Request: Add portable ESM build target

[Jamesernator]

💡 Feature description

Currently there's 3 main build targets all that require different platforms, however there is no module target that is platform independent.

I'd like to propose adding a new target --target module that generates an ESM module similar to web but does not include any web specific things like fetch/Request/etc that make it unportable to Node (once it supports ESM) or other environments in future.

In order to be as environment independent as possible I would propose outputting a module that imports no other modules (so that it doesn't tie down to specifier dependency) and also only rely on ecma262 builtin functions + WebAssembly globals.

In order to do this I would propose that this target would output a file similar to the current --target web but would instead of fetching the module itself in init it could instead just be passed a WebAssembly.Module directly and then the init function could then use the WebAssembly.instantiate function to instantiate it.

💻 Basic example

Using such a module from the browser:

import init, { method1, method2 } from './file.js';

async function main() {
  const response = await fetch('./file_bg.wasm')
  await init(await WebAssembly.compileStreaming(response))

  method1('blah')
}

Or using such a module in Node (under experimental modules):

import init, { method1, method2 } from './file.js'

async function main() {
  const module = await WebAssembly.compile(await fs.promises.readFile('./file_bg.wasm'))
  await init(module)

  method1('blah')
}

[fitzgen]

Sort of related is the idea of --target all which would generate a package that is portable, but would generate a copy for each different target rather than a single js glue file that supports everything.

#313

[ibaryshnikov]

I did some research, and found a surprising thing: it works almost as is with --target web
@Jamesernator everything as in your example, except you can't do import init from 'file.js' in node, you should use .mjs extension. Having both main.mjs and pkg/file.mjs, I ran

node --experimental-modules main.mjs

and got ReferenceError: Request is not defined
Ok, I went to file.mjs and changed

if (module instanceof URL || typeof module === 'string' || module instanceof Request) {

to

if (module instanceof URL || typeof module === 'string') {

and it worked!
I didn't even have to remove const response = fetch(module); from the file.mjs!

So we almost have this feature... almost, because of .mjs stuff

[Jamesernator]

except you can't do import init from 'file.js' in node, you should use .mjs extension.

Actually importing .js in Node is supported via the new package.json "type" field: https://nodejs.org/api/esm.html#esm_code_package_json_code_code_type_code_field

Regardless a portable module doesn't even necessarily need to know anything about that, it just needs to have some source text with the right exports.

---

If module instanceof Request is the only thing preventing the output being portable then could the output just be changed to?:

if (typeof module === 'string'
/* Node supports URL but not all hosts necessarily will e.g. XS */
|| typeof URL !== 'undefined' && module instanceof URL
|| typeof Request !== 'undefined' && module instanceof Request) {

[alexcrichton]

@Jamesernator I think that module instanceof Request is now expanded basically as you mentioned in the most recent version of wasm-bindgen

I think that means that this is actually fixed, so I'm going to close this.