce more concise diffs, testing could be easier if you're doing string matching, localization is more manageable with fewer scattered external dependencies on translation hooks or w/e

1. Exceptions can have codes, too, you know, and often do. 2. In the example, the exception is meant to bubble up all the way to the consumer. Different consumers can then handle as they see fit, even just displaying the exception message to the user.

Yes, exceptions can have codes. As far as I have experienced, their error instances don't do anything better than a plain object, and a non-exception error object can more clearly take a wide variety of shapes. It also limits potential confusion between errors or exceptions raised by your code and that of some dependency.

It is true that exceptions "bubble", and plenty of developers take advantage of that behavior. In my opinion, it is not helpful for errors that are expected to happen as part of normal application behavior. Bubbling of errors creates a sense of logical indirection. Errors as return values communicate that likely nothing " wrong" actually happened. Good communication through code reduces the amount of time developers spend using the wrong assumptions when debugging.

There is also no reason why errors in a return value can't be returned by the caller. When you learn to love objects/hashes as return values, you can get the same benefits of bubbling but with a more clear path to follow when getting a picture of where a value is coming from and why.

In the case of actual exceptions, like accessing a property on undefined, an error being thrown is appropriate because, like you say, it can be bubbled up. The nice thing about reserving exceptions for this sort of thing is you might get away with a single try-catch at the highest scope and have a single straight forward error modal for when an unexpected problem occurs. Then you can otherwise implement errors in your code without the possibility that they will escape through bubbling and trigger behavior that is not immediately obvious when following the path of execution.

This is not to say that the tradition of using errors and try-catch for normal application control flow is inherently bad. Its just another tool. I do subjectively believe they are counter productive when used that way, and encourage others to try something like my approach. I think we would benefit by making it a standard practice.

like Result in Rust

Haven't even looked at Rust code until just now, but yes, that looks like a more formalized version of this idea. Interesting!

Great book, looking for some info about main changes from 3rd edition but did not find it.

Any suggestions from the community on the best way to consume this site as an audio book? I know I could scrape and feed into a text-to-speech library but was wondering: is there is anything off-the-shelf?

First jquery, then React, now this book reaches a new version. Good times for JS

Frustrating that the online version uses weird format for it's code blocks, which leads reader view (Mozilla Readability) to not put code in a <pre> tag or a <code> tag and leaves it as plain text.

How is a `<pre>` tag for code blocks a 'weird format'?

I love the web version has a feature to link back to any given paragraph. I hadn't noticed this in any other site and found it delightful.

In my interactions with Marijn, he’s been very responsive. I’m happy to see this release and wish him continued success.

I learned JS from the second edition.

Incredible resource.

Definitely one of my favorites. It helped me level up when I was learning Javascript but also helped me understand how to be a better programmer.

Looks pretty solid.

CH 5 : "Higher Order Functions" seems a nice segway into one of my favorite, lesser known js util libraries Ramda.js

To learn js I recommend „WAT” tutorial.

Nothing changes in last few years. We still go in with what we incorrectly assumed at the start.

Does the author ever talk about why he is publishing a free version along with a more traditional paper version?

Makes it available to more people. Also probably helps paper books sales more than it hurts them, through exposure.

does anyone know where can i get the list of changes compared with the previous edition?

From a comment above thread, here's the diff of current state against the 3rd edition: https://github.com/marijnh/Eloquent-JavaScript/compare/f8f00...

many thanks, lioeters!

How does Eloquent JavaScript compare with Horstmann‘s JavaScript for the Impatient?

Its not for the impatient, its a deeper dive than that.

When is the print book coming ?

> Paper book will take a while longer.

According to the author, Marijn Haverbeke.


I wonder if such high quality free books are already fed to LLMs during training?

They're open source and easily accessible, so I would practically guarantee some/most of them are using it and other open source books.

Given so many are just on Github and very well known/linked-to I doubt they even put any special effort into getting these specific resources.

Where is the physical copy of the 4th edition? I can't find it.

Not yet, soon. It is in the making


The 'Eloquent JavaScript 4th edition (2024)' in Dark Mode

This is what really taught me JavaScript.

The best JavaScript book out there, IMHO

What if one is using TypeScript?

The rules still apply.

Great book

This is the book that helped me really understand JavaScript. Really great content and I can't thank Marjin enough for putting this out there for free.

Also how this person is so productive? I really would love to read on how he manages his time and priorities.

