Concerns with TypeScript 4.5's Node 12+ ESM Support

Andrew Branch (@andrewbranch)

For TypeScript 4.5, we've added a new module mode called node12 to better-support running ECMAScript modules in Node.js. Conceptually, the feature is simple - for whatever Node.js does, either match it or overlay something TypeScript-specific that mirrors thes same functionality. This is what TypeScript did for our initial Node.js support (i.e. --moduleResolution node and --module commonjs); however, the feature is much more expansive, and over the past few weeks, a few of us have grown a bit concerned about the complexity.

I recently put together a list of user scenarios and possibly useful scripts for a few people on the team to run through, and we found a few sources of concerns.

Bugs
UX
Ecosystem
User Guidance

Bugs

Most complex software ships with a few bugs. Obviously, we want to avoid them, but the more complex a feature is, the harder it is to cover all the use-cases. As we get closer to our RC date, do we feel confident that what we're shipping has as few blocking bugs as possible?

I would like to say we're close, but the truth is I have no idea. It feels like we'll have to keep trying the features for a bit until we don't run into anything - but we have less than 3 weeks before the RC ships.

Here's a few surprising bugs that need to get fixed before I would feel comfortable shipping node12 in stable.

Code changes breaking module resolution
- Unclosed 'new Set(' wrecks resolutions to packages in nodenext #46373
- nodenext alternates between finding and not finding the imported package #46396
Auto-imports not working: Debug failure on auto-import completion entry details #46332 (technically present in CommonJS)
resolveJsonModule can't be used with node12: Typescript 4.5 resolveJsonModule should also work with node12 and nodenext module and resolveTypes #46362
No errors on extensionless imports from .ts and .tsx files in node12 (unfiled, reported by Andrew Branch (@andrewbranch))
Strange errors under pnpm (unfiled, reported by Daniel Rosenwasser (@DanielRosenwasser))
package.json changes in packages not tracked (unfiled, reported by Daniel Rosenwasser (@DanielRosenwasser))

UX Concerns

In addition to bugs we found, there are just several UX concerns. Package authoring is already a source of confusion in the TypeScript ecosystem. It's too easy to accidentally shoot yourself in the foot as a package author, and it's too hard to correctly consume misconfigured packages. The node12 mode makes this a whole lot worse. Two filed examples of user confusion:

It's too hard to tell whether you're in an ESM or a CJS file: "moduleResolution": "node12" of Typescript 4.5 does not work as expected #46408
The export field is confusing to configure and diagnose: Surprising (or incorrect) priorities with package.json exports fields? #46334
Poor errors on extensionless imports: Provide better errors on module: node12 and extensionless imports #46152

While there might be a lot of "working as intended" behavior here, the question is not about whether it works, but how it works - how do we tell users when something went wrong. I think the current implementation leaves a lot of room for some polish.

But there are some questions about this behavior, and we've had several questions about whether we can simplify it. One motivating question I have is:

When a user creates a new TypeScript project with this mode, when would they not want "type": "module"? Why? Should that be required by default?

We've discussed this a bit, and it seems a bit strange that because we want to cover the "mixed mode" case so much, every Node 12+ user will have to avoid this foot-gun.

I would like to see a world where we say "under this mode, .ts files must be covered by a "type": "module"". .cts can do their own CommonJS thing, but they need to be in a .cts file.

Another motivating question is:

Why would I use node12 today instead of nodenext?

Node 14.8 added top-level await, but Node 12 doesn't have it. I think this omission is enough of a wart that starting at Node 12 is the wrong move.

Ecosystem

The ecosystem is CONFUSING here. Here's a taste of what we've found:

ts-node, Webpack, and Vite don't like imports with .js extensions, but TypeScript expects them. Not all of these can be configured with a plugin.
ts-node, Webpack, and Vite, and Deno are fine with .ts extensions, but TypeScript doesn't allow them!
Many packages that ship types have started supporting export fields, but don't have a types sub-field within export (e.g. RxJS, Vue 3).
Many packages have started supporting export fields, but their @types package might not reflect that.

