CommonJS modules are transformed twice on the registerHooks() path #820

On the module.registerHooks() path (Node.js ≥ 26.2), every CommonJS module that reaches the ESM loader is transformed twice: once by the load hook, whose output is then discarded, and again by the pirates hook.

This covers the entry point in a CommonJS package (because of the --import, Node.js loads it through the ESM loader) and every import/import() of a .cts file or of a .ts/.js file in a CommonJS package.

Where it happens

1. The synchronous default load (defaultLoadSync) always reads the file and hands its source to the hook. Unlike the asynchronous default load behind module.register(), it does not return null for CommonJS.
2. load in register.mjs passes that nextLoad straight to oxcLoad (register.mjs#L242). Because the source is present, transform_output takes the source-bearing branch and runs the full transform, codegen, and inline source map (src/lib.rs#L1611-L1685).
3. The hook then throws that output away for CommonJS and returns source: null (register.mjs#L253-L258).
4. Node.js' CommonJS loader reads the file again, and the pirates hook transforms it a second time (register.mjs#L148).

Measurement

The file is a CommonJS .ts entry of 1.8 MB (12,000 interfaces and functions, no import/export), in a package without "type". Results are on Node.js 26.5.0, macOS arm64, as the median of 15 runs:

The output is identical. The saving grows with the size and number of CommonJS modules.

The second row registers this hook before register.mjs, so it runs after it, directly above Node.js' default load:

// null-cjs-source.mjs
import { registerHooks } from 'node:module';

registerHooks({
  load(url, context, nextLoad) {
    const result = nextLoad(url, context);
    return result.format === 'commonjs' ? { ...result, source: null } : result;
  },
});

$ node --import ./null-cjs-source.mjs --import @oxc-node/core/register main.ts

Suggested fix

Do the same inside load: wrap nextLoad so a CommonJS result reaches oxcLoad without source.

function load(url, context, nextLoad) {
  if (isCommonJsRequire(context)) {
    return nextLoad(url, context);
  }
  const result = oxcLoad(
    url,
    context,
    (nextUrl, nextContext) => {
      const loaded = nextLoad(nextUrl, nextContext);
      return loaded.format === "commonjs" ? { ...loaded, source: null } : loaded;
    },
    HELPER_MODULE_NAME,
  );
  // ...unchanged
}

transform_output then takes the None branch, which is what the module.register() path already does. load_commonjs_esm only parses the file to detect ES module syntax, transforms it only when it flips to an ES module, and rejects a .cts with module syntax as it does today. Everything else is left to the CommonJS loader and pirates, as now.

commonjs-typescript, the format Node.js reports for a .ts file it strips itself, is not commonjs, so it keeps its source, as that translator requires.

Vite+ (voidzero-dev/vite-plus#2936) vendors oxc-node and mirrors register.mjs, so it has the same behavior. We'd rather fix it here and sync than diverge.