Sunday, March 30, 2008

js2-mode: a new JavaScript mode for Emacs

I've written a new JavaScript editing mode for GNU Emacs, and released it on

This is part of a larger project, in progress, to permit writing Emacs extensions in JavaScript instead of Emacs-Lisp. Lest ye judge: hey, some people swing that way. The larger project is well underway, but probably won't be out until late summer or early fall.

My new editing mode is called js2-mode, because eventually I plan to support JavaScript 2, also known as ECMAScript Edition 4. Currently, however, it only supports up through JavaScript 1.7, so the name is something of a misnomer for now.

It's almost ten thousand lines of elisp (just for the editing mode), which is more than I'd expected. So I figured I'd tell you a little about what it does, why I made certain choices, and what's coming up next. Even if you're not a JavaScript user, you might find the technical discussion mildly interesting.


In no particular order, here's what js2-mode currently supports.

M-x customize

All the user-configurable variables are defined as Custom variables for use with M-x customize. This means you can type

M-x customize-group RET js2-mode RET

to see a list of all the configuration options.

All the colors used for syntax highlighting are defined in the same js2-mode customization group, for convenience.

Many people complain that Emacs's Customize feature is lame. I thought so too, for a long time, and I'm certainly not claiming it's as good as a "real" UI. But I now appreciate that it gets the job done admirably for a text editor: there are no dependencies on GUI widgets, and you can use the Customize package over an ssh or telnet session. That's at least kind of cool.

Accurate syntax highlighting

This mode includes a recursive-descent parser that I ported from Mozilla Rhino. That means it's always right. It doesn't use heuristics or guesswork; it's exactly the same parser used by JavaScript engines. If it's ever wrong, then it's a bug in my code, and it's fixable.

The amount of syntax highlighting is configured by a variable called js2-highlight-level. It ranges from 0 to 3, with the default set at 2. Zero (or nil/NaN) means no highlighting. level 1 does basic syntax highlighting: keywords and declarations. Level 2 adds highlighting for Ecma-262 builtins (and SpiderMonkey extensions) such as Infinity, __proto__ and decodeURIComponent. Level 3 adds highlighting for all built-in functions and properties for all native JavaScript objects (Function, Date, Array and so on.)

The highlighting faces are my own choices, because I felt it was important for me to foist my personal style choices on the general public. Actually, that's only partly true: it's also because I like the color schemes employed by Eclipse and IntelliJ better than the default Emacs color scheme. So comments are green (not red!), keywords are blue, strings are soft blue, var decls are sea green, and so forth.

Fortunately for those (hopefully few) among you who love blood-red comments, you can add this to your .emacs file to make js2-mode honor your font-lock settings:
(setq js2-use-font-lock-faces t)

Or, alternately, M-x customize-variable RET js2-use-font-lock-faces RET and set the value to t, which is Lisp for "true". This particular variable requires an Emacs restart to take effect.

I just added a TODO item for myself: define Eclipse and IntelliJ color schemes that you can choose from. Should be pretty easy.

Asynchronous highlighting

Unlike most other Emacs modes (but like nXML mode), js2-mode does not use font-lock-mode, which is the standard Emacs infrastructure for doing syntax-coloring in buffers.

Although font-lock is quite fast and fairly flexible, it still uses heuristics to figure out what to highlight, and they can occasionally be wrong. If you've ever opened up prototype.js in Emacs and seen the second half of the file turned string-blue on account of being confused over a regular expression literal with a quote in it, you know what I'm talking about.

James Clark's nXML-mode does its own syntax coloring without using font-lock. It can do this because James wrote his own, fully-compliant, validating XML parser, so adding colors was a snap. I thought this was pretty macho, and since I also happen to have a full parser, I blatantly copied his idea.

For the OOD-loving and API-minded among you, the "beautiful" way to do syntax coloring would have been to finish parsing, then walk the AST using a Visitor interface, applying the coloring in a second pass. I tried it, and it was, as they say, "butt slow". In fact (perhaps not surprisingly) walking the AST takes exactly as long as parsing, so it was twice as slow as doing it inline.

So I bit the bullet and moved my syntax-coloring to happen inline with parsing. Fortunately it only introduced about 30 lines of code to the 4000-line parser/scanner, because most of the coloring happens in the scanner, at the token level. Go figure.

Unfortunately, my parser is asynchronous. It "sort of" happens on another thread, although what's really happening is that it waits for Emacs to become idle and parses until you hit a key or use the mouse. I wanted it to be synchronous, boy howdy I did, but it just wasn't quite fast enough. It can parse about 5000 lines a second, give or take, but for any file longer than 1000 lines or so, the parsing was happening every time you typed a key (that's what synchronous means, obviously), and the 0.2+ second delay became painfully noticeable.

I had two options: incremental parsing, or asynchrous parsing. Clearly, since I'm a badass programmer who can't recognize my own incompetence, I chose to do incremental parsing. I mentioned this plan a few months ago to Brendan Eich, who said: "Let me know how the incremental parsing goes." Brendan is an amazingly polite guy, so at the time I didn't realize this was a code-phrase for: "Let me know when you give up on it, loser."

