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Rushing up the JavaScript ecosystem

Rushing up the JavaScript ecosystem

2023-01-16 11:07:00

???? tl;dr: Whether or not you’re constructing, testing and/or linting JavaScript, module decision is all the time on the coronary heart of every thing. Regardless of its central place in our instruments, not a lot time has been spent on making that facet quick. With the adjustments mentioned on this weblog put up instruments will be sped up by as a lot as 30%.

In part 1 of this sequence we discovered a couple of methods to hurry numerous libraries utilized in JavaScript instruments. While these low degree patches moved the overall construct time quantity by an excellent chunk, I used to be questioning if there’s something extra basic in our tooling that may be improved. One thing that has a higher impression on the overall time of widespread JavaScript duties like bundling, testing and linting.

So over the subsequent couple of days I collected a few dozen CPU profiles from numerous duties and instruments which are generally utilized in our business. After a little bit of inspection, I got here throughout a repeating sample that was current in each profile I checked out and affected the overall runtime of those duties by as a lot as 30%. It’s such a vital and influential a part of our infrastructure that it deserves to have its personal weblog put up.

That vital piece known as module decision. And in all of the traces I checked out it took extra time in complete than parsing supply code.

The price of capturing stack traces

It began after I observed that probably the most time consuming facet in these traces was spent in captureLargerStackTrace an inside node operate answerable for attaching stack traces to Error objects. That appeared a bit out of the abnormal, on condition that each duties succeeded with out displaying any indicators of errors being thrown.

Result table in speedscope showing captureLargerStack traces taking up to 29% time in various traces

After clicking by a bunch of occurrences within the profiling information a clearer image emerged as to what was occurring. Almost all the error creations got here from calling node’s native fs.statSync() operate and that in flip was referred to as inside a operate referred to as isFile. The documentation mentions that fs.statSync() is mainly the equal to POSIX’s fstat command and generally used to examine if a path exists on disk, is a file or a listing. With that in thoughts we must always solely get an error right here within the distinctive use case when the file doesn’t exist, we lack permissions to learn it or one thing comparable. It was time to take a peek on the supply of isFile.

operate isFile(file) {
attempt catch (err) {
if (err.code === "ENOENT" || err.code === "ENOTDIR") {
return false;
throw err;

From a fast look it’s an harmless wanting operate, however was displaying up in traces nonetheless. Noticeably, we ignore sure error instances and return false as a substitute of forwarding the error. Each the ENOENT and ENOTDIR error codes in the end imply that the trail doesn’t exist on disk. Perhaps that’s the overhead we’re seeing? I imply we’re instantly ignoring these errors right here. To check that idea I logged out all of the errors that the attempt/catch-block caught. Low and behold each single error that was thrown was both a ENOENT code or an ENOTDIR code.

A peek into node’s documentation of fs.statSync reveals that it helps passing a throwIfNoEntry choice that stops errors from being thrown when no file system entry exists. As an alternative it can return undefined in that case.

operate isFile(file) {
const stat = fs.statSync(file, { throwIfNoEntry: false });
return stat !== undefined && (stat.isFile() || stat.isFIFO());

Making use of that choice permits us to do away with the if-statment within the catch block which in flip makes the attempt/catch redundant and permits us to simplify the operate even additional.

This single change decreased the time to lint the challenge by 7%. What’s much more superior is that checks obtained the same speedup from the identical change too.

The file system is pricey

With the overhead of stack traces of that operate being eradicated, I felt like there was nonetheless extra to it. , throwing a few errors shouldn’t actually present up in any respect in traces captured over the span of a few minutes. So I injected a easy counter into that operate to get an thought how continuously it was referred to as. It grew to become obvious that it was referred to as about 15k instances, about 10x greater than there have been recordsdata within the challenge. That smells like a possibility for enchancment.

To module or to not module, that’s the query

By default there are three type of specifiers for a software to find out about:

  • Relative module imports: ./foo, ../bar/boof
  • Absolute module imports: /foo, /foo/bar/bob
  • Package deal imports foo, @foo/bar

Essentially the most attention-grabbing of the three from a efficiency perspective is the final one. Naked import specifiers, those that don’t begin with a dot . or with a slash /, are a particular type of import that sometimes confer with npm packages. This algorithm is described in depth in node’s documentation. The gist of it’s that it tries to parse the package deal title after which it can traverse upwards to examine if a particular node_modules listing is current that accommodates the module till it reaches the foundation of the file system. Let’s illustrate that with an instance.

