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Comment by s_tec | Hacker News Reader

I wonder why they kept the ugly .mjs extension. This is a hold-over from an earlier plan, and doesn't seem necessary in this iteration.

This new plan seem to be going down the path that `import` is for modules while `require` is for everything else. If that's true, then they shouldn't need a special file extension for modules - it should always be clear from the context which is which.

The only exceptions I can think of are executable scripts (which could use a command-line option) and packages in node_modules (which can use a special package.json field).

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They say that it's required for ecsmascript modules that neither import nor export (for instance, modules that place things in the global namespace), but I would argue those are mostly shims for a pre-moudule era and they should just be `required()`. Or perhaps they could use some special comment at the top to let Node know it should be treated as a module even though it doesn't look like one.

Otherwise, just work down the `import` tree from index.js and anything that is imported is a escmascript module, everything that is required() isn't. Of course, this is just an armchair quarterback opinion, but I'm curious to know why that approach wouldn't work.

Exactly! You have to know whether something is a script or a module before you can parse it, but with this plan, that's always clear from the context.

This is exactly how browsers do it, by the way. If you `import` something, is a module, period. The `<script>` tag is the only place that supports both, so they use a `module="true"` flag to indicate which is which.

Small correction: it’s <script type="module">, as distinct from <script> or <script type="text/javascript">.

> This is exactly how browsers do it, by the way. If you `import` something, is a module, period. The `<script>` tag is the only place that supports both, so they use a `module="true"` flag to indicate which is which.

This is not entirely true. If you import something and it's served with a content type associated with JavaScript modules, then it's interpreted as a JS module. But if they are served with a wasm content-type, they may in the future interpreted as wasm modules. In node there's the additional content-type of "CommonJS file" which has to be handled somehow as well.

Non-module script tags aren't really relevant because node itself never supported scripts (at least not what the browser and the ECMA standard calls scripts).

>I would argue those are mostly shims for a pre-moudule era and they should just be `required()`

Modules are always strict, so loading something intended to be a module as commonjs could break the code within, which expects strict mode.

Consider the following source text: `delete Object.freeze({ a: 1 }).a`

It will return `false` in sloppy mode, and throw in strict mode. So loaded as commonjs, it will do nothing, and as a module it will be an uncaught exception.

>perhaps they could use some special comment at the top to let Node know it should be treated as a module even though it doesn't look like one.

This was discussed, but one of our goals is to be able to know what type of file something is before reading its contents. our ESM implementation, unlike CJS, doesn't assume that everything it uses comes from the local fs.

Right. In other words, if you `import` a file with that line, it should crash. If you `require` the file, it will do nothing. The context can determine the file type unambiguously, and you don't need an extension.

> This was discussed, but one of our goals is to be able to know what type of file something is before reading its contents.

The file extension came from an earlier plan where `require` and `import` could be used on both types of Javascript. Now that the plan uses `import` only for modules and `require` only for scripts, you can get rid of the extension.

> our ESM implementation, unlike CJS, doesn't assume that everything it uses comes from the local fs.

Node is `import`ing from HTTP, it can use content-type like browsers do. This is only a problem for local file systems, which don't have content-type. For that, you can just assume everything is a module. If people need to load CJS, they can use `require` (as the proposal above already says).

Bare import specifiers like `import { foo } from 'lodash'` aren't URI's, so you don't need to maintain compatibility with the browser. For this, you can just use the existing [package.json/module](https://github.com/rollup/rollup/wiki/pkg.module) field to determine whether to enter the ESM world or the CJS world.

I know Node rejected the package.json solution back when they were trying to make `import` and `require` interchangeable, but now that they aren't, I think the original performance worries should no longer apply.

>In other words, if you `import` a file with that line, it should crash. If you `require` the file, it will do nothing

This is the situation we don't want to be in, where the consumer is choosing. The consumer doesn't know. Even if you, a human, read the file, you don't know what the author's intention was.

> you can just use the existing pkg.module field to determine whether to enter the ESM world or the CJS world.

We really can't though, because its already full of files that are intended to be used with rollup/babel/etc, which have incomplete implementations of esm.