Someone asked about integer support in JavaScript. JS now supports BigInt!


And in some JavaScript engines (eg V8) bigint multiplication is asymptotically fast, unlike say the default CPython. It’s a very pleasant surprise if you happen to be a number theorist (say).

How do you mean? CPython uses karatsuba's for large numbers which should be asymptotically fast


Oh thank god.

I've been having to do some hacky things to represent numbers on an astronomical scale for my star system/planet generators.

Yes, I was asking about that and I'm very happy to see this exists now.

What's new in this edition?

you can see the differences on github: https://github.com/marijnh/Eloquent-JavaScript/compare/3rd_e...

From a quick browse:

- # for private properties

- ESM imports in node

- hasOwnProperty -> hasOwn (TIL: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...)

- Math.pow -> **

- coverage of `function*` generators (https://eloquentjavascript.net/11_async.html#h-o+cFzGGhnz)

Here's a diff without the first commit that changed all linebreaks: https://github.com/marijnh/Eloquent-JavaScript/compare/d8290...

It should be mentioned in the "Introduction" but it seems your question is not covered there.

Could've used better commit messages, but:


This book took me from having nothing -- literally being a political science major grocery store employee -- to making 2xx,xxx YOY.

It was challenging and made me question whether or not I had it in me to code. Turns out, I could, and, vis-a-vis my peers, exceptionally well.

Well, I took me, but this book was my first real introduction to computer science.

Funnily enough, I didn't then and don't now personally care for the author (he seems untalented as a teacher), but I feel he accidentally made a good resource in his hubris.

You're welcome

Thumbs up emoji.

Where are you making $200k+ with limited experience and do you need a senior partner? :-D

6 years is sufficient experience, and we're good, thanks!

It’s enough to do well, but certain we all grow for quite a while after that. The improvement typically springs from discipline at that point rather than calculative ability however.

I'm not entirely convinced.

I've seen interns that are better than some of my 15+ year colleagues.

I'm better than most of them myself, but I've also seen interns better than me -- better problem solving skills and attention to detail I suppose. Very inconsistent correlation in terms of time spent outside of filtering people. Of course, you used to have a knowledge advantage that would hold for a bit, but intelligent usage of GPT-4 removes even that.

I'm pretty sure the whole enterprise is a combination of being gifted intellectually, with maybe a sprinkle of actual effort and abilify to sustain your attention on supremely boring tasks -- but mostly just your innate gifts at work.

When I say "better than"... well, here's my opinion: In programming, someone more talented can be up to 10x more productive at each tier of competence, shall we say. My least productive (senior and junior) colleagues are 10-100x less productive than me. My most productive colleagues (senior and junior) are probably 10x more productive than me.

This is an interesting discussion to me because I used to believe the whole tiered conception of programming knowledge until I learned that this wasn't the case through experience.

It's all just problem solving, and you're either smarter or less smart and you can't change this with even 1000 years of study.

You’re comparing with others, however I meant compared with yourself. I look at my Uni code and see a quantum leap from that to now. Some of it is better tooling but a significant part is discipline and experience.

Knowing what rabbit holes not to go down is the true 10x. Not the faster typing part.

In the job market, it's all about how you stack up to other people.

That kid that blows you away will likely stagnate like all of us, he might pick up a few tricks, though.

I’ve never met a kid that blew me away, but would like to. Presumably they are shuttled off to an underground bunker somewhere.

Also, if one hasn’t read the historical literature such as Petzold’s Code, Mythical Man Month, High Output Management, McConnell *, etc they have a long way to go. No amount of bit-twiddling proficiency can make up for that. Unless you’re in the twiddling business. :-D

I don't really think reading books is the way to go generally to improve at the job of making things, at least, not any of those books.

Making things seems to do the trick there.

Hah, I let a few silly things slide as youthful exuberance but this one will be laughable even to you in not too many years. Zuck learned that one in public, so you’re in good company.

This is my favorite book about JS, and I always recommend it to people. It occurs to me for the first time that the lack of TypeScript might be a problem, because if I’m making recommendations to someone learning, I am definitely going to recommend they write TS instead of JS. On the other hand it may actually be helpful to learn the concepts in this book without the additional syntax overhead of type annotations, plus the more webby content doesn’t really have much to do with types anyway.

I very much concur that it is better NOT to introduce TS when explaining JS fundamentals. I've seen smart engineers with a C++ background get tripped up on and very confused working with TS, because it's not clear to them what concepts are "language fundamentals" and what concepts are "the TS transpiler".

