I tested the hook in a Typescript project, using Typescript 5.7 (w/tsx), Mocha v.10, Sinon v.17, and most important Chai v.5. Many old CommonJS projects are barred from moving to Chai v.5 if they use nyc for coverage, because Chai v.5 is ESM only. For me moving to c8 is a no-no, because there are cases in which the coverage is not as accurate as istanbul/nyc (tbh I don't remember the exact scenarios, I tested it a while ago).

My working configuration is as follows.

• "type" : "module" in package.json
• Mocha config in package.json:

  • "mocha": {
      "require": [
        "./test/register-loader.js",  // this would just register `esm-loader-hook`, see my snippet below.
        "tsx"
      ],
      "color": true,
      "full-trace": true,
      "bail": true,
      "spec": [
        "test/**/*.test.ts"
      ],
      "enable-source-maps": true
    }

• nyc config in package.json:

  • "nyc": {
      "include": [
        "src/**/*.ts"
      ],
      "reporter": [
        "text-summary",
        "lcov"
      ],
      "sourceMap": true,
      "instrument": true,
      "check-coverage": true,
      "lines": 100,
      "statements": 100,
      "functions": 100,
      "branches": 100
    }

• ./test/register-loader.js

  • import { register } from "node:module"; 
    import { pathToFileURL } from "node:url"; 
    register("./test/esm-loader-hook-edited.js", pathToFileURL("./"));  // this should point to the library once this PR is merged

• added "allowSyntheticDefaultImports": true, in tsconfig.json for babel compatibility

With the above, just running npx nyc mocha gives me the correct coverage output, and works with decorators as well.