The basic idea behind incremental parsing (at least, my version of it) was that I already have these little functions that know how to parse functions, statements, try-statements, for-statements, expressions, plus-expressions, and so on down the line. That's how a recursive-descent parser works. So I figured I'd use heuristics to back up to some coarse level of granularity — say, the current enclosing function – and parse exactly one function. Then I'd splice the generated syntax tree fragment into my main AST, and go through all the function's siblings and update their start-positions.

Seems easy enough, right? Especially since I wasn't doing full-blown incremental parsing: I was just doing it at the function level. Well, it's not easy. It's "nontrivial", a word they use in academia whenever they're talking about the Halting Problem or problems of equivalent decidability. Actually it's quite doable, but it's a huge amount of work that I finally gave up on after a couple of weeks of effort. There are just too many edge-cases to worry about. And I had this nagging fear that even if I got it working, it would totally break down if you had a 5,000 line function, so I was kinda wasting my time anyway.

So, without telling Brendan (and don't you dare mention it to him), I switched to asynchronous parsing. Actually, first I went around to my Eclipse- and IntelliJ-using friends, and I forced them to give me live demonstrations of Java editing on large files. This is why I have so few friends. It turns out that Eclipse and IntelliJ both use asynchronous parsing as well, which made me feel better about the basic approach.

Asynchronous parsing is pretty simple in principle: when the user is typing, don't do anything. Just let 'em type. When they stop, start a timer for, say, a 200 to 500 millisecond delay, and when the timer expires, start parsing. Every once in a while, see if they typed anything. If so, stop parsing and let them type.

The main downside of this approach is that for some programmers, the 500-ms timer fires between every keystroke, so the file never actually finishes parsing. (Yes, that was a mean joke. I have a blog on this subject coming up; I'm declaring war on people too lazy to learn to type.)

Actually, now that I think about it, I did mention my change of heart (and asynchronous approach) to Brendan a week or two ago, and he jumped immediately to the smart-guy conclusion: I need continuations. Fortunately I'd thought of this, albeit not in the 200 milliseconds it took him to arrive at that conclusion (over wine, no less!), so I was able to retort: "um, yeah... it's in my to-do list. Right now I hack it."

And hack it I do! I rely on the fact that my parser does 5000 lines a second, so if the parse gets interrupted, at some point even the fastest, most dedicated typist will have to pause for a second, and I'll finish the parse (which in turn finishes the highlighting and error/warning reporting – see below).

Unfortunately (as Brendan instantaneously concluded), this means that if the parse gets 99.9% complete, and you hit the up-arrow, it abandons the entire parse (and parse tree built so far), starting from scratch again when Emacs goes idle. So if you open a big file (like prototype.js) and start navigating around it, you may not see any results until you stop typing or scrolling.

The proper fix will be to record where I'm at, and pick up where I left off when I restart the parse. That's what nxml-mode does, but I'm forced to concede that James Clark is way cooler than I am. If you have a multi-threaded system, then it's trivial, and if your system supports continuations, it's also trivial. But Emacs has neither of these.

Instead, I pause every 100 statements or so (this is a lame heuristic, I agree) and check for user input. Since I'm pausing at the top level of the parser, in the loop where it consumes whole statements, I really don't need to store that much information to fake a continuation, so this problem is eminently fixable.

But I had to release this thing eventually, which meant drawing the line somewhere. So for now it has asynchronous full-restart parsing. This means that as you edit the file, just like in Eclipse and IntelliJ, it can take a second or two for the parser to catch up with you after you pause.

It doesn't (or shouldn't) interfere with your editing, though, so hopefully this isn't a big issue.

Missing highlighting

It's on my js2-mode TODO list to highlight E4X literals. E4X is a JavaScript language extension (an official Ecma standard, in fact) that allows you to embed XML literals in your JavaScript code and provides various XML operators and functions that let you do DOM-style manipulations and XPath-style queries, but with JavaScript-style syntax and semantics.

I parse these properly, but don't highlight them yet. The Rhino parser just parses them as strings, so to get more accuracy I'll need to make my own little XML parser. It must (I think) be my own little parser because E4X permits embedding arbitrary JavaScript expressions in curly-braces as a form of templating. This complicates the XML parsing because you can find one or more {javascript-expr} expressions in the middle of any XML element name, attribute name, attribute value, text node, or just about anywhere else that doesn't cross a quote or angle-bracket boundary.

I'll get around to it eventually.


I would have been publishing this article at least a month ago if it weren't for indentation. No, six weeks, mininum.

See, I thought that since I had gone to all the thousands of lines of effort to produce a strongly-typed AST (abstract syntax tree) for JavaScript, it should therefore be really easy to do indentation. The AST tells me exactly what the syntax is at any given point in the buffer, so how hard could it be?

It turns out to be, oh, about fifty times harder than incremental parsing. Surprise!

Just to give you a feel for the size of the problem, the package cc-engine (including its cc-* helper packages) bundled with GNU Emacs 22 is approximately 27,000 lines of lisp code, and it's all dedicated to indentation. There's a teeny tiny smattering of maybe 500 lines dedicated to filling, and sure, it supports several C-like languages, but let's face it: 25k lines for indenting? 27k lines of Lisp code? (Meaning it would be, like, five times that much Java?)