(Like expecting that just because you declared something as type X, that it guarantees at runtime it will always be type X, but it won't. You may get data from an API that _says_ it returns type X but the contents don't match. That can be valid code that compiles and has weird runtime behavior)

> I've seen smart engineers with a C++ background get tripped up on and very confused working with TS, because it's not clear to them what concepts are "language fundamentals" and what concepts are "the TS transpiler".

This is exactly why I never recommend TypeScript to new developers.

A similar problem (that used to be worse) with people learning JavaScript is the lack of separation between JavaScript and the DOM + browser APIs. 10+ years ago, people have told me how much they hated JavaScript and when probed about it further, would admit that it’s actually the DOM or new/inconsistent browser APIs that have caused issued.

JS has a number of its own flaws and quirks, yes, but there are two fundamental issues that make it harder to approach as a new learner (as opposed to, to say, Python) are how tightly coupled it has historically been to it’s primarily application case and how high or low level is this language?

What helped me understand the Runtime behaviour of TS is to understand that TS doesn't actually have strong typing. If it was named "LintScript", its name would be much closer to the truth.

This statement is quite questionable starting from the fact that there is no definition for what strong typing is.

Care to provide what you mean?

Strict TS won't compile pretty much any type-unsafe operation.

It's not perfect, the standard libraries are a bit too unsafe type wise, exceptions are untyped and need Either/Result-like data types to be handled but it's an extremely powerful language if you know it and it's ecosystem.

Most people though don't even bother reading the docs and can't tell you what a mapped type is or what is a disjointed union, etc.

If you are in a completely self-defined world, then yes you can trust the compiler. But I once built a react component which was called from library code and it simply did not adhere to the method signature (this was for a material UI table extension).

As soon as you have third party code calling your methods all bets are off. It could be JS calling your methods, or there simply is a hidden cast somewhere.

This is the moment that you realize that type annotations really are just a compile-time fiction, much like Python. At least in my definition, this is weak typing as the variable is physically capable of changing to any type at runtime, despite its type annotations.

TBF, this can also be true in C++, C#, probably most languages that can interoperate with other systems not written 100% in the same language + same runtime. After a while, you just get used to not trusting anything at the boundary - types are for your convenience and your internal logic, nothing more.

You are talking about something entirely different. Having weird interop is one thing. But having your internal state compromised because the language does not perform runtime checks is something entirely different.

In case of C++ this would lead to desastrous memory corruption. If all data is dynamic you can't have a safe program as data on the stack must be monomorphic or you corrupt nearby memory.

Naturally, you can defend against hostile input via excessive defensive programming (asserting against nulls, asserting against wrong types etc.). Or you simply use a strong static typed language.

Yes, that's why I used C++ as an example. It's very easy to create a scenario where the memory layout your logic (and your type-checker) assume exists just... doesn't. Core dumps from the field with "impossible" values, vtables missing entries, etc.

Do you mean runtime type checking? Coming from compiled type safe languages, it was difficult to wrap my head around full type erasure too. Your name would have definitely helped.

> I've seen smart engineers with a C++ background get tripped up on and very confused working with TS,

The idea that someone is good at another language so they'll automatically be good at another is a common misconception. In fact, they're likely to be worse because they're less likely to spend time trying to learn things from the ground up and less likely to write idiomatic code.

It's especially bad with js/ts because of it's popularity (so lots of new programmers that complain about how NaN doesn't equal itself), and because it's the defacto web language so lots of people are forced to use it as a secondary language that don't want to spend time learning it.

NaN doesn’t equal itself in any language I’m aware of, fwiw.

I wholeheartedly agree. At most, I introduce JSDoc[1] to newer developers as standardising how parameters and whatnot are commented at least gets you better documentation and some safety without adding any TS knowledge overhead.

[1] https://jsdoc.app/

It is (or should be) common practice to parse/validate any external data you depend on. You should be doing this for Javascript too. I find that the library https://zod.dev/ is quite helpful for this

Agree. I wish tools like zod/valibot were first class in typescript.

Also effect schema if you're looking for something much more powerful and in effect-land.

> I am definitely going to recommend they write TS instead of JS

Why is that? If you want them to learn JS, teach/recommend them to learn JS?

Compile-to-JavaScript languages come and go, but JavaScript has remained. First learning vanilla JavaScript makes sense, and then add TS on top if you really have to. At the very least they'll be prepared for when TypeScript goes out of favor.

