Shared library development with ts-node

When developing a large project, it can be useful to put some of the code into a shared library and use local file paths to load it. That can be harder than it sounds with Typescript so here I will share how I did it with much trial and error.

Project set up

My project is fairly simple, but this approach will work if you have a more complex project with lots of libraries and applications.

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ProjectRoot
 --> mylibrary
        --> package.json
        --> src/
 --> myproject
        --> package.json
        --> src

The mylibaray project has the name @andrew/mylibrary and it contains a function called reallyGood. I need to import and call that function from myproject.

Here is the code for myproject.

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// src/index.ts
import {reallyGood} from '@andrew/mylibrary';

function run() {
    const outcome = reallyGood();

    console.log(outcome);
}

run();

I am using ts-node to run it locally. The problem is that when I run it I get this error:

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Error: Cannot find module '@anndrew/mylibrary'

The problem is that ts-node has no idea where to find a module called @andrew/mylibrary.

Adding paths to tsconfig.json

In myproject I can open the tsconfig.json and manually provide the path.

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{
    "compilerOptions": {
        "paths": {
            "@andrew/mylibrary": ["../mylibrary"]
        }
    }
}

This will tell the build process where to find the import. It’s not ideal if you want to push versions of your library to a repository, but for local development, it works fine. That allows me to build the code with tsc but it will still fail when run with ts-node. The problem is that ts-node does not respect the paths object by default.

Making ts-node use paths

To make ts-node use the paths object from the compiler, you need to add some additional configuration. First install the tsconfig-paths library in your project.

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npm i -D tsconfig-paths

Then register it for ts-node in the tsconfig.json file.

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{
  "ts-node": {
    "require": ["tsconfig-paths/register"]
  },
  "compilerOptions": {
    // paths etc.
  }
}

Now ts-node will use the paths and everything should work.

Summary

Building local libraries with ts-node is a great way to get code working quickly but some of the error messages are not very clear. This took me a while to look up and get working, so hopefully, it will help you too.