What the hell is so hard about indentation?

For starters, in order to provide user-configurable indentation for every possible syntactic context, you need to name all the syntactic contexts. cc-engine defines about 70 syntactic positions in a data structure called c-offsets-alist. This is a map of {context-name : indent-level}, where indent-level can be a number (a multiple of the variable c-basic-offset), or a symbol specifying some multiple of c-basic-offset, or even a function to call to figure out how to indent.

It's pretty darn flexible. And people still complain about it! Apparently 70 different syntactic contexts isn't enough to let you specify your indentation exactly the way you like it.

Anyhoo, most existing JavaScript editing modes for Emacs use cc-engine and try to coerce it into indenting JavaScript properly. This usually meets with lackluster results, since JavaScript is gradually drifting further and further from C. So is Java, but someone actually bothers to try to keep cc-engine up to date for Java.

Here's the deal: the cc-engine code for interpreting that c-offsets-alist data structure (with all the indentation configuration options) is pretty small. Most of the code goes to parsing and trying to figure out the current syntactic context.

You can probably guess what I tried to do. I wanted to let people customize their js2-mode indentation much the same way they can customize their c-mode or java-mode indentation, using c-offsets-alist. So I figured I'd use the exact same configuration data structure, and use my parse tree to replace c-guess-basic-syntax (and the 25k lines of lisp code for implementing it!)

(time passes...)

Approximately one month later, I threw in the towel. I renamed my js2-indent.el to doomed-indent.el, and my js2-indent-test.el unit-test file to doomed-indent-test.el, and gave up on this approach for the forseeable future. 1500 lines of painfully crafted lisp code down the drain.

Ugh. Sure, it was only a few hours a week, but it still felt like a lot of work. And it was a lot of calendar time.

Amazingly, surprisingly, counterintuitively, the indentation problem is almost totally orthogonal to parsing and syntax validation. I'd never have guessed it. But for indentation you care about totally different things that don't matter at all to parsers. Say you have a JavaScript argument list: it's just (blah, blah, blah): a paren-delimited, comma-separated, possibly empty list of identifiers. Parsing that is pretty easy. But for indentation purposes, that list is rife with possibility! You might want to indent it like this:
blah, blah)

or this:

or this:

or this:
blah, blah)

Let's face it: you could be a total lunatic, and Emacs has to make you happy. So instead of simply parsing a plain argument list, you need to determine and capture the (a) the fact that it looks like an argument list, (b) the position and indentation of the open-paren, (c) whether the cursor is before or after the open-paren, (d) whether the arg list is nonempty, (e) whether the cursor is before the first list element, (f) whether the cursor is on the line containing the closing paren, (g) whether there are any block or single-line comments interspersed between any of the list elements or parentheses, (h) whether the AAAAUGH, I can't stand it anymore!

The problem is, this explosion of "one case to arbitrarily many cases" occurs for every single grammatical construct in your language. So if you have 70-ish such constructs (as JavaScript does - Java has almost double that, because of the type system), and each one expands to 5 to 10 possible indentation situations, well, you've got an awful lot of edge cases to deal with.

Worse, having a rich AST doesn't help you much. You can figure out that it's an argument list, and possibly where the cursor is in the list, but you still have to grope around in the buffer looking for other contextual cues that matter for indentation but which the parser threw away. So each syntactic case in the 700-odd scenarios I had to handle expanded to anywhere from 2 to 10 lines of lisp code.

I was about 1500 lines into my doomed-indent.el (plus unit tests), and maybe (optimistically) 35% finished, when it occurred to me: "is there a better way?"

Karl Angalsdkjfadslkfj to the rescue

I remembered that there are several javascript editing modes out there already, and none of them does a very good job (or I wouldn't be working on js2-mode). But one of them, "javascript.el", I remembered as being pretty good at indentation. It wasn't perfect, and I'd had to write some custom hacks for it here and there, but it was actually pretty decent. How did it work?

I went and looked at it. It's written by a guy named, according to the comment header, Karl Landström. I'd always assumed that this was just some my-font-doesn't-support-Unicode gobbledygook, and that his name was actually something more reasonable like Karl Landstr\301^HB^P\302\301!\204^0^@. But upon closer inspection, I think he may be a fan of the artist formerly known as the artist formerly known as Prince, aka "Prince", because the "ö" in his name shows up pretty consistently across platforms and fonts. So it may be intentional. Perhaps his parents were ardent mathematicians.

In any case, Karl Landstrlaksjdflaksjd is an amazingly clever guy, because his indenter, which beats the pants off all the JavaScript modes based on cc-engine, is only about 200 lines of elisp. 200!? How does he do it?

Well, in a nutshell, he makes the inspired assumption that indentation is almost always a function of brace/paren/curly nesting level, and he uses a little-known built-in Emacs function called parse-partial-sexp, written in C, which tells you the current nesting level of not only braces, parens and curlies, but also of c-style block comments, and whether you're inside a single- or double-quoted string. How useful! Good thing JavaScript uses C-like syntax, or that function would have been far less relevant.

