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Parens And Performance

Years ago, some smart folks that worked on JS engines realized that not all JS that's loaded into a page/app initially is needed right away. They implemented JIT to optimize this situation.

JIT means Just-In-Time, which means essentially that the engine can defer processing (parsing, compiling) certain parts of a JS program until a later time, for example when the function in question is actually needed. This deferral means the engine is freer to spend the important cycles right now on the code that's going to run right now. This is a really good thing for JS performance.

Some time later, some JS engine devs realized that they needed to get some hints from the code as to which functions would run right away, and which ones wouldn't. In technical speak, these hints are called heuristics.

So they realized that one very common pattern for knowing that a function was going to run right away is if the first character before the function keyword was a (, because that usually means the function is an IIFE (immediately invoked function expression):

(function IamAnIIFE() {
   // .. some code

They need a hint at the beginning of the function, rather than waiting to find the () call operator at the end of it, because by they time they get there, they've either parsed or not.

One hidden complication is that if a function is going to be parsing-skipped for JIT reasons, it still has to sorta be parsed (so they know where the ending } is, for example), though it won't be deeply parsed. Later, the full parsing happens on that snippet when the JIT says it's ready to do so.

This sorta leads to some extra work, because there's like 125% of the time spent on overall parsing of that function (25% now, 100% later) rather than just 100% now. That's the tradeoff cost.

Even given this tradeoff, JS engine devs decided to avoid JIT and immediately parse functions if they are wrapped in ( ), because the assumption is that this will only be the case in the function that is going to execute right away (have a trailing () call operator). In these cases, it's best to go ahead and parse it fully now.

Of course, they need smarter heuristics than just that, because this code is not an IIFE and definitely could (should?) be deferred with JIT:

setTimeout(function IamNotAnIIFE() { /* .. */ }, 1000);

They also recognized that JS minifiers like Uglify don't use ( ) for IIFEs, they usually prefix with !, like !function IamAnIIFE(){..}(). So, the heuristics need to identify more than just ( -- actually, all unary operators -- followed by function, but you get the idea.

The key take-away here is that they developed these heuristics by surveying real code in real apps spread across the entire web. They tested and found that by not-JIT-deferring (aka immediately parsing) such heuristically-identified functions, they would improve the overall speed of current, untouched applications executing in current browsers. In some cases, the speed up was quite noticeable. They rolled out these changes and they've been silently in play in all our apps for a long time.


One wrinkle in this whole plan is that modern module packaging tools have for years had the entire code in a file wrapped in a function, but not looking like an IIFE, like:

define("foobar",function IamNotAnIIFEButIWillRunRightAway(){
   // ..

Inside of define(..), it's going to run that function right away, but it's not obvious syntactically that this is going to happen. So it's a miss on the heuristic.

Keep in mind that this practice of bundling and delivering code in that format has been around for many years, longer than the heuristics the JS engines identified to speed up apps by immediately executing certain functions. They did not immediately parse such full-file function wrappers.

You might wonder why they can't identify such patterns with better heuristics. It's really hard to do so, because any old function passed as a callback is not necessarily going to get immediately invoked. Also, what if that function is like megabytes worth of JS, because you've done the common thing and wrapped all 200 of your JS files into a single bundle.

The engine can't really know at that point if this function would be small or really huge. If it immediately parses a really huge function, it's likely it might lose all the benefits of having the capability to JIT in the first place.

So understandably, they want to be conservative. If they immediately execute all the code and have no JIT, performance will probably suffer.

Second Guess

Some smart folks decided recently they'd try to second-guess the browser and trick the heuristic into faster parsing, by making their module-bundling tool automatically insert the ( ) around those wrapped functions. Like this:

define("foobar",(function IamNotAnIIFEButIWillRunRightAway(){
   // ..

Look closely. The only difference is the ( ) around the function. Everything else is the same. But that's enough to kick in the engine's heuristic and "force" it to immediately parse instead of JIT parse.

Various benchmarks have shown that this makes the contents of that file parse anywhere from a few milliseconds to 40ms faster.

In fact, someone else wrote another tool to automatically go wrap all (*deliberately being vague/generalistic) your functions in ( ) even if you're not using one of those module bundling tools.

Yay, right!?!? Right? Look how clever we are, outsmarting the browser at its own game. Humans 1, Computers 0.

Reality Check

You can proably guess that where I'm headed is to suggest this is a terrible idea. Let's take a step back and consider the cost.

