Package export map & self-referencing not possible

[Knorgias]

1. Description

Hey there 👋

I want to be able to self-reference my package without lint errors while using packages eslint & eslint-plugin-import.

I have a repo tree:

.
├── foo-bar.js
├── index.js
├── node_modules
├── package-lock.json
├── package.json
└── resolver.cjs

with file contents…

index.js :

export const foo = 1;

foo-bar.js :

import { foo } from 'demo-export-map';

console.log(foo);

package.json :

{
  "name": "demo-export-map",
  "version": "0.0.1",
  "description": "",
  "main": "index.js",
  "type": "module",
  "exports": {
    ".": "./index.js"
  },
  "scripts": {
    "lint": "eslint ."
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "eslint": "^8.26.0",
    "eslint-plugin-import": "^2.26.0"
  }
}

2. Expected behaviour

npm run lint runs without errors

3. Actual behaviour

npm run lint throws error:
error Unable to resolve path to module 'demo-export-map' import/no-unresolved

You can reproduce it via:

git clone https://github.com/Knorgias/demo-export-map.git
npm i
npm run lint

Note: I also spotted this related issue, and so it seems that the browserify’s resolver doesn't support “exports” mapping & self-referencing, by default.

4. Possible solution/workaround

First, create a eslint-resolver.cjs file and add contents:

'use strict';
const { createRequire, builtinModules } = require('module');
exports.interfaceVersion = 2;
exports.resolve = function (source, file, config) {
  try {
    if (builtinModules.includes(source)) {
      return { found: true, path: null };
    }
    const myRequire = createRequire(file);
    const resolvedPath = myRequire.resolve(source);
    return { found: true, path: resolvedPath };
  } catch (err) {
    return { found: false };
  }
};

Then use it as import resolver in .eslintrc.js by adding:

settings: {
    'import/resolver': {
      [path.resolve('./scripts/eslint-resolver.cjs')]: {},
    },
  },

You may also need to import path, by adding const path = require('path'); to top of file.

You can reproduce it via:

git checkout fix/exports-resolver
npm run lint

View fix on github

Note: this fix only works in node (not in browser).

[ljharb]

This plugin uses resolve, which does not yet support exports - and thus, doesn't support self-referencing. When it does, so will we.

Duplicate of #2430.

[toFrankie]

exports field not supported?

[ljharb]

@toFrankie no, it's not supported yet.

Since packages should be using exports in a backwards-compatible way, this shouldn't be an obstacle, but if packages deviate from this best practice, it'll be annoying until resolve adds this support.

[daKmoR]

I'm curious why is it using a custom-coded resolver when you could use the "built in" resolver from node?
e.g. why use the resolve package and not require.resolve(source);?

does eslint always run via node?

[ljharb]

node's built-in resolver can't consistently be used in the way we need it; and regardless, having this plugin behave unpredictably across different node versions would be undesirable.