The rest of his code handles cases where you have a JavaScript keyword such as if, while or finally (a "possibly braceless keyword"), where you can optionally leave off the curly-brace, and it should still indent one basic step for the nested statement.

The results are actually pretty darn good, and assuming you're reasonably flexible about where you position your parens and curly-braces, you can exert at least some control over the indentation. (E.g. you can move a curly down to its own line and manually indent it, and subsequent lines will indent from that curly.)

Go Karl!

Unfortunately, it's not perfect (no solution so elegant could ever be, at least for a language based on the inelegant syntax of C), so I was faced with a dilemma: should I pile hack upon hack until it becomes the new cc-engine? Or is there another way?

Well, I've always been vaguely admiring of python-mode's Emacs indentation, which chooses among various likely indentation points when you press TAB repeatedly. Why not use that approach for JavaScript?

So that's what I wound up doing. I put a few tweaks into Karl's original indenter code to handle JavaScript 1.7 situations such as array comprehensions, and then wrote a "bounce indenter" that cycles among N precomputed indentation points.

For any given line, there are some obvious possible indentation points:

- whatever position Karl's guesser wants to use
- the beginning of the line
- after the '=' if the previous line is an assignment
- same indentation as the previous line
- first preceding line with less indentation than the preceding line

I wrote a function that computes all these positions, based on heuristic parsing (NOT on my AST, which might not even be available yet if the parse is taking a while), and the TAB key cycles among them.

This moved the accuracy, at least for my own JavaScript editing, from 90% accurate with Karl's mode up to 99.x% accurate, assuming you're willing to hit TAB multiple times for certain syntactic contexts.

There are still plenty of user-defined situations (e.g. parts of Google's internal JavaScript style guide) that my guesser doesn't compute. You don't want to compute every possible indentation point, or the TAB key degenerates into the space key modulo the line length, so at some point I'll add a customization hook that lets you write a function to help decide the right indentation.

Anyway, where was I. Oh yeah. Indentation is a real pain in the b-hind. I'm glad to be (mostly) done with it. At least hopefully you now understand why my mode isn't configurable the way other C-like modes are, and you sympathize with me. Next time I have time to write ten thousand lines of indentation-related guessing, I'll fix it.

Meanwhile, if you find points where it doesn't do what you want, let me know (or post them on the Wiki), and I'll either hack them in or write that customization hook.

Other Stuff

I didn't expect to spend so long on just syntax highlighting and indentation. It's just the beginning! Unfortunately I'm out of patience, and I'm guessing you are too. So here's a short list of other features.

Code folding

I support hiding function bodies and /*...*/ block comments as {...}. It's in the menu. Turn on menu-bar-mode, or right-click in the buffer, to invoke these functions.

At some point I'll generalize it to hiding any curly-brace construct, the way Eclipse does. This was just an experiment to see how easy it would be. (Answer: pretty easy! Emacs has good built-in support for this kind of thing.)

Comment and string filling

One neat trick I stole from Eclipse: if you hit <Enter> inside a string literal, it will autoamtically turn it into a multi-line string concatenation.

You can also hit Alt-q (fill-paragraph) inside a comment or a string to see hopefully useful things happen. Let me know if it doesn't do what you expect.

Syntax errors

The mode highlights syntax errors in red. This can be annoying as you type, but I'm told (by Eclipse/IntelliJ users) that you get used to it.

You can control this behavior via a customization variable.

Strict warnings

JavaScript defines a whole bunch of strict-mode warnings: things like "don't have a trailing comma in an Array or Object literal", or "your variable name conflicts with one of the function parameters". I've implemented some of them, with more to come. They get underlined in orange.

I actually found some bugs in live code I'd written with this feature. Pretty cool!

jsdoc highlighting

There's a program similar to javadoc called "jsdoc" that lets you do documentation comments for your JavaScript functions and other declarations. It defines a similar set of @whatever tags. We use it at Google, albeit with limited success because it's a Perl program that core-dumps on most of our JavaScript code base. My mode highlights the various tags in jsdoc comments, if you happen to use them.

It's possible that you'll notice it highlighting curly-brace constructs inside jsdoc comments, such as:
@return {SomeType} my return value

Googler Bob Jervis has written a type-inferencing engine for our JSCompiler, in his 20% time, that uses the type-tags we've defined in an enhanced version of jsdoc comments. It's still pretty new, and we're planning to open-source it and integrate it with Mozilla Rhino at some point, but since it's 20% time, there's no telling when it'll be released. But hopefully that'll explain the bizarre highlighting you might sometimes see.

If this isn't good enough for you...

Well, you have three options.

First, you can whine about it. If you whine in the appropriate places, such as the Wiki, then I'll eventually notice and try to fix whatever it is that's bothering you.

Next, you can offer to help. I haven't uploaded the original source code, but I can certainly start doing so. (The file js2-<datestamp>.el is generated from a little build script I wrote, to make installation easier.) If you're a good Emacs-Lisp programmer, and you want to help make this mode better, let me know and we can get you hooked up!

Finally, if you can afford it (or if your company can afford it), consider using IntellIJ IDEA. Yes, it's commercial, but if you spend even 30 seconds on their site it becomes apparent that "commercial" means "better". Their JavaScript support is way better than mine, and is as far as I can tell the gold standard for JavaScript editing today.