I disagree. Knowing what format a module is written in is no different than knowing it's API. You can't blindly start using a module without knowing stuff about it and how it's intended to be used

Node disagrees. You can `require('./a')` without knowing if its `a.json` or `a.js` or `a.node`. All you need to know about `./a` is the interface it exposes.

I wouldn't miss this feature, actually

Right, that's an antifeature that destroys performance for no real gain (packages are the only instance where the file name is abstracted from the importer).

> This is the situation we don't want to be in, where the consumer is choosing. The consumer doesn't know. Even if you, a human, read the file, you don't know what the author's intention was.

If I am importing a file URI, then I am the author, and I know what it is. File URI's are primarily used for linking together stuff in the same git tree. If I am using third-party code, it will be a bare module import specifier from node_modules, not a file URI.

I agree that there are weird situations where this isn't the case, but you don't need to worry about those. Why? Because module support in Node is a new feature, and therefore opt-in. The people who choose to opt-in know what they are doing, and are probably running a pure ESM codebase already. The goal should be to make something useful to the 99%, and let the other 1% keep doing it the old way.

If you are already using Rollup, then this will be plug & play. Rollup already assumes that everything you `import` is a module, and puts them in strict mode.

> We really can't though, because its already full of files that are intended to be used with rollup/babel/etc, which have incomplete implementations of esm.

That's a legitimate concern. Even if using "module" itself is too risky, but the approach itself seems solid. If I want to publish something to NPM that supports native ESM in Node, it's easy for me to add something to package.json that indicates my support, without renaming all my files.

My packages on NPM have both CJS and ESM entry points. The ESM entry point is for tools like Webpack & Rollup, which do not understand the .mjs extension. I tried using the .mjs file extension for this, and it just broke things. There is no way we can re-write every Webpack and Rollup config file in existence to be compatible with this.

Even in the same project, you might have multiple people working. The specifics end up not mattering, because the goal here is to always be unambiguous by default. If you want to have your own setup in your project, you can use a loader resolve hook and come up with your own rules.

Assuming that everything you import is esm is doable, but we want cjs to continue being a first-ish-class citizen, since so much code uses it.

Right now you can opt into a dual mode package by having a file.js and a file.mjs and setting main to `./file` without an extension, and it will "just work". You can also have index.js and index.mjs without a main field in package.json.

This disagrees with my experience working in this space every single day.

Most projects in the Node ecosystem will continue to use CJS. If you are writing "Node software", like web servers, there is no reason not to just use CJS. It will remain a well-supported and well-used option for as long as Node stays relevant. CJS stays a "first-class citizen" by not changing anything, so you get your wish by default.

On the other hand, activating Node ESM support is a conscious choice. This will typically happen on a project-by-project basis. When I say "project", I mean a single GIT repository, a single NPM module, a single Web site, or something like that. If your organization is big enough to have multiple teams (mine is), each project typically has its own team.

The interfaces between these projects are usually package.json files. This is true even for giant code-bases like Babel that live in a single GIT repo but have dozens of tiny sub-projects. Within a project, people use local file references like `import('./util/foo.js')`. Between projects, people use bare import specifiers like `import('lodash')` or `import('@babel/parser')`.

So, I think it's safe to say that a team decides to switch on ESM support, they know what is going on within their own little world. Their `import` statements can be ESM-only for local files, and they will have zero trouble. Meanwhile, if bare module specifiers like `import('lodash')` rely on package.json metadata, nobody needs to care what the other teams are doing. Any project can enable / disable ESM support at any time, and nobody will know the difference.

This all goes back to an old organizational principle that the boundaries within a company often manifest themselves as boundaries within the software architecture. This has to do with the communication overhead of coordinating changes between teams. It's easier to codify the organization structure into a versioned interface than to face the chaos of just "winging it". In the Node ecosystem, this translates practically into versioned package.json files and bare module specifiers. If your requirements don't take this into account, then I am genuinely curious in hearing what your workplace culture is like.

If it's not needed, then I'm also questioning it. Requiring that extension breaks a lot of previously "very nice" things.

