ESM loader cannot find a module detected by rollup

[pravi]

• Version: v12.7
• Platform: linux
• Subsystem:

With the following test code (test.mjs),

import { encode } from 'sourcemap-codec';
 console.log(encode);

And rollup.config.js

import resolve from 'rollup-plugin-node-resolve';
const plugins = [ resolve(), ];
export default [                                        /* umd */                                       
{                                                       input: 'test.mjs',                              
plugins,                                        
output: {                                               
file: 'bundle.js',                              
format: 'umd',                                  
name: 'Test',                                   
sourcemap: true                         
}
        
}
];

Run npm install sourcemap-codec

Then rollup -c

test.mjs → bundle.js...

created bundle.js in 41ms

There are no warnings and

$ node bundle.js
[Function: encode]
$

But

$ node --experimental-modules test.mjs                   
(node:26761) ExperimentalWarning: The ESM module loader is experimental.
file:///home/praveen/forge/test-rollup/test.mjs:1

import { encode } from 'sourcemap-codec';
         ^^^^^^                                 SyntaxError: 
The requested module 'sourcemap-codec' 
does not provide an export named 'encode'
 
   at ModuleJob._instantiate 
(internal/modules/esm/module_job.js:93:21)                          
  at async ModuleJob.run 
(internal/modules/esm/module_job.js:108:20)                             
 at async Loader.import 
(internal/modules/esm/loader.js:134:24)

[himself65]

try add this to rollup config
external: ['sourcemap-codec']
or edit format to cjs

and this is not wrong with nodejs

[devsnek]

once could make a loader hook for node which duplicates whatever the behaviour of rollup is, but node core itself isn't trying to match any specific bundlers.

[pravi]

@devsnek aren't both rollup and node implementing the same standard? Do you think rollup implementation is wrong?

[devsnek]

@pravi i don't know quite how rollup's works, but there are a few things to note here. Firstly, esm doesn't specify how implementations perform resolution at all, so node and rollup can have completely different ways of deciding if a file is an es module and still be standards compliant. Looking at the sourcemap-codec package, it is likely that rollup is using the "module" field of sourcemap-codec's package.json. However, many usages of the "module" field in package.json have been found to be incompatible with spec compliant esm (there's a long story here about babel handling graph validation incorrectly, but not very relevant at this point). Because of this, node does not use the "module" field at all, so it will end up resolving the umd bundle that sourcemap-codec ships, which is decidedly not esm.

[pravi]

@devsnek thanks for the clarification, I have opened an issue with sourcemap-codec.

[devsnek]

@pravi i would recommend against asking random packages to make themselves work with node's esm system until it is stable. we might change everything about it, and then the packages will still be incompatible.

[pravi]

@devsnek OK, I will stop with this issue for now. Is there a list of blockers or a roadmap for this to be declared stable?

[devsnek]

more or less https://github.com/nodejs/modules/issues

[pravi]

But if they add "module": " es6-entry.mjs", and other modules explicitly calling "module/es6-entry.mjs", won't it work for both loaders?

[PoojaDurgad]

@pravi , is this still an issue ?

[gireeshpunathil]

inactive, closing