Eventually I hope to be able to reach feature parity with IntelliJ, and it's certainly possible, but it'll be some work. In the meantime, if you can't wait, give them a try!


At this point I have to go to the bathroom so bad that I don't care what other features I've added. You can look at the Wiki!

If you habitually (or even occasionally) use GNU Emacs to edit JavaScript, please give this mode a try! It's probably got a fair number of bugs and usability issues, since it's brand-new, but it'll improve more quickly if you play guinea pig for a while.

Feel free to email me directly with comments, suggestions, or bug reports, or you can go to the Wiki and add your comments there.



Blogger Natalie said...

My javascript.el/configuration/font combination has the author as "Karl Landström" (that's an o with an umlaut).

This looks very cool. Next time I have some Javascript hacking to do, I'll definitely grab it.

10:32 PM, March 30, 2008  
Blogger Unknown said...

I would be interested in seeing the source code as an example of how a competent coder would do unit testing in emacs. Hence this nudge to try and persuade you to upload it.

12:46 AM, March 31, 2008  
Blogger Wesley Walser said...

If only it weren't so late...

I will be playing with this tomorrow, looking forward to it.

1:17 AM, March 31, 2008  
Blogger Baishampayan said...

Highlighting still breaks when I try to open JQuery 1.2.3 (uncompressed).

Nevertheless, pretty nifty.

1:45 AM, March 31, 2008  
Blogger Adam said...

This sounds great and I'm definitely going to be your guinea pig!

For some reason my formatting conventions are really whacky ATM so hopefully I can be a real pain.

2:03 AM, March 31, 2008  
Blogger Unknown said...

James Gosling (one of the authors of emacs) has the following to say: "Emacs was a really good idea in 1978. That was like 30 years ago. Stop using it!",339028020,4000198303o,00.htm

Power to ya Stevey.

4:12 AM, March 31, 2008  
Blogger jrockway said...

Interestingly, I am doing something similar for Perl (using PPI, a Perl-parser written in Perl). Anyway, to avoid porting PPI to Lisp, I do the highlighting in an external process and send the offsets and face names back to emacs. This is nice because it doesn't block emacs at all, although I admit it's sad to see your syntax highlighter using 100% CPU while you're typing in your code. (Right now, the naive interface doesn't cancel requests when a new one comes in, so you end up highlighting "f" "fo" "foo", ... when you type "foo". That can be worked around though.)

Anyway, it would be interesting to see the your javascript syntax highlighting happen in an external process.

*sigh* I miss the days when font-lock could efficiently handle all the popular languages :)

My code is at:;a=summary

4:36 AM, March 31, 2008  
Blogger Unknown said...

Very nice piece of software, indeed.

I see in the comments (in the code, not here) that you're planning to support script tags inside HTML. I hope you give a nice priority to that.

@julio: don't pay attention to Gosling, he's getting old.

4:47 AM, March 31, 2008  
Blogger AriT93 said...

Thank You Steve!

Have you considered subimitting this for inclusion to emacs? I don't know how google would treat the copyright assignment or if they would allow it at all. I think it would be really great if this came stock with emacs!

6:03 AM, March 31, 2008  
Blogger Aemon Cannon said...

Hey, interesting post.

On parsing languages in emacs, I wonder if you also considered handing the parse off to an external process? My language mode helper, flyparse-mode, works this way (similiar to what jrockway describes above, actually):

* On idle, if the current buffer is dirty, it gets written to a file.

* An external ANTLR parser is invoked on that file. The output of the parser, a sexp-encoded AST, is written to another temporary file.

* I use emacs's (load .. ) to load the AST (this is the fastest way I could figure how to get it out of the parser, into emacs).

* Once the AST is in emacs, language mode helper functions can query it for useful information. The tree includes buffer-offset information so cursor positions can easily be translated to logical positions.

* The buffer-offsets are stored as relative offsets, on the nodes of the tree, so that between parses the tree is easily (and inexpensively) kept up-to-date with the current state of the buffer.

This all works well in practice, with no noticeable delays. I haven't tried to use it for syntax highlighting, though.

6:22 AM, March 31, 2008  
Blogger Marijn said...

Concerning keeping parsing and highlighting separate, and doing parsing asynchronously, you might find CodeMirror's approach interesting.

6:30 AM, March 31, 2008  
Blogger Jarosław Rzeszótko said...

Two things:

1. I would really like it to highlight some minor errors like those JSLint detects, ie. missing the end semicolon in "var x = function() { doSomeThing(); };" Things like that break the browser after you concatenate all your javascripts together and minify it.

2. The indentation works badly in one particular case, when mixing hashes and functions etc. Try to construct an Ajax.Request with a hash of options, and defining an onComplete handler inside of the hash - you will end up with the whole hash indented far to the right, where it would be much more reasonable for it to be indented only one level more than the base "new Ajax.Request" statement.

6:41 AM, March 31, 2008  
Blogger Barry Kelly said...

Not to be snarky, but 5000 lines/sec... what are you running that on, a 90MHz Pentium? Or is emacs elisp really that bad for performance? (I guess it must be.)

