import.meta.main

[MylesBorins]

Deno just added this.

Should we?

denoland/deno#1835

[ljharb]

I'm a bit confused what it does - does it mean "it was the main in package.json"? it was the primary entry point? what about import()?

[devsnek]

it's true if the file was the entrypoint. for node i'd rather it be called something else cuz the term main already has package-scoped meaning.

[ljharb]

Right, but anything that's import()ed is also an entry point, and I'm very unclear on why you care what that value is. Currently you can look at process.argv and __filename to determine that (and import.meta.filename or similar is quite well motivated).

Also yes, main is a terrible name for multiple reasons, not the least of which is that it's a package.json field.

[guybedford]

I really like the import.meta.main concept. Currently in Node.js it is common for CLI tools to do the following check:

if (require.main === module) {
  console.log('This is a CLI tool! Usage: ...');
}

We currently have no version of the above check in ECMAScript modules, so this is a problem that comes up, and there is currently no easy way to do this without it feeling like a hack.

As an already-paved use case, users are familiar with the main terminology in this context already.

The above check then becomes:

if (import.meta.main) {
  console.log('This is a CLI tool! Usage: ...');
}

and the really nice thing about this pattern is that it works in browsers and other environments completely fine without any need for compatibility API layers or compilation.

For these reasons I'm 100% supportive of import.meta.main.

[ljharb]

Doesn’t that use case predate the current community preference, which is to have separate packages for a library and a CLI?

[guybedford]

@ljharb this use case is explicitly documented in https://github.com/nodejs/node/blob/master/doc/api/modules.md#accessing-the-main-module.

[ljharb]

Sure but so is every aspect of the require algorithm :-) the better question is, is it still a common or desired use case to determine if something is the entry point, and how does that change now that dynamic import allows multiple entry points?

[guybedford]

@ljharb yes it is a desired feature. The require.main === module was something we had to explicitly support in the ncc project as users were using it in CLI tools. This came up pretty early in the project and had multiple users asking about it here - vercel/ncc#224. Dynamic import does not affect this, as what we are distinguishing is the CLI entry point, not the module graph entry point.

[ljharb]

The feature seems fine then (it’d only be true in the top-level process entry point); the name, definitely not (but we can bikeshed that).

[bmeck]

Does this absolutely need to be in our first iteration or can we get feedback / focus on other things?

[zenparsing]

Thanks for bringing this up @MylesBorins. I (think) we can say this is post-MVP but it's good to have on our radar.

[MylesBorins]

can definitely be post MVP. Just wanted to document what's going on in ecosystem

[shian15810]

Well, I'm not part of the organization, but as a cli maintainer, this proposal seems so interesting.

Can I start to try implementing this in https://github.com/nodejs/node and possibly a PR now?

[devsnek]

I'm not a huge fan of the concept of main. imo you should have separate files for bin and lib, which is something other languages do just fine, and installers like npm and yarn already let you specify separate bins.

[guybedford]

@shian15810 thanks for showing an interest in the modules work! Contributions are very much welcome to Node.js core and the module work here.

We do have some consensus issues for this feature due to our not wanting to provide unnecessary features without seeing a strong need for them in the initial modules implementation. import.meta is a very widely used namespace (every module!) with strong backwards compatibility needs, so we do also need to be very cautious about what we put on it.

A PR to Node.js core would certainly drive discussion, and may even sway consensus. Also hearing more about your use case as a CLI maintainer and how this is useful could help us better understand the importance of the feature to you (including for example the points raised by Gus above). Even if the PR sits without approval, we may be able to come back to it in a few months even as well.

[shian15810]

The use case of mine is that both bin and main fields in package.json point to the same file. Using require.main === module, as mentioned in the official docs here, to tell whether the script has been run directly or not, so that to know if the main function should be executed or exported.

As far as I can tell, with a package.json having "type": "commonjs", there are three ways to tell if a file is run directly (aka not being required):

1. require.main === module or process.mainModule === module
2. require.main.filename === __filename or process.mainModule.filename === __filename
3. !module.parent

However, with a package.json having "type": "module", require and module are not available, as well as process.mainModule is undefined.

What's left is just import.meta.url, but the problem is what to compare with to know if it is the main script.

In addition, according to this, module.parent in a cjs script will be undefined if the parent is a mjs script. After some testing, require.main and process.mainModule are both undefined too in this context.

In my imagination, if import.meta.main is implemented, I could use it in mjs script, while retaining require.main === module in cjs script. But this still pose a serious problem to a project with mixed cjs and mjs scripts importing each other, as mentioned above.

So yeah, I agree with @devsnek in this case, that is pointing bin and main to separate scripts in the first place, which I now think is the right way to solve this problem.

Also, I agree with @guybedford regarding strong backwards compatibility, even though TC39 states that import.meta object will be extensible.

As a last note, I've read somewhere in this repo stating that in the context of esmodule, there is no concept of main such as in commonjs, correct me if I'm wrong. But I certainly think that there must be some other use cases of determining the main script, just like require.main === module in commonjs. It is good to have import.meta.main as the counterpart in esmodule, though should it be named main is pretty much debatable.

[MylesBorins]

Going to abandon this for now, please feel free to reopen

[guybedford]

I still believe this is an important feature, to be put in the same basket as CJS features not available in ESM like require.resolve.

It's not a common feature, but it is definitely a heavily ingrained one where it is used, and has a small collection of users that absolutely do expect this functionality (mainly CLI developers).

[devsnek]

i am still against the concept of a main module in general.

[SMotaal]

@guybedford Which labels are relevant here? Can you add them to make it easier to related to the context please — ie cjs anr/or esm... etc.