Let’s say that we now have a file situated at /Customers/marvinh/my-project/src/options/DetailPage/parts/Structure/index.js that tries to import a module foo. The algorithm will then examine for the next places.

  • /Customers/marvinh/my-project/src/options/DetailPage/parts/Structure/node_modules/foo/
  • /Customers/marvinh/my-project/src/options/DetailPage/parts/node_modules/foo/
  • /Customers/marvinh/my-project/src/options/DetailPage/node_modules/foo/
  • /Customers/marvinh/my-project/src/options/node_modules/foo/
  • /Customers/marvinh/my-project/src/node_modules/foo/
  • /Customers/marvinh/my-project/node_modules/foo/
  • /Customers/marvinh/node_modules/foo/
  • /Customers/node_modules/foo/

That’s numerous file system calls. In a nutshell each listing can be checked if it accommodates a module listing. The quantity of checks instantly correlates to the variety of directories the importing file is in. And the issue is that this occurs for each file the place foo is imported. Which means if foo is imported in a file residing someplace else, we’ll crawl the entire listing tree upwards once more till we discover a node_modules listing that accommodates the module. And that’s a facet the place caching the resolved module enormously helps.

But it surely will get even higher! A number of initiatives make use of path mapping aliases to save lots of just a little little bit of typing, with the intention to use the identical import specifiers in all places and keep away from numerous dots ../../../. That is sometimes accomplished through TypeScript’s paths compiler choice or a resolve alias in a bundler. The issue with that’s that these sometimes are indistinguishable from package deal imports. If I add a path mapping to the options listing at /Customers/marvinh/my-project/src/options/ in order that I can use an import declaration like import {...} from “options/DetailPage”, then each software ought to find out about this.

However what if it doesn’t? Since there is no such thing as a centralized module decision package deal that each JavaScript software makes use of, they’re a number of competing ones with numerous ranges of options supported. In my case the challenge makes heavy use of path mappings and it included a linting plugin that wasn’t conscious of the trail mappings outlined in TypeScript’s tsconfig.json. Naturally, it assumed that options/DetailPage was referring to a node module, which led it to do the entire recursive upwards traversal dance in hopes of discovering the module. But it surely by no means did, so it threw an error.

Caching all of the issues

Subsequent I enhanced the logging to see what number of distinctive file paths the operate was referred to as with and if it all the time returned the identical outcome. Solely about 2.5k calls to isFile had a novel file path and there was a robust 1:1 mapping between the handed file argument and the returned worth. It’s nonetheless greater than the quantity of recordsdata within the challenge, but it surely’s a lot decrease than the overall 15k instances it was referred to as. What if we added a cache round that to keep away from reaching out to the file system?

const cache = new Map();

operate resolve(file) {
const cached = cache.get(file);
if (cached !== undefined) return cached;

const resolved = isFile(file);
cache.set(file, resolved);
return file;

The addition of a cache sped up the overall linting time by one other 15%. Not unhealthy! The dangerous bit about caching although is that they could grow to be stale. There’s a time limit the place they normally need to be invalidated. Simply to be on the secure facet I ended up selecting a extra conservative method that checks if the cached file nonetheless exists. This isn’t an unusual factor to occur should you consider tooling typically being run in watch mode the place it’s anticipated to cache as a lot as attainable and solely invalidate the recordsdata that modified.

const cache = new Map();

operate resolve(file) {
const cached = cache.get(file);

if (cached !== undefined && isFile(file)) {
return cached;

for (const ext of extensions) {
const filePath = file + ext;

if (isFile(filePath)) {
cache.set(file, filePath);
return filePath;

throw new Error(`Couldn't resolve ${file}`);

I used to be truthfully anticipating it to nullify the advantages of including a cache within the first place since we’re reaching to the file system even within the cached state of affairs. However wanting on the numbers this solely worsened the overall linting time solely by 0.05%. That’s a really minor hit as compared, however shouldn’t the extra file system name matter extra?