The last two can be easily fixed over time, though it would be nice to have the team pitch in and help prepare a bit here, especially because it's something that affects our tooling for JavaScript users as well (see #46339)

However, the first two are real issues with no obvious solutions that fall within our scope.

There's also other considerations like "what about import maps?" Does TypeScript ever see itself leveraging those in some way, and will package managers ever support generating them?

Guidance

With --moduleResolution node, it became clear over time that everyone should use this mode. It made sense for Node.js apps/scripts, and it made sense for front-end apps that were going to go through a bundler. Even apps that didn't actually load from node_modules could take advantage of @types in a fairly straightforward way.

Now we have an ecosystem mismatch between Node.js and bundlers. No bundler is compatible with this new TypeScript mode (and keep in mind, back-end code also occasionally uses a bundler).

Here's some questions I wouldn't know how to answer confidently:

Is our guidance to always use this mode for plain Node.js apps, and let tools "catch up"?
Should new projects that use this mode pick node12 or nodenext?
There's a big foot-gun with forgetting "type": "module" - should we always recommend .mts?

Next Steps

I see us having 3 options on the table:

A mad dash to fix everything - I think this is too hard in the next 2 weeks to pull off.
Keep the feature in, but make it inaccessible until we're ready - I think temporarily removing the feature would be impractical. It would probably take more work to remove it than to fix the issues I've already mentioned. So we might as well keep it in.
Ship with some "experimental" labeling - I think this makes the most sense, but with some caveats to what I mean by "ship". It would make sense to just ship this as "experimental", but I think we should make this feature only work in nightly releases so that people can continue to easily use it, but not depend on a stable version until it's ready.

I think this issue is a good example for the long needed plugin (hook) system.

The Solution to the first 2 Problems is rollup at present you can use it with plugin typescript to resolve anything correct and then inject it into the typescript program.

i am already researching how i could maintain and release a typescript-rollup version which would be typescript + rollup hooks and plugins.

Conclusion

a Plugin/Hook System is the Solution for the Resolve Problem. The Only one that is flexible and adjustable enough to cover every case.

Conclusion

a Plugin/Hook System is the Solution for the Resolve Problem. The Only one that is flexible and adjustable enough to cover every case.

There is already a hooks system built into package.json... it's bad, but the whole point is to get rid of it as soon as dependencies merge the PR's to fix the issues.

I have the following install hook as a bandaid while upstream applies the fixes to default imports and reachable types/modules.

#!/bin/bash
set -euo pipefail

sed -i '2s/import express/import \* as express/' node_modules/\@feathersjs/express/index.d.ts
sed -i '1s/import http/import \* as http/' node_modules/\@feathersjs/socketio/index.d.ts
sed -i '2s/import io/import \* as io/' node_modules/\@feathersjs/socketio/index.d.ts
sed -i '8s/"source",/"\.\/source\/index\.js",\n"types": "\.\/index\.d\.ts",/' node_modules/chalk/package.json

I'd rather have 4.5 stable sooner, than to wait for yet another workaround.

rayfoss

rayfoss we can take your example to again show that a plugin/hook system like the one from rollup is badly neeeded.

you mixed linux shell script into your package.json as workaround.

Now we have an ecosystem mismatch between Node.js and bundlers. No bundler is compatible with this new TypeScript mode (and keep in mind, back-end code also occasionally uses a bundler).
Is our guidance to always use this mode for plain Node.js apps, and let tools "catch up"?

Yeah, so the trouble is if TypeScript doesn't encourage the use of node12/nodenext then the package author will be unable to use any packages that use Node ESM. So no matter which choice the author makes some things will be invariably broken.

This is something I've mentioned in the past on some related issue, but has it been considered not to have a distinct node and node12/nodenext mode at all, but rather just use --module node and require the presence of "type": "module" or "type": "commonjs" in package.json? (And regardless of this being present, .mts/.mjs/.cts/.cjs would work always, this would only be required for using .ts/.js in the new mode).

By having both --module nodenext and "type": "module" people are gonna be essentially double-configuring in a lot of cases anyway (most cases?), given people have to add --module nodenext to their tsconfig to enter the mode anyway, I don't see that it would be significantly worse to have them add "type": "module"/"commonjs" to their package.json instead.