TypeScript isn't really a "compile-to-JavaScript" language in the same way that, say, Rescript is. Modulo a few corner cases (e.g. enums) which are now considered anti-features, TypeScript has the exact same runtime semantics as JavaScript - you can (almost) convert TypeScript to JavaScript just by stripping away the type annotations (and, if various ECMAScript proposals go through, you won't even need to do even that).

This is both a curse and the secret behind its success - it's arguably not a separate language, but instead an annotation layer on top of the existing language for encoding and checking your static reasoning and assumptions.

I guess by that same definition, Coffeescript isn't a compile-to-JavaScript language either, as it has the same semantics as vanilla JavaScript?

I think that's besides the point. My point was more that people who are supposed to learn JavaScript, should do so by learning vanilla JavaScript first, then they can move on to learning whatever is currently hyped by the zeitgeist (which happens to be TypeScript currently).

No, CoffeeScript doesn't have the same semantics as vanilla JavaScript; not in any meaningful sense I can think of, certainly not in the way that TypeScript does.

Most CoffeeScript will simply syntax error if you feed it into a JS interpreter (and vice versa), and there's no trivial syntactic transform to get around that (i.e. it's not just a new surface syntax over JavaScript). Various features in CoffeeScript have no equivalent in JS, so new code needs to be synthesized for them; even fundamental features that are shared by the two languages are different - for just one example, this [0] article shows that how a function is compiled in CoffeeScript is non-local - the compiler is tracking variable scopes to get around the fact that CoffeeScript and JavaScript have different scoping semantics.

With TypeScript, the "compilation" process is (almost):

  parse(typeScriptCode) -> treeMap(removeTypeAnnotation) -> write
That's it! TypeScript code is syntactically valid JavaScript code, just with added type annotations (and, as mentioned, soon that'll still count as syntactically valid JavaScript code); JavaScript code is syntactically valid TypeScript code without any type annotations - which doesn't make it syntactically invalid! Indeed, if you turn down the strictness settings on tsc to permit untyped code, the compiler doesn't even complain about it.

[0] https://donatstudios.com/CoffeeScript-Madness

CoffeeScript has quite a lot of code generation features, while TypeScript largely just removes code in order to transpile to JavaScript. The only exception I can think of is enums, and I suspect they wouldn't have put those in if they had it all to do over again.

Which further exposes the irony of casual hipsters who say things to me like "Thank God for Typescript, it's like a whole other language than JS". For example, a designer, who writes a quick one off plug-in for Figma in Typescript without realizing 99.9% could have been just written/pasted into the JS file (which is ultimately what Figma runs).

To be honest, in the appropriate context I will contradict myself and say that that you should think of TypeScript as a separate language. There's a lot of dynamic things you can do in JavaScript that will interact poorly with attempts to statically reason about your code with TypeScript; even if it's the approach you'd take in JS, when you add typing you should recalibrate how you approach the problem - that, to me, is an argument for thinking of it as separate language.

...but that's mostly just what I say to JavaScript devs, to be polite - to not tell them that their JavaScript code is bad, too. Code that's difficult to reason about statically is strictly worse, in my opinion, than code that's easier to reason about statically. And in that sense, TypeScript is just guiding you to write better JavaScript, which maybe undercuts its claim to being a language of its own.

Because TypeScript is the de facto standard way of writing JavaScript for most of the industry and it has killed all of the other compile-to-js languages.

The chances of it going out of favour are very slim, there's a giant ecosystem built on it and the language is very well loved by devs (should be second only to Rust).

Microsoft has 50 people on payroll working only on TS. Any competitor needs a gargantuan investment.

> Because TypeScript is the de facto standard way of writing JavaScript for most of the industry

It really isn't, but I guess that's really context specific. What country and sector are you talking about? For US-SaaS, what you're saying is probably true, but there is a whole world outside of that, and JavaScript is with 99% certainty much more wildly used than TypeScript.

> and it has killed all of the other compile-to-js languages.

Also it hasn't. I've been writing ClojureScript for the last 5 years, almost exclusively. And while the community is small, I wouldn't say it's "dead" or been killed. There are a bunch of compile-to-JS languages that haven't "been killed", besides ClojureScript.

But it serves basically the opposite niche compared to TypeScript.

> The chances of it going out of favour are very slim

Same has been said about everything, always, and it's always not true. Winds change, and surely so shall the winds of TypeScript. Not being able to see that it's possible, will put you at disadvantage.

