Towards a JavaScript Binary AST

Aug 3, 2017 23:31 · 2271 words · 11 minutes read JavaScript Mozilla parsing ast compiler performance SpiderMonkey Babel newsletter

In this blog post, I would like to introduce the JavaScript Binary AST, an ongoing project that we hope will help make webpages load faster, along with a number of other benefits.

A little background

Over the years, JavaScript has grown from one of the slowest scripting languages available to a high-performance powerhouse, fast enough that it can run desktop, server, mobile and even embedded applications, whether through web browsers or other environments.

As the power of JavaScript has grown, so has the complexity of applications and their size. Whereas, twenty years ago, few websites used more than a few Kb of JavaScript, many websites and non-web applications now need to deliver and load several Mb of JavaScript before the user can start actually using the site/app.

While the sound of “several Mb of JavaScript” may sound odd, recall that a native application such as Steam weighs 3.1Mb (pure binary, without resources, without debugging symbols, without dynamic dependencies, measured on my Mac), Telegram weights 11Mb and the Opera updater weighs 5.8Mb. I’m not adding the size of a web browser, because web browsers are architected essentially from dynamic dependencies, but I expect that both Firefox and Chromium weigh 100+ Mb.

Of course, large JavaScript source code has several costs, including:

  • heavy network transfers;
  • slow startup.

We have reached a stage at which the simple duration of parsing the JavaScript source code of a large web application such as Facebook can easily last 500ms-800ms on a fast computer – that’s before the JavaScript code can be compiled to bytecode and/or interpreted. There is very little reason to believe that JavaScript applications will get smaller with time.

So, a joint team from Mozilla and Facebook decided to get started working on a novel mechanism that we believe can dramatically improve the speed at which an application can start executing its JavaScript: the Binary AST.

Introducing the Binary AST

The idea of the JavaScript Binary AST is simple: instead of sending text source code, what could we improve by sending binary source code?

Let me clarify: the Binary AST source code is equivalent to the text source code. It is not a new programming language, or a subset of JavaScript, or a superset of JavaScript, it is JavaScript. It is not a bytecode, rather a binary representation of the source code. If you prefer, this Binary AST representation is a form of source compression, designed specifically for JavaScript, and optimized to improve parsing speed. We are also building a decoder that provides a perfectly readable, well-formatted, source code. For the moment, the format does not maintain comments, but there is a proposal to allow comments to be maintained.

Producing a Binary AST file will require a build step and we hope that, in time, build tools such as WebPack or Babel will be able to produce Binary AST files, hence making switching to Binary AST as simple as passing a flag to the build chains already used by many JS developers.

I plan to detail the Binary AST, our benchmarks and our current status it in future blog posts. For the moment, let me just mention that early experiments suggest that we can both obtain very good source compression and considerable parsing speedups.

We have been working on Binary AST for a few months now and the project was just accepted as a Stage 1 Proposal at at ECMA TC-39. This is encouraging, but it will take time until you see implemented in all JavaScript VMs and toolchains.

Comparing with…

…compression formats

Most webservers already send JavaScript data using a compression format such as gzip or brotli. This considerably reduces the time spent waiting for the data.

What we’re doing here is a format specifically designed for JavaScript. Indeed, our early prototype uses gzip internally, among many other tricks, and has two main advantages:

  • it is designed to make parsing much faster;
  • according to early experiments, we beat gzip or brotli by a large margin.

Note that our main objective is to make parsing faster, so in the future, if we need to choose between file size and parsing speed, we are most likely to pick faster parsing. Also, the compression formats used internally may change.


The tool traditionally used by web developers to decrease the size of JS files is the minifier, such as UglifyJS or Google’s Closure Compiler.

Minifiers typically remove unused whitespace and comments, rewrite variable names to shorten then, and use a number of other transformations to make the program shorter.

While these tools are definitely useful, they have two main shortcomings:

  • they do not attempt to make parsing faster – indeed, we have witnessed a number of cases in which minification accidentally makes parsing slower;
  • they have the side-effect of making the JavaScript code much harder to read, including renaming unreadable names to variables and functions, using exotic features to pack variable declarations, etc.

By opposition, the Binary AST transformation:

  • is designed to make parsing faster;
  • maintains the source code in such a manner that it can be easily decoded and read, with all variable names, etc.

Of course, obfuscation and Binary AST transformation can be combined for applications that do not wish to keep the source code readable.


Another exciting web technology designed to improve performance in certain cases is WebAssembly (or wasm). wasm is designed to let native applications be compiled in a format that can both be transferred efficiently, parsed quickly and executed at native speed by the JavaScript VM.

By design, however, wasm is limited to native code, so it doesn’t work with JavaScript out of the box.

I am not aware of any project that achieves compilation of JavaScript to wasm. While this would certainly be feasible, this would be a rather risky undertaking, as this would involve developing a compiler that is at least as complex as a new JavaScript VM, while making sure that it is still compatible with JavaScript (which is both a very tricky language and a language whose specifications are clarified or extended at least once per year). Of course, this task ends up useless if the resulting code is slower than today’s JavaScript VMs (which tend to be really, really fast) or so large that it makes startup prohibitively slow (because that’s the problem we are trying to solve here) or if it doesn’t work with existing JavaScript libraries or (for browser applications) the DOM.