Conclusion

a Plugin/Hook System is the Solution for the Resolve Problem. The Only one that is flexible and adjustable enough to cover every case.

This seems like the absolute worst conclusion to ever reach. I'm really sorry for butting in on this issue as not a huge avid typescript users but the one thing I like about typescript is precisely it doesn't require 9001 packages like soo many other builders and bundlers to actually get into a working state.

Typescript is standalone that just-works and doesn't require the end programmer to have to build-their-own-compiler themselves. Add plugins is just gonna be that, making it more and more complicated while adding nothing really. Especially something like this issue that REALLY needs to work out of the box but isn't. And saying to people "oh you're using typescript but typescript is dumb like bundlers, you need to hook plugins to get it working" is something you don't want to say to developers or people.

I really strongly advice against any case of adding plugins to this package. If it doesn't work, we can fix it. If it works, why would you need a plugin just to make configging even more complicated?

Best regards:
TT

P.S.
what is it with developers and wanting plugins in literally everything?

Jonatan Nilsson (@TheThing)

Jonatan Nilsson (@TheThing) in my case it is simply needed because there are many package authors with total diffrent opinions and i do not want to get blocked by them. I also do not want to hardfork everything and so on.

The only Alternativ to a plugin system in typescript is the usage of dev bundels that are typescript compatible.

also my conclusion is driven from the fact that there are tons of other environments not only nodejs

i only vote for resolve hooks and plugins because of all the diffrent environments as also package managers.

Native support for PnP #35206 (comment)

Node 14.8 added top-level await, but Node 12 doesn't have it. I think this omission is enough of a wart that starting at Node 12 is the wrong move.

What about removing Node12 and starts from Node 14?

One note here from https://github.com/microsoft/TypeScript/issues/46550#issuecomment-954348769—for users who have declaration emit enabled, error messages like this are probably going to be a common symptom of a dependency that needs to update their export map to include "types" conditions:

error TS2742: The inferred type of 'T' cannot be named without a reference to '../../../node_modules/async-call-rpc/out/full'. This is likely not portable. A type annotation is necessary.

It feels pretty non-obvious that that’s what’s going on so I wanted to make a note of it here. Basically, the declaration emitter wants to print a type like import("async-call-rpc/out/full").T, but it can’t because the package has an export map that doesn’t specify any "types". So the only way it can reach that module is with a relative import through node_modules, which is not allowed to be synthesized by declaration emit. If you look at the package’s package.json, you can see that the actual specifier that should be generated will probably be "async-call-rpc/full", but each of those entrypoints in the export map needs a "types" if it’s going to be resolvable by TypeScript.

Jack Works (@Jack-Works)

☝️ Actually, I was wrong about this particular example. This may be a common symptom for some packages if their typings are stored in a separate folder from the JS that the export maps point to, like RxJs does. However, if the .d.ts files are colocated with the .js/.mjs/.cjs files pointed to by the export map, this should “just work.” This is the case for async-call-rpc, so the fact that the declaration emitter is complaining here is probably indicative of a module specifier resolution bug. (cc Jack Works (@Jack-Works))

Oh. Yes, I found that package actually exports the correct typing and JS file at /full, not /out/full.

There are some rough edges with the new resolution mode. I've been trying out this mode for two weeks.

The main issues I've hit are:

unstable import resolution issues in IntelliJ IDEA (looks to be related to nodenext alternates between finding and not finding the imported package #46396 )
libraries missing a types declaration (@types/koa-route, raw-body)
libraries with an export field but no types condition while the type declarations are in their own directory (rxjs)

Despite all of these issues, I still hope that the new resolution algorithm becomes available on stable TypeScript: the core seems good and bugs/UX can be iterated. The new resolution mode lets me get rid of tons hacks and makes it finally ergonomic to use ESM natively.

I no longer have to manually watch out for explicit extensions in relative imports
I can use exports with subpath mapping to expose my outputDir directly for deep-imports; while being fully compatible with npm|yarn link and project references, and without having to abuse typeVersions.
For complex cases where types are generated directly in the build directory (e.g. using wasm-bindgen), I am now able to use import maps and remove dummy files used just so TypeScript does not complain about missing files.
Other various quality of life improvements such as proper handling of import.meta or __dirname.

