Use typescript only library

Question

I'm trying to publish and consume typescript only library(w/o dist folder with compiled .js files)

I created simple library:

src/test/index.ts:

export const test = 42;

src/index.ts:

import {test} from "./test";

export {
  test
};

package.json:

{
  "name": "test",
  "version": "1.0.0",
  "type": "module",
  "main": "src/index.ts"
}

and published it to local npm registry.

Next, I created simple app where I installed this library as dependency.

src/index.ts:

import {test} from "test"

console.log(test);

package.json:

{
  "name": "foo",
  "version": "1.0.0",
  "type": "module",
  "main": "src/index.ts",
  "scripts": {
    "start": "node --experimental-specifier-resolution node --loader ts-node/esm src/index.ts"
  },
  "dependencies": {
    "test": "^1.0.0",
    "ts-node": "^10.4.0"
  }
}

tsconfig.json:

{
  "compilerOptions": {
    "module": "esnext",
    "moduleResolution": "node",
    "esModuleInterop": true,
    "target": "esnext",
    "sourceMap": true
  }
}

When I run it via npm run start I got TypeError [ERR_INVALID_RETURN_PROPERTY_VALUE]: Expected string to be returned for the "format" from the "loader getFormat" function but got type object.

Denis · Accepted Answer · 2021-11-01 13:39:05Z

1

Why it happens

The issue is that ts-node won't transpile your node_modules folder.

Solution

So you need to explicitly tell ts-node to handle your lib. In your case you have to change your start command in the root of the project from

node --experimental-specifier-resolution node --loader ts-node/esm src/index.ts

to something like (to transpile ALL node_modules. Be careful, it's kinda bad practice)

set TS_NODE_IGNORE='' && node --experimental-specifier-resolution node --loader ts-node/esm src/index.ts

or a little bit better - with cross-env

cross-env TS_NODE_IGNORE='' node --experimental-specifier-resolution node --loader ts-node/esm src/index.ts

answered Nov 1, 2021 at 13:39

Denis

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