Node.js with esm and app-module-path (using mocha as test runner)

Node.js with `esm` and `app-module-path`

This is a short post on how to use ESM with app-module-path in a Node.js (6+) project.

Q: Why?

Lots of benifits, easier to export/import modules.

Instead of:

const foo = require('app/foo');

module.exports = { foo, bar: 2 };

We can now use:

import foo from 'app/foo';

export default { foo, bar: 2 };

And we don't need to do:

exports = module.exports = Foo;

exports.bar = 'bar';

We can just:

export default Foo;

export const bar = 'bar';

Q: Why not just update to Node 9+ ?

A1: Not possible to run anything higher than Node 8.2 on AWS Lambda.
A2: Perhaps don't wan't/can't start Node process with flag --experimental-modules

Install `esm` and setup with `app-module-path`

# install esm
npm i esm -S

Add a new entry point (entry.js):

// Entry point for our application.

// Setup esm
require = require('esm')(module);
// Setup app-module-path
require('app-module-path/register');

module.exports = require('./old-entry-point');

Migration

The app with require calls and module.exports will continue to work, but here is a shortcut for how to convert to import and export. Of course, manual work is also going to be needed.

Replace `require` with `import`

(VS Code is used as editor, don't know how to search/replace in other editors)

In VS Code RegExp Search Replace:

Search: const\s+(\w+)\s+=\s+require$'([^']+)'$;?
Replace: import $1 from '$2';

Replace `module.exports` with `export default`

Search: module.exports =
Replace: export default

Replace `exports.<variable> =` with `export <variable>`

Search: exports.(\w+)\s+=\s+([^']+)?
Replace: export $1 $2

Manually replace destructing require

-const { sleep } = require('src/utils/promise-utils');
+import { sleep } from 'src/utils/promise-utils';

-const { expect } = require('chai');
+import { expect } from 'chai';

Mocha

To be able to setup esm with mocha, it was not enough to just --require esm in mocha.opts or try to initialize esm in the test/bootstrap.js file.

Had to change the test script in package.json from:

-"test": "_mocha --recursive",
+"test": "node -r esm node_modules/.bin/_mocha --recursive",

to make it work.

Links

https://medium.com/web-on-the-edge/es-modules-in-node-today-32cff914e4b - ESM release post
https://nodejs.org/api/esm.html#esm_enabling
https://github.com/standard-things/esm
https://github.com/patrick-steele-idem/app-module-path-node

nekman/node-esm.md