I was able to use node12 with Node, webpack, mocha, wasm-bindgen, native node modules and Angular 13.

In my personal experience, all of these improvements strongly outweight the current issues: it makes things so much simpler as it realigns TS behavior's with Node's. I hope that the current issues get fixed soon and I hope that the concerns will not delay the new resolution too much.

Charles Samborski (@demurgos)

unstable import resolution issues in IntelliJ IDEA (looks to be related to #46389)

Charles Samborski (@demurgos) did you get the right issue number here? This doesn't sound like it would be related to what you're talking about 🤔

I wanted to link the issue 46396: "nodenext alternates between finding and not finding the imported package"

I've double checked my message: I linked it as #46396 but it looks like GitHub failed to resolve it properly. Editing my message to add a single space to force a refresh fixed the rendering.

Are you using bundled TypeScript from vxc (It could be outdated)? Make sure to use your workspace tsdk with this settings:

Even compiling with tsc wasn't working. But re-installing TypeScript did fix it. Thanks :)

What is the recommended way to upgrade large codebases to ESM? I haven't found anything in the docs. The most painful part is the no extensions imports to .js imports. Are there any codemods or tools to migrate? Ideally some transpiler or setting that allowed both .js and no extension so you can gradually migrate large codebases.

By the way there should be a much bigger and clearer warning that you should use .js and not .ts, since this is really counterintuitive. Especially if you use ts-node.

What is the recommended way to upgrade large codebases to ESM?

There isn't one. "Upgrading" to esm is a breaking change for a node package, no way around it.

Yes, but a codemod or similar could exist to add the .js to imports to aid migration. Or a setting to allow imports without extension to work alongside .js so you can gradually migrate imports.

Both of these things are completely feasible and I'm asking if there's already something like that. There's no reason to manually add .js to a million imports if you can automate that.

I mean, there are a bunch of community codemod tools that do stuff like that. We don't exactly bundle tools to make moving from target to target easier; it's just not a thing we do.

I understand not bundling codemods, but I was wondering if there was an official or unofficial recommendation on how to migrate to ESM.

Now that some popular packages are moving to ESM only, you can probably expect more and more people going from CJS to ESM. Expanding the docs with a page featuring a guideline or recommendations for migrating to ESM would be very helpful, it will avoid many future Github Issues.

Currently this page covers the technical basics regarding TS and ESM, but a page full on dedicated to migrating would be helpful to many people in the future.

Such page could mention the existence of tools that aid migration, or TS settings that help. From what I've seen the TS team has decided against any setting that allows .js and no extension imports from coexisting. Which is a pity and I hope you reconsider since this would be extremely useful for migrations.

After all the vast majority TS codebases currently use CJS, and in the future if they want to keep up with the npm ecosystem they'll presumably move to ESM. So good docs on this would help numerous projects.

TS team has decided against any setting that allows .js and no extension imports from coexisting

There is a setting, and it’s called “using CommonJS” 🙃. We allow this wherever your module loader does. ESM in Node does not, so we do not.

It may sound like we’re being pedantic here, but our point of view is derived from two principles which may not be obvious to many people reading up on the new nodenext mode:

We don’t believe you should migrate to ESM for the sake of migrating to ESM. (Hence Wesley’s use of scare quotes on your characterization of this as an “upgrade.”)
If you actually have reasons to migrate to ESM, TypeScript should get out of your way. There should basically be no TypeScript-specific steps in your migration beyond using --module nodenext. Once you’ve done that, literally everything you do to make TypeScript happy (including putting .js extensions on your relative imports) is precisely what you would have to do to make Node happy.

This is core to why we don’t have a migration guide. If folks are set on migrating to ESM and seeking guidance, I would encourage them to look for general JS/Node guides on the subject—it shouldn’t have to be specific to TypeScript.