Brief History Lesson

First, do you recall from CSS the !important feature, which was added because CSS devs needed to force the engine to override its normal behavior? It was cheat, a hack, but it was effective and pragmatic and it let CSS devs ship "working" code. Today, most developers would tell you that !important is a wart on our history. It's terrible and should never be used. But we're stuck with it.

What about when JS devs were told, "always cache the length of an array before looping over it"... you know, for performance. Outsmart the engine, you're a human after all. The problem is, that advice worked so well that most everybody started doing it.

Eventually, browsers got smart and put in optimizations to automatically cache the length of the array in those cases where it would actually help. Guess what effect that had? If you're manually caching the length of your arrays, you're actually doing duplicate work and your code goes (a teeny bit) slower now. Whoops.

Or what about the CSS "zero-z-transform" hack for "forcing" the rendering engine to use hardware acceleration on some animation (because it tricked it into thinking it was 3-d)? That turned out great, didn't it? We had to invent a whole new feature set to get away from that crap.

I call all these sorts of "just do it now" kinds of hacks: "betting against the future". You're saying, I'm clever and smart and I bet that I'll always be better than the computer at this thing. Guess what!? No, you won't. The computer will eventually do it better than you. It's always a terrible bet to wager against the future of computing technology.

Predictions of Future Reality

Here's some predictions I have for where this ( ) hack is going to lead us.

  1. More tools will start automatically doing this ( ) wrapping, meaning more and more deployed code will have them, meaning more and more functions will be parsed in immediate mode instead of JITing. In the short term, many people will perceive wins in performance, especially on mobile.

  2. Over time, JS engines will start to realize that most code is being parsed immediately, and not much JITing is happening. They'll realize the heuristics need to change, to get back to a better balance. Either they'll remove the ( ) heuristic entirely, or make it more sophisticated, but either way, all of a sudden, all that code that used to be "forcing" the immediate parse no longer will.

    This shifting and chasing of heuristics is dangerous because it creates cult myths about performance that aren't actually true, and it actively makes it harder for the JS engines to really get the best performance for your applications. Cult myths and old code last for years and years beyond when they've been busted or made moot.

  3. So, eventually, tools will adjust and chase after that moved goalpost.

  4. Over time, blog posts will be written that tell developers that they should do the ( ) hack themselves (in the cases where the tools are incapable or unsuitable), just like we have posts from the Bluebird folks telling people that to become elite at JS programming performance, you should avoid closures and use objects, etc etc.

  5. New developers/learners will not understand the nuance of balance that is required, to make the proper decisions on when to use ( ) wrapping and not. They'll probably do what we always seem to do: just do something absolutely and always.

    So when a dev sees that they have a function declaration like function foo() { .. }, they'll realize that they can't just wrap ( ) around it to "just make it run faster". So they'll refactor them to let foo = (function foo() { .. }); style, meaning that effectively we'll be losing out on an important readability characteristic: function hoisting. All functions will have to be defined at the top before being used, instead of (as I prefer) definining functions at the bottom with executable code above.

  6. Somewhere, someday, a JS teacher will have a student ask why we always put the ( ) around functions. They'll talk about how functions are much nicer in non-JavaScript languages. That teacher will have to re-count this whole ugly mess, and instead of enlightening students, that narrative will just confuse and frustrate learners.

  7. Someday, the tide will shift and people will talk about the bad old days when we used to do stupid stuff like wrap ( ) around functions just so the engine runs faster. The facepalm cycle will be fully complete at that point.


So, what's the alternative(s)? I can think of a few:

  1. JS engine devs can admit that the heuristic is not sufficient as-is, in that there's all these major libraries that are currently still parsing slower than necessary. They'll quickly work on fixing that heuristic. In the meantime, they'll advise against people doing short term hacks.

    Once those fixes are in place, the need for this whole ( ) crap will go away, and tools should roll back and stop advertising the technique at all.

  2. JS engine devs can admit that there's no way the heuristic can really be smart enough, so they'll officially do away with the heuristic entirely. Instead they'll say that JS devs should make explicit code declarations for the engines to know whether to parse immediate or not. For example, rather than use implicit syntax, they should propose an explicit feature to TC39 like a "parse immediate"; pragma (strict-mode like) that we can all agree on, learn, teach, and consistently apply.

    This is actually consistent with the same mindset that drove the proposals to move from implicit PTC (tail calls) to explicit STC (syntactic tail calls). If we want something (even performance oriented) for developers to participate in, let's be explicit about it and standardize it.