Technically Haxe is still a thing, though its typical use case these days is almost 100% game development (including the JS target).

And JavaScript has a bigger ecosystem and more entrenchment, not to mention an international standard.

Not sure how's that relevant.

Any valid JavaScript is valid TypeScript and you're gonna write and read TS anyway in the industry.

They aren't in competition, but coming back to the first comment it sounds reasonable to start directly with TS for many users.

Your experience is not the industry. JS is widely used, almost certainly more than TS.

My experience may not, but the annual JS dev survey points that TS is the main way to write JS from years.

and not to mention terrible.

I use TS, I like it.

I'm not sure you're 100% right about it overtaking JS for most of the industry.

I wouldn't recommend it to beginners until they've learned JS because it's a lot of stuff to learn on top of JS to achieve basically the same outcomes (with fewer bugs). Chapter 5 in the Eloquent JavaScript book gets to higher order functions, which in TS means Generics. Nobody needs to learn Generics in Chapter 5 of their programming journey.

You also don't really appreciate how useful TS is until you've battled at least one project in plain JS.

(Arguably you can go a long way without HoF too, but perhaps not if you're hoping to understand other people's code.)

Flow has about a dozen of important features that typescript lacks (while being 10x smaller in terms of LoC).

Flow is basically a dead project outside of Facebook.


Not that I think download metrics is the best metric to decide that, but even with that, flow-bin has almost half a million of downloads per day. That's far away from dead, at least in my world. And I'm guessing that doesn't count anything from Facebook as they most likely run their own registries.

flow-bin has substantially fewer downloads than coffeescript and both are declining. That is a dead project.


It's not as popular as typescript but not dead, it's consistently active for a decade [0].

Compare it with typescript [1] contributions if you want.

[0] https://github.com/facebook/flow/graphs/contributors

[1] https://github.com/microsoft/TypeScript/graphs/contributors

Don't you find it interesting that node-fetch is slightly less-dead otherwise has exactly the same "aliveness" as typescript [0] from your source (click 5 years)?

[0] https://npmtrends.com/node-fetch-vs-typescript

I don't want them to "learn JS." Most likely the person in question is trying to write web applications, and I want them to learn what is most useful for that. After writing web applications for years without TypeScript and then for years with it, I cannot imagine going back. I can make complex refactors with very little worry. Thanks to type inference, most of the TypeScript code I write is indistinguishable from JavaScript, so the idea that learning TS takes away from learning JS is ridiculous. When I suggested that the extra syntax might be distracting at first, I meant for like... a week.

It's better if they learn the core language and then graduate to TypeScript (if at all, as there's active discussion of adding type annotations to the core language [1]).

An anecdote: I was sold a similar story on CoffeeScript back in the day, stressed myself out learning it, only to discard it a couple years later. TS won't have an identical fate as its shepherded by Microsoft, but eventually it will go the way of the dodo (whereas core JS will still be humming along).

[1] https://youtu.be/SdV9Xy0E4CM?feature=shared&t=380

This was the approach one of my first mentors recommended and it's served me well. Learn the core language and its features and then add tools/frameworks as needed. Lately it feels like we're teaching new devs popular frameworks and completely ignoring the fundamentals.

Yep. But business requirements dictate what happens and these frameworks are the fastest way to achieve business goals, which sucks as when the frameworks lose their appeal, those devs are gonna struggle to transfer their skills to something else.

It is useful to know vanilla JS. Right now I am learning an integration platform that uses JS for its scripting language, but you cannot use TS.

ts-node with ts can do script the same as node with js? surprised why TS can not be used as it's 'safer' for scripting.

Serious question - has Javascript the language evolved any features for handling integer math correctly so expressions like 2*57 aren't silently rounded? I realize that in most cases Javascriopt is "good enough" for useful work but I've seen even senior engineers get tripped up by Javascript's corner cases when it comes to its handling of integer vs float.

There's BigInt [0]. Probably less useful for this specific case, but possibly useful as well, are typed integer arrays [1].

[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guid...

Ah, very cool. Thanks.

> handling integer math correctly

JavaScript doesn't have integers. Everything is a float. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

Although it has BigInt now.

Yes, I'm aware that internally it is using floating point representation and that is one of the main reasons I avoid using it for anything math related. However, I am now aware that BigInt() exists so thanks for the link.


Sounds like a joke, an oxymoron.

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