I couldn't figure out why ts-node isn't resolving the alias when esm is enabled

I made a tiny project trying to isolate the issue as much as possible

package.json

{
  "type": "module"
}

tsconfig.json

{
  "compilerOptions": {
    "module": "es2020",                                
    "baseUrl": "./",                                  
    "paths": {
      "$lib/*": [
        "src/lib/*"
      ]
    },
  },
  "ts-node": {
    "esm": true
  }
}

test.ts

import { testFn } from "$lib/module"

testFn()

lib/module.ts

export function testFn () {
  console.log("Test function")
}

command

ts-node -r tsconfig-paths/register src/test.ts

Here's a minimal repo

Solution from: https://github.com/TypeStrong/ts-node/discussions/1450#discussion-3563207

At the moment, the ESM loader does not handle TypeScript path mappings. To make it work you can use the following custom loader:

// loader.js
import {
  resolve as resolveTs,
  getFormat,
  transformSource,
  load,
} from "ts-node/esm";
import * as tsConfigPaths from "tsconfig-paths"

export { getFormat, transformSource, load };

const { absoluteBaseUrl, paths } = tsConfigPaths.loadConfig()
const matchPath = tsConfigPaths.createMatchPath(absoluteBaseUrl, paths)

export function resolve(specifier, context, defaultResolver) {
  const mappedSpecifier = matchPath(specifier)
  if (mappedSpecifier) {
    specifier = `${mappedSpecifier}.js`
  }
  return resolveTs(specifier, context, defaultResolver);
}

Then use the loader with: node --loader loader.js index.ts

Caveat: This only works for module specifiers without an extension. For example, import /foo/bar works, but import /foo/bar.js and import /foo/bar.ts do not.

Remember to install these packages as well:

"ts-node": "^10.9.1",
"tsconfig-paths": "^4.1.2",

Updated: 09/11/2023

The new loader will automatically resolve the index.(js|ts):

import { resolve as resolveTs } from 'ts-node/esm'
import * as tsConfigPaths from 'tsconfig-paths'
import { pathToFileURL } from 'url'

const { absoluteBaseUrl, paths } = tsConfigPaths.loadConfig()
const matchPath = tsConfigPaths.createMatchPath(absoluteBaseUrl, paths)

export function resolve (specifier, ctx, defaultResolve) {
  const match = matchPath(specifier)
  return match
    ? resolveTs(pathToFileURL(`${match}`).href, ctx, defaultResolve)
    : resolveTs(specifier, ctx, defaultResolve)
}

export { load, transformSource } from 'ts-node/esm'

Example:

Path: /src/modules/order/index.ts
Resolve: import orderModule from '@/modules/order';

Updated: 11/04/2024

For the node version 20, we need to set the arg: --experimental-specifier-resolution=node

node --experimental-specifier-resolution=node --loader ./loader.js src/main.ts