That deeply sucks. For one thing, you don't need to create a full AST for syntax highlighting. For basic lexical highlighting, a lexer that can track its line starting states will do; for semantic markup, such as highlighting methods, classes, etc., and providing some kind of code insight / intellisense, you do need to parse declarations, but you can largely skip tokens between '{' and '}' at the top level.

To be frank, though, if elisp could only do 5000 lines/sec for highlighting, I think I'd rather write my own editor & macro system. It would be less depressing.

6:51 AM, March 31, 2008  
Blogger Manc Trader said...

Great blog Steve, I have been reading all your post for the last few years this past month.
I am not programmer but a trader. I was a cs/ee geek who loves LISP.
I always install emacs at the workplace to the chagrin of my colleagues.
Keep up the informative posts. If i get a chance to learn some javascript for kicks i'll give your mode a try.

7:03 AM, March 31, 2008  
Blogger Steven Atkinson said...


I loaded it up this morning with a javascript file swimming in regexs. Every other extension I threw at it would barf.

So writing a mode which actually worked was at the bottom of my todo list. Until now.

Maybe you should consider hosting it with git (or mercurial). Getting your users hacking on it may speed up development.

8:38 AM, March 31, 2008  
Blogger Dan Shoutis said...

Hi Steve!

I just skipped to the bottom here when you started talking about continuations and the like. Writing recursive-descent parsers that use continuations is not actually that much work if you have a language w/ higher-order functions and closures -- the trick is to use a combinator library for parsing rather than writing it out by hand.

The really sweet thing about this approach is that you add features (e.g., incremental parsing) to your parser by adding them to your combinators: the parser definition itself remains the same!

8:52 AM, March 31, 2008  
Blogger Unknown said...

You don't have to restart the parser if it hasn't reached where you're changing things. Heck, you don't even have to stop the parser until it reaches that point.

9:11 AM, March 31, 2008  
Anonymous Anonymous said...

Stevey, that you are using an obsolete Latin-1 encoding is not my problem ;-) Guess you're on windows, right? Calling myself Karl Landstrom may still be the only safe alternative... but it feels so 20th century.

10:11 AM, March 31, 2008  
Blogger Natalie said...

I second Steve Atkinson's git suggestion. I'd love to be able to clone this and hack on it.

10:14 AM, March 31, 2008  
Blogger izaak said...

You've got an alert in your page that prints "you fixed me big boy". thought you might like to know about

Firebug finds it in your scribat around this:
this.i[uid] = i; this.q(); alert('you fixed me big boy')},

Hope this helps

10:39 AM, March 31, 2008  
Blogger Deepak said...

Thanks Steve! This is very useful.

I currently use a mishmash of ECB (Emacs Code Browser), Senator and the Javascript mode that comes out of the box with Aquaemacs to emulate an IDE, and it works pretty well. Your mode is definitely more interesting than the default but for some reasons doesn't play nicely with ECB. For example it doesn't show a list of variables in the ECB's Methods window and shows all the methods as collapsed by default. Is integration with ECB something you'll be looking at at all?

10:51 AM, March 31, 2008  
Blogger Steve Yegge said...

Thanks all - this is why I released it early; I wanted a new stream of bug reports and feature requests.

Baishampayan - I'll look at JQuery.

AriT93 - I think it's still a long way (stability-wise, and integration-wise) from inclusion in Emacs. Maybe in a year or two.

sztywny - I'll add in a missing-semicolon warning asap.

barry kelly - it can't be as bad as you think (or perhaps as bad as I think), since my parser has the same responsiveness as the ones in Eclipse and IntelliJ, and they're written in Java. They don't do it synchronously either.

Karl - sorry, and I'll make a note of the proper spelling. :) And thanks for the indenter!

James Gosling - Java was a great idea fifteen years ago...

11:33 AM, March 31, 2008  
Blogger Marcus Frödin said...

Hey! Nice. As a Swede, I well sympathize with Karl Landström. For some reason, that bastardization happens every now and then.

Anyway, his URL is


12:16 PM, March 31, 2008  
Blogger Eric Ludlam said...

A note about the ECB comment. ECB uses either the CEDET/Semantic javascript parser, or the imenu parser.

ECB support could be handled by changing the configuration for contrib/wisent-javascript.el in CEDET to point at the new js2 mode hook. This may not work if the syntax table is too different though.

Alternately, this javascript mode could probably generate a very nice set of CEDET/Semantic tags which could be used instead, thus enabling local context parsing and smart completion via the semantic APIS.

Also, a note on the incremental parser discussion in the blog post. Semantic handles incremental parsing by chopping up the buffer under overlays in the first pass. Incremental parses group individual changes under the overlays, reparses those overalys, and splices the results back to gether. It handles most cases well, and is fast even in big buffers.

12:31 PM, March 31, 2008  
Blogger Andrei Popov said...

Since you brought up prototype.js early in the post -- are there plans to add bla = function() {...} parsing to IM-JavaScript-IDE scanning? Otherwise a 2000+ lines of Prototype seem to define only 2 functions ($H and $).

1:24 PM, March 31, 2008  
Blogger Stuart Hacking said...