Being able to create a file like `thing.js` and require it by doing `require('./thing')` made it easy to be able to switch that out with a directory later and put an `index.js` in there without having to change any import statements. That is something i've used countless times and it tends to really improve my development experience since modules can grow more naturally and it avoids lots of one-file directories, and with the `.mjs` it's entirely gone.

The .mjs extension isn't required in browsers, but absolute paths are.

So even if the .mjs extension requirement is dropped, I don't think the implication is that './foo' can successfully resolve to './foo/index.js' as there is no such thing as a directory in HTTP and you don't want browsers to make multiple guesses for which path exactly exists.

Isn't the absolute path (not exactly true, since dots are supported) thing just a matter of punting till later? If I recall correctly, they couldn't agree on resolution rules and perhaps more specifically how node-like rules would translate to the web, so they punted and went for an "absolute or bust" kind of approach till someone comes up with a better idea rather than painting everyone into a corner. Mapping identifiers and path resolving functions have been suggested as ways to make identifiers "nicer" I think.

This seems reasonable. I reckon the reason why node's behind on figuring out how to make ESM work is because of all this implicit loading magic, and a reluctance to break existing code. (A commendable goal I think.)

> you don't want browsers to make multiple guesses

The webserver does that work.

But that would mean that every single webserver would have to implement that logic. Consistently.

Except no? Only servers where stuff making requests expects the server to behave that way. The interesting thing about this is that every server in the world already works like this: they expect the person calling the server to know how the server works.

You know what, I had completely forgotten about that, and it completely changed my view.

Sure, we can have webservers serve up different thing based on parsing the url (a GET for `/users` could return `/users/index.mjs` if it wants), but having that required by the spec would be a mess, and not having it in the spec but still allowing it in node would cause annoying bugs when trying to use a library on both sides.

This is a pretty ubiquitous web server behaviour already.

If you link to /foo and it happens to be a directory, the server redirects you to /foo/. It then looks within the corresponding directory for an index file and serves that.

Most web servers have this as the default configuration, but the default list of potential index files doesn't include index.js (it's usually just index.html or index.html index.htm default.htm etc). It's usually a one line change to include index.js. This has worked fine for decades; there's no need for including this in the JavaScript spec. – it's up to whoever wants their server to respond that way to configure it appropriately.

But if node.js were to use a different default of how to do filesystem lookups (look for foo.js, then fallback to foo/index.js if not found), then you'd start seeing modules which wouldn't work on the web without the web server setup to use that exact same system. That's now an implicit contract.

This is in all honesty a small problem in the grand scheme of things, and it's one that can be equally solved by better editor tooling (for auto-refactor filenames abilities that also change all imports, since imports are for the most part statically analyzable now)

> But if node.js were to use a different default of how to do filesystem lookups (look for foo.js, then fallback to foo/index.js if not found)

They won't do that because they are clearly intent on having code work between environments. They repeatedly tell people this whenever people complain about .mjs. So we can discount this as a possibility.

> you'd start seeing modules which wouldn't work on the web without the web server setup to use that exact same system. That's now an implicit contract.

The inverse is true for their .mjs solution, which is essentially "Use Node's .mjs on the web or shared code will break in Node". This requires adopting the Node naming convention on the web and altering web servers to serve .mjs files as application/javascript. The .mjs file extension isn't a standard and doesn't make sense on the web; ESM modules are both of those things. If you think changing web server configurations for an implicit contract is not acceptable, you should be against .mjs.

It isn't required when importing something, its required for the filename. You can continue to do `import './thing'` where you have `thing.mjs`.

That's not what the linked article says, and even if it is true, it still won't do a lookup for `index.mjs` inside the `thing` folder, so it still hurts that usecase.

It sounds to me like it is a temporary holdover from the earlier plan to speed up the implementation of Phase 1. The idea is that it's better not to add the complexity to parsing while the minimal kernel is figured out, and then layer it on in Phase 2 once the most important bits are working.

I do find it a little annoying personally, and it will hurt my personal adoption slightly...but I understand why they would tackle it in that order.

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