[SMotaal]

@devsnek Can you elaborate more? Main here is the main entrypoint, so it is like window.location where there is a Window context or self.location where there is not (workers... etc.) but that does not apply the same way in CommonJS (ie not URLs).

Aside — I don't think adding a global for ESM only is a good idea personally, and otherwise neutral to import.meta.main.

[SMotaal]

@guybedford Awesome, that really helps 💯

[devsnek]

@SMotaal i dislike the idea of differentiating entrypoints. you can just use a separate file for your bin.

[devsnek]

being an entrypoint doesn't inherently mean it's a cli (for example, cf workers). additionally, some hosts like browsers don't map well to the concept of entrypoints. depending on how you think about it, a webpage may either have several or no entrypoints. Even if you choose to think about a browser having several instead of none, none of the several would be a cli.

[SMotaal]

@SMotaal i dislike the idea of differentiating entrypoints. you can just use a separate file for your bin.

@devsnek certainly it is one style, I am not saying your style is wrong, but others also have justifications that cannot be open to judgement because this all comes down to matters of opinion.

As it happens, any given module in a package can be called as a result of at least two paths, a main entrypoint in the package, or an entrypoint for which the package is a direct or indirect dependency… some packages may actually consider this a functional parameter depending on their purpose.

depending on how you think about it

Yes, I think this is the bottom line… And, so no disagreement or disapproval is intended, just wanting us to consider the more diverse landscapes that we may not ourselves be interested in (yet).

[guybedford]

To clarify some of the above, the only module that gets import.meta.main set is the module passed into node main.js it is only the module corresponding to process.argv[1]. And no, it doesn't apply to browsers.

This is the Node.js application entry point main, not package entry points, or realm / worker entry points.

An equivalent way to achieve this check would be to do path.resolve(process.argv[1]) === fileURLToPath(import.meta.url).

Perhaps the above explicit check would be enough though.

[aduh95]

Since Node 20.11.0:

if ( import.meta.filename === process?.argv[1] ) {
  // Node main script
}

Another limitation of this is it would return false positive if the same module is loaded twice (e.g. entry point is file:///module.js and file:///module.js?notEntryPoint is imported later).

Would it make more sense to expose this as something like process.cliEntrypoint or process.cliMain so that the same API can be used everywhere?

What would be the value of process.cliEntryPoint? How do you envision using it?

[egasimus]

🦗 🦗 🦗

So what's wrong with using the default export of the file passed to node as the entrypoint?

[aduh95]

@egasimus you can probably find tons of modules that have been written with a default export which they don't expect to be run when module is the entrypoint. #32223 (comment) suggested using a named main export for that, which seems less dangerous than using the default one. In any case, it is not going to happen until someone opens a PR implementing it.

[nex3]

What would be the value of process.cliEntryPoint? How do you envision using it?

The value would be the same as import.meta.main as proposed here, plus the ability to access it consistently in any module whether it's CJS or ESM.

[aduh95]

What would be the value of process.cliEntryPoint? How do you envision using it?

The value would be the same as import.meta.main as proposed here, plus the ability to access it consistently in any module whether it's CJS or ESM.

The glaring difference is that import.meta is module-specific (each module receives a different import.meta object), while a global object such as process is the same for all modules (and non-modules) on the same realm, so we couldn't pass a boolean value there.

[nex3]

I see, I misunderstood the intended meaning of import.meta.main since there was a considerable amount of discussion of process.argv[1]. I'll open a separate issue (#51840).

[sosoba]

Welcome in 2024

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[arslivinski]

I read all the comments but I can't understand why this issue is still open after 3 years but it's not going anywhere.

Are you still stuck deciding whether you want this feature at all, or what else is blocking this? After longer than three years and a hundred comments I think it's time to decide: please close the issue if you really don't want it.

As @Beedeebee said almost 3 years ago, can we have an official position if this is going forward or not?

I see that the problem is that no one wants to commit to decide and being responsible if the outcome goes sideways. That will always be the problem with decisions by committee. However, as both Deno an Bun have import.meta.main, it's only logical to just behave the same.

Otherwise, we could just close this issue and discuss this on WinterTC (TC55) wintercg/proposal-minimum-common-api#46 probably for more 3 years.

[iki]

Would be great to have a proper solution, because it should be easy to use, but it's not as simple to even get it right currently.

For example, es-main works ok, but is-main does not.

import { createRequire } from 'node:module'
import { fileURLToPath } from 'node:url'

const extRegExp = /\.[^./\\]*$/
const stripExt = (path: string) => path.replace(extRegExp, '')
export const isMain = (meta: ImportMeta, cmd = process.argv[1]): boolean =>
  // Unlike path.resolve, require.resolve returns a real path,
  // so it is compatible with package bins in node_modules/.bin,
  // e.g. when running via npx/pnpx.
  !!meta && !!cmd && stripExt(fileURLToPath(meta.url)) === stripExt(createRequire(meta.url).resolve(cmd))

And yes, having import.meta.main compatible with Deno and Bun would be optimal, IMO.

[voxpelli]

Should be reopened, but not sure one can transfer ownership of this issue though? As @MylesBorins seems to wants all of his GitHub issues closed now I guess that should also be respected?

[MylesBorins]

If you want to work on making this happen I'd say open another issue and reference this one. GitHubs design makes any open issues I've created a never ending Todo list / noise.

Think of it like forking 😄

[rivy]

Understandable.
It has been 5 years.
I also close my long standing, "non-resolving" issues to clear my "to-do" list periodically.

[rhyek]

Ugh, considering moving back from bun for my scripts and thought most of the friction would be gone after the release of --experimental-strip-types, but hit this immediately.