Flag for scope-hoisting in serve/watch mode

[sergey-shpak]

🐛 bug report

I'm trying to import node package into my app, package has 'main' and 'module' props in it's package.json:

"main": "dist/index.js",
"module": "src/index.js",

as result with parcel@v1 it works ok - package is imported as module,
but using parcel@next or parcel@nightly gives me following error:

@parcel/resolver-default:
"main": "dist/index.js",
>    |           ^^^^^^^^^^^^^^^^^^^^^ './dist/index.js' does not existundefined'

even though dist/index.js doesn't exist, why it's loaded instead of 'module' source?

🎛 Configuration (.babelrc, package.json, cli command)

no config

🤔 Expected Behavior

importing package as module

😯 Current Behavior

importing package main dist

[mischnic]

In development mode, "main" is used because it has to be transpired to commonjs anyway.

[sergey-shpak]

just tested and looks like parcel build works fine, it fails only with parcel watch

I had to walk through the core and found this piece of default-resolver:

// If scope hoisting is enabled, we can get smaller builds using esmodule input, so choose `module` over `main`.
// Otherwise, we'd be wasting time transforming esmodules to commonjs, so choose `main` over `module`.
if (dependency.env.scopeHoist) {
  mainFields.push('module', 'main');
} else {
  mainFields.push('main', 'module');
}

that means that scopeHoist is disabled for parcel watch as result order of entries is different (and that probably the root case of my issue)?

[sergey-shpak]

ok, the issue comes from disabled scopeHoist option (--no-scope-hoist) for watch/serve as result using 'main' package property instead of 'module'. Any explanation why es module exports cannot be scope hosted with 'watch' or 'serve'?

[mischnic]

Does the comment in the code snippet you posted still not answer your question?

Related issue: #4879

[sergey-shpak]

@mischnic it answers about scope hoisting in general and its performance, but not why scope hosting (for es 'module') is disabled by default for watch/serve and there is no option to enable it

[mischnic]

Ah, two reasons:

• it's very slow
• HMR (with scope-hoisting, individual files cannot be swapped out at runtime)

[sergey-shpak]

@mischnic ok, thanks! Anyway, it would be nice to have more transparency and control over scope hoisting, something opposite param to --no-scope-hoist, like --with-scope-hoist (default for build, and optional for watch/serve)

[devongovett]

Isn't this a bug in the package that there is a "main" field pointing to a non-existent file?

[sergey-shpak]

@devongovett it's a main problem for sure, but it has raised what we just discussed here - --no-scope-hoist disables scope hoisting by silently switching package entry point, which looks not so obvious and is not expected.

Anyway, changing it to --with-scope-hoist and giving more control to user or not, it would be nice to mention about changing entry point somewhere in the docs