  3. Developers can just ignore the current heuristic and tools can, too. Go on about your day business as usual. Let the engine keep doing what it does best, and stop trying to hack to bet against the future of their capabilities. The 3ms you might see saved in some artificial benchmark is not actually worth the real cost as this plays out over time.

You can probably guess by now that I'd rather see #3, or some combo of #3 and #1.

Stop doing silly stuff for the sake of saving a few milliseconds. Code is about communication. We don't need any help adding extra clutter to it to make the communication worse. We actively need to eschew all this premature optimization fascination that's become popular of late.

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:ten thumbs up:

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There are heaps of things that happen to code in order to minify/mangle it for production use, and almost none of them make the code more readable. At that build step, we consciously make code harder to communicate, in order to make it smaller, or to take advantage of the engine optimisation quirks du jour.

I've never once hand-written an IIFE preceded by a !, but it's always been deployed as such, and I've not picked up any bad habits in the process. Who actually teaches JavaScript classes and instructs people to write!function...? If it comes up, it's usually because someone has spotted it in production code, and then they get to have the fun, educational side conversation about uglifiers, browsers and such.

I just see this latest () trick as another one of those things that should be applied at the build step, and should never appear in source code. Once it's no longer effective, that's when it should be removed from the build step, and we move on.

I totally get what you're saying about browser vendors reacting to the way we code. But that's a two way street, and it's part and parcel of how the web works. This is how we eventually figure out best practices and propose new stuff which become browser experiments and eventually standards. This is why the language evolves. This is why we have progress. Embrace it!

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ojacobson commented Sep 19, 2016

This is great.

One thing that developers - especially junior developers - tend to miss is that real, measurable performance gains like a 40ms improvement in initial rendering time by delaying function parsing is not the end of the story. It's very hard to convince someone who is faced with a measurement like that that they're making the wrong call by relying on that measurement to justify a programming choice. This article is a great tool towards communicating why that 40ms gain in one spot is not a free lunch.

You've clearly articulated three independent costs that need to be paid for that up-front gain:

  • Medium-term maintenance costs as JIT optimizations shift out from under your code,
  • Ongoing communication overhead spent explaining (possibly even incorrectly explaining) why the program makes some non-obvious syntactic choices, and
  • Extra runtime cost performing the parsing and analysis delayed by forcing a specific optimization path.

Now, I tend to believe that our reliance on load-time parsing is a major contributor to this problem space. Cmd-R development itself is a nice up-front win, but to pay for it we rely on browsers to re-parse our code for us every time it's loaded, just in case it's changed. Maybe up-front compilation for the web is out of reach now, but can we get there? Edit: and can we avoid paying for it by losing the ability to read the source code to the apps we use?

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bmaurer commented Sep 19, 2016

Hey --

I didn't realize we'd trigger so much discussion from this bug on rollup.

I completely agree that just randomly going and adding parentheses around functions isn't good for the web. At Facebook we often find that in the short term we have to do terrible hacks (like adding parens around functions) that aren't best practices. Our goal is to still do those things, so that our users get immediate benefit, but start discussion with the community about what's best for the long term.

IMHO all of the options that are available today or envisioned here have a fundamental flaw -- they require a JS engine to do some, minimal parsing of every byte of JS on a site. Currently the average web page has about 400 KB of compressed JS (according to web page archive). Even if you don't fully build and AST that's a lot of lexing to do.

Long term we really want to see some kind of format where you only start analyzing the inside of a function once you execute it. Web Assembly is a step in this direction, but it doesn't target the full JS language (dom access, strings, GC, etc). Internally at FB we're doing dome exploration on what a web-assembly-like solution would look like it it targeted the full language.

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linus-amg commented Sep 20, 2016

"parse immediate"; thats the first thought that came to my mind also, after reading till paragraph 3, go for it babel!

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kangax commented Sep 22, 2016

I entertained the idea of a directive as well but v8 devs are hesitating to do it (in private discussions). For understandable reasons. It's kind of like when everyone started forcing engines into hardware accelerated CSS transforms with indirect "translate3d(0,0,0)" hints. We shouldn't have to worry about these things, but we do. It's an indication of a problem; in this case the fact that parsing is (relatively) slow. I agree that for many apps this is a premature optimization. When you're serving megabytes of JS to 1.7 billion people, it's a bit different.

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Thank you for this highly thoughtful and truthful analysis.

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Thanks @getify for this epic explanation. You should know that your predictions are pretty great! As I read the top part of this, I was thinking most of those things, and then read the part where you predicted I would say that. I LOL'd when I read that section because you got in my head man.

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ar5had commented Jan 4, 2017

Great stuff, thanks for sharing this!

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