AVA requiring @std/esm does not work

[billyjanitsch]

Here is a minimal test repo using AVA and @std/esm. To reproduce the issue, run npm install then npm test.

lib/ contains an ESM build. index.js activates @std/esm and re-exports the default export of lib/. In package.json, module points to lib/, and main points to index.js with "@std/esm": "cjs" for CJS compat. As I understand it, this is a standard setup.

AVA is configured to require @std/esm, and to inherit the Babel config in .babelrc to transpile its test files (which would be necessary, for instance, if the test files used JSX). This config does not transpile ESM -> CJS. However, npm test fails:

/Users/billy/work/ava-esm/test.js:1
(function (exports, require, module, __filename, __dirname) { import test from 'ava';
                                                              ^^^^^^

SyntaxError: Unexpected token import
    at createScript (vm.js:80:10)
    at Object.runInThisContext (vm.js:139:10)
    at Module._compile (module.js:599:28)
    at extensions.(anonymous function) (/Users/billy/work/ava-esm/node_modules/require-precompiled/index.js:13:11)
    at Object.require.extensions.(anonymous function) [as .js] (/Users/billy/work/ava-esm/node_modules/ava/lib/process-adapter.js:100:4)
    at Module.load (module.js:554:32)
    at tryModuleLoad (module.js:497:12)
    at Function.Module._load (module.js:489:3)
    at Module.require (module.js:579:17)
    at require (internal/module.js:11:18)

This suggests that @std/esm is not working correctly on the test file.

If you remove the line which tells AVA to use .babelrc, npm test passes because AVA falls back to its default Babel config which transpiles ESM -> CJS.

[jdalton]

Thanks @billyjanitsch!

Our scenario test missed the "inherit" option. It looks like ava/lib/process-adapter.js is the issue:

const precompiled = opts.precompiled[filename];

if (precompiled) {
  sourceMapCache.set(filename, path.join(cacheDir, `${precompiled}.js.map`));
  return fs.readFileSync(path.join(cacheDir, `${precompiled}.js`), 'utf8');
}

If precompiled was null then it would allow @std/esm to be used... but as it is, it gets trapped by the precompiled block. The --no-cache option and config equiv has no affect either. \cc @motss.

It doesn't look like there is a way to disable this precompilation of ava at the moment.
Removed ava from the tips section for now.

[billyjanitsch]

That's too bad. Thanks for looking into it, @jdalton.

[jdalton]

Ah, it's not that bad. I mean, packages using @std/esm still work with AVA. It's just AVA handles ESM tightly on test files at the moment. I think they've eventually get more configurable but as it stands ESM still works with AVA so it's all good.

[jamestalmage]

See avajs/ava#1618

[jdalton]

Thank you @jamestalmage!