See Also

The file extension guessing sport

The factor with modules in JavaScript is that the language didn’t have a module system from the get go. When node.js got here onto the scene it popularized the CommonJS module system. That system has a number of “cute” options like the flexibility to omit the extension of the file you’re loading. While you write a press release like require("./foo") it can robotically add the .js extension and attempt to learn the file at ./foo.js. If that isn’t current it can examine for json file ./foo.json and if that isn’t accessible both, it can examine for an index file at ./foo/index.js.

Successfully we’re coping with ambiguity right here and the tooling has to make sense of what ./foo ought to resolve to. With that there’s a excessive probability of doing wasted file system calls as there is no such thing as a manner of understanding the place to resolve the file to, upfront. Instruments actually need to attempt every mixture till they discover a match. That is worsened if we have a look at the overall quantity of attainable extensions that exist as we speak. Instruments sometimes have an array of potential extensions to examine for. In the event you embody TypeScript the complete listing for a typical frontend challenge on the time of this writing is:

const extensions = [

That’s 8 potential extensions to examine for. And that’s not all. You basically need to double that listing to account for index recordsdata which may resolve to all these extensions too! Which means that our instruments haven’t any different choice, apart from looping by the listing of extensions till we discover one which exists on disk. After we wish to resolve ./foo and the precise file is foo.ts, we’d have to examine:

  1. foo.js -> doesn’t exist
  2. foo.jsx -> doesn’t exist
  3. foo.cjs -> doesn’t exist
  4. foo.mjs -> doesn’t exist
  5. foo.ts -> bingo!

That’s 4 pointless file system calls. Positive you possibly can change the order of the extensions and put the most typical ones in your challenge initially of the array. That will improve the probabilities of the proper extension to be discovered earlier, but it surely doesn’t get rid of the issue completely.

As a part of the ES2015 spec a brand new module system was proposed. All the small print weren’t fleshed out in time, however the syntax was. Import statements rapidly took over as they’ve very advantages over CommonJS for tooling. As a consequence of its staticness it opened up the house for tons extra tooling enhanced options like most famously tree-shaking the place unused modules and and even features in modules will be simply detected and dropped from the manufacturing construct. Naturally, everybody jumped on the brand new import syntax.

There was one downside although: Solely the syntax was finalized and never how the precise module loading or decision ought to work. To fill that hole, instruments re-used the present semantics from CommonJS. This was good for adoption as porting most code bases solely required syntactical adjustments and these could possibly be automated through codemods. This was a implausible facet from an adoption viewpoint! However that additionally meant that we inherited the guessing sport of which file extension the import specifier ought to resolve to.

The precise spec for module loading and determination was finalized years later and it corrected this error by making extensions necessary.

import { doSomething } from "./foo";

import { doSomething } from "./foo.js";

By eradicating this supply of ambiguity and all the time including an extension, we’re avoiding a whole class of issues. Instruments get manner quicker too. However it can take time till the ecosystem strikes ahead on that or if in any respect, since instruments have tailored to cope with the anomaly.

The place to go from right here?

All through this complete investigation I used to be a bit shocked to search out that a lot room for enchancment with reference to optimizing module decision, on condition that it’s such a central in our instruments. The few adjustments described on this article decreased the linting instances by 30%!

The few optimizations we did right here will not be distinctive to JavaScript both. These are the identical optimizations that may be present in toolings for different programming languages. In relation to module decision the 4 foremost takeaways are:

  • Keep away from calling out to the file system as a lot as attainable
  • Cache as a lot as you’ll be able to to keep away from calling out to the file system
  • While you’re utilizing fs.stat or fs.statSync all the time set the throwIfNoEntry: false
  • Restrict upwards traversal as a lot as attainable

The slowness in our tooling wasn’t attributable to JavaScript the language, however by issues simply not being optimized in any respect. The fragmentation of the JavaScript ecosystem does not assist both as there isn’t a single customary package deal for module decision. As an alternative, there are a number of and so they all share a unique subset of options. That’s no shock although because the listing of options to help has grown over the years and there’s no single library on the market that helps all of them on the time of this writing. Having a single library that everybody makes use of would make fixing this downside as soon as and for all for everybody lots simpler.

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