Now, exploring this would definitely be an interesting work, so if anybody wants to prove us wrong, by all means, please do it :)

…improving caching

When JavaScript code is downloaded by a browser, it is stored in the browser’s cache, so as to avoid having to re-download it later. Both Chromium and Firefox have recently improved their browsers to be able to cache not just the JavaScript source code but also the bytecode, hence side-stepping nicely the issue of parse time for the second load of a page. I have no idea of the status of Safari or Edge on the topic, so it is possible that they may have comparable technologies.

Congratulation to both teams, these technologies are great! Indeed, they nicely improve the performance of reloading a page. This works very well for pages that have not updated their JavaScript code since the last time they were accessed.

The problem we are attempting to solve with Binary AST is different: while we all have some pages that we visit and revisit often, there is a larger number of pages that we visit for the first time, in addition to the pages that we revisit but that that have been updated since our latest visit. In particular, a growing number of applications get updated very, very often – for instance, Facebook ships new JavaScript code several times per day, and I would be surprised if Twitter, LinkedIn, Google Docs et al didn’t follow similar practices. Also, if you are a JS developer shipping a JavaScript application – whether web or otherwise – you want the first contact between you and your users to be as smooth as possible, which means that you want the first load (or first load since update) to be very fast, too.

These are problems that we address with Binary AST.

What if…

…we improved caching?

Additional technologies have been discussed to let browsers prefetch and precompile JS code to bytecode.

These technologies are definitely worth investigating and would also help with some of the scenarios for which we are developing Binary AST – each technology improving the other. In particular, the better resource-efficiency of Binary AST would thus help limit the resource waste when such technologies are misused, while also improving the cases in which these techniques cannot be used at all.

…we used an existing JS bytecode?

Most, if not all, JavaScript Virtual Machines already use an internal representation of code as JS bytecode. I seem to remember that at least Microsoft’s Virtual Machine supports shipping JavaScript bytecode for privileged application.

So, one could imagine browser vendors exposing their bytecode and letting all JS applications ship bytecode. This, however, sounds like a pretty bad idea, for several reasons.

The first one affects VM developers. Once you have exposed your internal representation of JavaScript, you are doomed to maintain it. As it turns out, JavaScript bytecode changes regularly, to adapt to new versions of the language or to new optimizations. Forcing a VM to keep compatibility with an old version of its bytecode forever would be a maintenance and/or performance disaster, so I doubt that any browser/VM vendor will want to commit to this, except perhaps in a very limited setting.

The second affects JS developers. Having several bytecodes would mean maintaining and shipping several binaries – possibly several dozens if you want to fine-time optimizations to successive versions of each browser’s bytecode. To make things worse, these bytecodes will have different semantics, leading to JS code compiled with different semantics. While this is in the realm of the possible – after all, mobile and native developers do this all the time – this would be a clear regression upon the current JS landscape.

…we had a standard JS bytecode?

So what if the JavaScript VM vendors decided to come up with a novel bytecode format, possibly as an extension of WebAssembly, but designed specifically for JavaScript?

Just to be clear: I have heard people regretting that such a format did not exist but I am not aware of anybody actively working on this.

One of the reasons people have not done this yet is that designing and maintaining bytecode for a language that changes all the time is quite complicated – doubly so for a language that is already as complex as JavaScript. More importantly, keeping the interpreted-JavaScript and the bytecode-JavaScript in touch would most likely be a losing battle, one that would eventually result in two subtly incompatible JavaScript languages, something that would deeply hurt the web.

Also, whether such a bytecode would actually help code size and performance, remains to be demonstrated.

…we just made the parser faster?

Wouldn’t it be nice if we could just make the parser faster? Unfortunately, while JS parsers have improved considerably, we are long past the point of diminishing returns.

Let me quote a few steps that simply cannot be skipped or made infinitely efficient:

  • dealing with exotic encodings, Unicode byte order marks and other niceties;
  • finding out if this / character is a division operator, the start of a comment or a regular expression;
  • finding out if this ( character starts an expression, a list of arguments for a function call, a list of arguments for an arrow function, …;
  • finding out where this string (respectively string template, array, function, …) stops, which depends on all the disambiguation issues, …;
  • finding out whether this let a declaration is valid or whether it collides with another let a, var a or const a declaration – which may actually appear later in the source code;
  • upon encountering a use of eval, determine which of the 4 semantics of eval to use;
  • determining how truly local local variables are;

Ideally, VM developers would like to be able to parallelize parsing and/or delay it until we know for sure that the code we parse is actually used. Indeed, most recent VMs implement these strategies. Sadly, the numerous token ambiguities in the JavaScript syntax considerably the opportunities for concurrency while the constraints on when syntax errors must be thrown considerably limit the opportunities for lazy parsing.

In either case, the VM needs to perform an expensive pre-parse step that can often backfire into being slower than regular parsing, typically when applied to minified code.

Indeed, the Binary AST proposal was designed to overcome the performance limitations imposed by the syntax and semantics of text source JavaScript.

What now?

We are posting this blog entry early because we want you, web developers, tooling developers to be in the loop as early as possible. So far, the feedback we have gathered from both groups is pretty good, and we are looking forward to working closely with both communities.

We have completed an early prototype for benchmarking purposes (so, not really usable) and are working on an advanced prototype, both for the tooling and for Firefox, but we are still a few months away from something useful.

I will try and post more details in a few weeks time.

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