Wouldn't it be better to use the default emacs highlight faces as far as possible? One of the many things I like about emacs is consistency and having the same highlighting scheme applied to all languages is nice. (and less variables to change if I want to adjust it)

1:32 PM, March 31, 2008  
Blogger Phil said...

+1 for distributed version control; I would love to clone your repo.

I'm quite curious as to how you're handling unit tests. I've written a handful of unit testing frameworks in elisp (for my own sadly deprecated rhtml-mode), and the language seems to lend itself to a very different testing approach from your classic xUnit style.

I also haven't used any of my frameworks on large elisp projects, so I'm sure making it useful on that level involves its own set of challenges.

1:34 PM, March 31, 2008  
Blogger Michael R. Head said...

Hmph... you keep pimping for ECMAScript. I came of age before AJAX and Web 2.0 and as such have been prejudiced against that language. I may to look into it, especially if there's some decent elisp for it.

2:50 PM, March 31, 2008  
Blogger Doug Orleans said...

Doesn't seem to work with XEmacs. Is XEmacs dying? Should I just give in and switch back to GNU Emacs? After 15 years?

4:12 PM, March 31, 2008  
Blogger Steve Yegge said...

It would probably be better to continue these discussions in the Wiki -

In any case...

I uploaded a new version today with some fixes and new features based on all your comments so far, here and in the Wiki. Nothing big, but progress is nice.

XEmacs: it's not dead, but it appears to be dying. GNU Emacs (as of version 22 and even more so with 23) has essentially caught up with XEmacs and surpassed it in some ways. There are minor differences here and there, but it's worth switching.

Supporting XEmacs is hard for GUI stuff because it diverges dramatically from FSF Emacs in handling for input events, keystrokes, fonts, colors, widgets and other UI-related stuff. At this point it would be best for Emacs in general if XEmacs users could wean themselves off it. (With permanent props to the XEmacs developers for pushing the envelope for so long.)

ceesaxp: yes, I'm working on better imenu support, including parsing down into idiomatic declarations such as those found in prototype.js. look for it in an upcoming release.

andy freeman: emacs has no threads, so all I can do is pause and check for user input occasionally. It tells me if there is any input pending, but NOT what kind of input it is, so I don't have enough info to know whether to stop parsing. If someone knows the Emacs input system well enough to tell me how to look into the input queue, I'll make use of it.

Still looking into JQuery, mmm-mode, ECB, etc.

As for Semantic, although I think it's a great idea in principle, I find it to be one of the most annoying packages on the planet. It's complicated to install, isn't bundled with its dependencies, leaves crap everywhere in your filesystem, isn't smart enough to know it's trying to write to read-only filesystems, and so on ad infinitum. I needed a full Ecma-compliant parser for the bigger project anyway, so I figured I might as well use it for the IDE.

4:48 PM, March 31, 2008  
Blogger Eric Ludlam said...

Re: Semantic is annoying

If you were frustrated by CEDET, it would have been nice if you had contributed to the mailing list.

As far as Javascript support is concerned, you are right. I don't include the mode for it. CEDET doesn't include major modes, and I only stick in stuff folks explicitly agree to.

As for the other stuff, the CVS version probably fixes all that stuff. I have limited testing options, so I'm dependent on others to let me know if different platforms don't build or install correctly.

5:45 PM, March 31, 2008  
Blogger Watmough said...

There's an interesting paper here on Pretty Printing that might be of some interest.

A Prettier Printer - Philip Wadler

8:49 PM, March 31, 2008  
Blogger John Connors said...

This is interesting because I've had several recent encounters with the highly scary cc-mode.el. So far it's the most viable parsing option I've found. I'm using it in conjunction with ectags to give myself a half - decent shot of extracting useful semantic information from a large C++ project.

The main problem I'm having is resolving scope: when mutiple tags with the same scope exist in the project such as Fooozle::GetWidget, Fortle::GetWidget and Zarkon::GetWidget, and my cursor hovers over a GetWidget, which GetWidget am I looking at?

Amazingly cc-mode can actually help with this, and with some tweaking it appears it can get it right (mostly by correctly identifying the enclosing scope), although I've found no way to get it to treat Thing::Thong as a single identifier, so I'm having to write a lashup out of the functions cc-mode.el gives me.

When it works, though, it should kick etags out of the park, as I've found exuberant ctags simply to be the best practical parser out there, despite it needing tweaking.

It's not incremental though :(

Sorry to go on, but you are one of the few people on the planet who can truly appreciate a cc-mode war story ;-)

9:52 AM, April 02, 2008  
Blogger Jeff Walden said...

I feel obligated to note that, given a little knowledge of UTF-8, you could have figured out the penultimate character in the name "Karl Landström". I'll dump a bit of it here in the following paragraphs.

First, take the last name as displayed and get the character codes:


From that I get:


The two that correspond to our mystery character(s) are 195,182 by pulling off the "Landstr" and the "m".

Since all the cool kids use UTF-8 because it's reasonably efficient and doesn't half-pretend to be a fixed-width encoding like UTF-16 does (let's just ignore all that non-BMP stuff outside the first 65536 characters, eh? NOT), it's probably UTF-8 (also because that's the default across Linux distros, and this is Emacs). (There's also the not-so-little matter of the rest of the name being ASCII, which also might rule out UTF-16 unless you translated when doing the blog post, but UTF-16 seems unlikely for the previous reason anyway.)

