NodeJS custom loader for CommonJS

Question

I'm playing with NodeJS loaders where I want to intercept all calls to require and modify the source (transpile). When using ESM modules with import it works, but with CommonJS it doesn't trigger the loader when calling require.

Here is the loader code:

async function load(url, context, nextLoad) {
  console.log({ url, context })
  return nextLoad(url)
}

async function resolve(specifier, context, nextResolve) {
  console.log({ specifier, context })
  return nextResolve(specifier)
}

module.exports = { load, resolve }

I'm injecting the loader by running node --loader=./loader.js hello.js from terminal.

And here is the CommonJS code for hello.js:

var lib = require('./lib.js')

The require('./lib.js') line neither triggers the loader resolve nor the load functions in the loader.

If I rewrite using ESM .mjs and import it works as expected.

Any ideas?

Answer 1

Loaders in Node.js for ECMAScript Modules (ESM) and CommonJS modules. The loader you have defined works with ESM imports, but it doesn't automatically handle CommonJS require calls because they use a different mechanism for module resolution and loading.

If I didn't wan't to use a build tool, I would use Node.js’s require hook mechanism; but it’s more complex and less flexible IMHO.

const Module = require('module');
const fs = require('fs');

Module._extensions['.js'] = function (module, filename) {
  const content = fs.readFileSync(filename, 'utf8');
  const transformedContent = transformContent(content); 
  module._compile(transformedContent, filename);
};

function transformContent(content) {
  // Implement your transpiling logic here
  return content;
}