If you hit pain-points that are TypeScript specific (reminder: this does not include adding extensions to module specifiers; this is literally a Node requirement), we’re interested in hearing those—they may be good candidates for docs (if not actual fixes in our code). (FWIW, the area I’m most concerned with here is dealing with dependencies—it’s going to take some time before all the widely used npm packages who use conditional exports get their typings properly configured. For a user migrating to ESM who runs into dependency issues of this nature, there’s not much they can do beyond recognize what the problem is and file an issue with the library, but maybe we can help with that diagnostic step somehow.)

We don’t believe you should migrate to ESM for the sake of migrating to ESM.

I don't think anybody with a large codebase wants to migrate for the sake of migration. TS already gives you import. It's because of dependencies becoming ESM only that people might want to migrate. In fact I assume most people would prefer a flag that was "emit whatever makes Node ESM happy so I can use my ESM dependencies, without changing my current TS imports", because that would be the least amount of work.

But I understand TS is against that and I don't want to start another flamewar with something that has been discussed many times.

So given that some people will need to migrate to ESM, and such people will need to change their imports, I think there should be docs regarding this. And these docs could rephrase what you just explained, that you must change your imports so that they are imports that make Node happy.

But it is a typescript specific step because people are used to TS handling their imports. Sure, once you know how TS with ESM works you can say imports are now not a step specific to TS. But where are you going to get that knowledge?

People looking at general JS guides won't know how TS decided to handle ESM. They can at most make assumptions. And a lot of people, if they assume that now you have to use extensions with tsc too, they might assume that you have to use .ts instead of .js.

I assumed that and many people will assume that too. I'm not arguing against that decision, just stating that there should be good documentation so people don't make these wrong assumptions.

What I propose is having some documentation with something like a checklist for migrating, something like

Make the following changes to your package.json and tsconfig.json.
Change your imports so they have a .js file extension (NOT .ts). Consider using some tools to automate most of this process
Some smaller things to take into account

I see no downside to having such documentation, and while you can argue that you can infer all that from here, having this info concisely presented to those wanting to migrate will help a lot of people, and we'll avoid a lot of erroneous github issues.

In NodeJS ESM is only a good Authoring format not a good Runtime format at all. The Pros of ESM while coding

static analyzeable imports.
easy transpilable / downleveling
when used as is it works in the browser most of the time and even inside nodejs

The parsing and loading speeds of ESM and CJS do total depend on the overall project variables like size module graph time of instantiation.

Cons ESM:

The ESM System to be more exact the specifier lookup is done in the Host that embeds the ES Engine so out of control without engine hooks or modifications. This is defined in the Specs will never change it was a mistake. Workarounds get invented that allow you to create user space ESM Systems.
see: https://nodejs.org/api/vm.html#class-vmmodule there comes next support for interaction with the Module Loader see also other methods containing the word Module.
no access to the Module Cache without Restarting the engine no cleanup possible by design.
inside nodejs internal loading of dlls and .node modules so interaction with native code is always Sync in the CJS context
the engine instation is always Sync and in CJS even with ESM as Entrypoint that results in
- v8::isolate create context (CJS) => run nothing on first run and start reading ESM Imports => Secund loop iteration of the engine starts to init ESM Code see: https://nodejs.org/api/vm.html#class-vmmodule again for the 3 phases that a ESM Module walks through by design ever phase gets runned after the whole engine stack is empty

Update because thumbs down

I did not design that module system that was ECMA i only say it . Thats how it is. i do not say that ESM Got well designed.

I don't really get it, you've made a new major version that doesn't support the most trending JS language, TypeScript?

Here is a single-file script that automatically adds .js extensions (or /index.js) to the import.

https://gist.github.com/Jack-Works/6ebba47c3fdef50637b10819f3e7e68a

I just want to give a little update:

this introduces the next problem:

Feature Request: tsconfig option "conditions=x,x,x," "exportConditions" with moduleResolve: node<any> and nodenext and or importMap support #47931

Andrew Branch (@andrewbranch)

Hey Andrew Branch (@andrewbranch) what are the remaining concerns keeping this issue open? Most sub tasks seem closed now but not sure if I’m missing something.

Update dependency got to v13 (#66)

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Concerns with TypeScript 4.5's Node 12+ ESM Support #46452

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frank-dspeed commented on Oct 24, 2021

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