In UTF-8, every character from U+0000 to U+007F is just the equivalent byte, like ASCII -- easy to deal with. Everything above that has the high bit set in a first byte and then follows with some number of trailing bytes, noted by the value of the first byte. In binary, the two values are:

0b11000011 0b10110110

The number of 1s after the leading 1 in the first byte say how many further bytes to read, so we only have one more. In the leading byte, bits indicating the character follow the first 1 and precede the first 0, so we have 00011. Trailing bytes start with 10 and are then followed by bits indicating the character, so we have 110110. Concatenating, we get:


In decimal this is 246; in hexadecimal it's 0xF6. This is the Unicode code point U+00F6, which when I plug into JavaScript gets me:


...returning an o with an umlaut. For me this was the intuitively obvious choice for a character to fit there, if one were to be there, so I declare victory and assume the name is:

Karl Landström

Unicode and UTF-8 are awesome; if only those fool Windows and OS X people hadn't deigned to anoint UTF-16 as their platforms' wide character encoding we might all be happily using UTF-8 today.

7:28 PM, April 03, 2008  
Blogger Xah Lee said...

Fantastic job. Thanks Stevey. I haven't been coding js but will try your js mode next time.

Regarding your reply to Karl Landström:
«Unicode and UTF-8 are awesome; if only those fool Windows and OS X people hadn't deigned to anoint UTF-16 as their platforms' wide character encoding we might all be happily using UTF-8 today.»

Not sure what you mean exactly... but as far as i know, OS X espouses utf-8, not utf-16. (so, in your phrasing, Mac OS X has actually not deigned to anoint UTF-16, but Windows has. (and, not quite sure “deigned” is the proper word here, since you seem to regard utf-8 is superior. So, Windows has not deigned to utf-8. (further, the use of deign and anoint in “deigned to anoint ...” is contradictory)))

I lookup on wp ( ) and it seems verify that mac os x is utf-8.

Also, regarding the choice of utf-16 and utf-8... if your text is mostly euro langs, utf-8 is more efficient in storage. But if you write in Chinese, utf-8 incleases the storage by a factor of 1.5 or so, in comparison to utf-16.

The reason that mac os x chooses utf-8, from my guess, is largely compability with the unixes. i.e. unix tools are almost all utf-8.

9:41 AM, April 05, 2008  
Blogger Unknown said...

Thanks Steve, you rock! Now I don't have to quit emacs to do javascript editing. Great Work!

8:17 PM, April 09, 2008  
Blogger Steve Yegge said...

Turning off comments since I started to see some spam come in, and it's tedious to clean up.

Please use the Wiki at for further discussion. I've been releasing updates with major bug fixes, enhancements and usability tweaks about once a week.

3:01 AM, April 18, 2008  
Blogger smarks said...

I suspect that Karl Landström's parents were typographers, not mathematicians.

6:31 PM, April 21, 2008  
Blogger tomyandel said...

There is also a js+rhino rewrite of jsdoc called jsdoc-toolkit. We recently started using the version 2 beta, which seems pretty good.


12:13 PM, May 14, 2008  
Blogger Unknown said...

Amazing work! Thank you thank you thank you!

I'm using a modified version of Karl's mode for a long time but I'm not too happy with indentation. Spent all day yesterday trying to figure out how CC Mode indentation works, then asked a question on their mailing list and someone pointed me to your page.

I still think that the best idea for indentation is to use CC Mode... It works pretty damn well and it's heavily customizable. As far as I can see, it fails only in one case: when you write literal objects/arrays or anonymous functions in an argument list. I think this can be fixed and I'll continue to look into it.

Anyway, thanks again for this great work and I'm eagerly waiting for the "bigger project"! :-D

1:24 AM, May 24, 2008  
Blogger Alex V Koval said...

Amazing, excellent, thank you very much! :-)

12:21 AM, June 12, 2008  
Anonymous Anonymous said...

his looks very cool. Next time I have some Javascript hacking to do,

1:08 AM, September 18, 2008  
Blogger Dmitry said...

Hello, Steve!

Could you please tell how do you test your elisp code? I didn't find any tests in svn repository.. Do you use some library, or maybe assert from 'cl library?

11:53 PM, April 27, 2009  
Blogger rocky said...

@Tomtt, @Phil, @Dmitry etc.

For largish emacs code I've been writing, I have been using elk-test.el for unit testing.

It's not fantastic nor is my use of it, but it's probably a step in the right direction.

8:05 AM, May 19, 2009  
Blogger Michael Dotterer said...

Wow, great work! This fixes all the gripes I have with my emacs setup for javascripts and have been meaning to deal with for ages.

8:16 AM, September 09, 2009  
Blogger Unknown said...

Why does this interefere with yassnippet?

Yas is incredibly useful, and I'd like to be able to use it alongside js2mode.

Trouble is, js2-mode grabs the tab for changing indentation, do I can't expand my snippets while editing js.

12:30 AM, June 27, 2010  

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