Expose parts of runtime for dynamic module loading

[skeggse]

🚀 Feature Proposal

Expose enough functionality from the runtime to allow dynamically constructing a module (and optionally loading it into the module registry) from generated source.

In particular, this would involve a new function (e.g. jest.loadModuleFromSource) that hooks into _execModule with a little additional glue to create the InitialModule.

Motivation

When testing bundler-related functionality, it can be useful to evaluate the generated source and check its validity against a well-defined module implementation. It's possible to do this currently by instantiating a Module from the builtin module module (ahem) and using _compile, but this doesn't get access to the current testEnvironment (i.e. there's no document available in the dynamically loaded module under jsdom).

Example

Please provide an example for how this feature would be used.

import { rollup } from 'rollup';
import customPluginBeingTested from '..';

it('should parse and execute', async () => {
  const code = await rollup({
    // e.g. makes use of browser globals and thus needs jsdom present to work.
    input: 'fixtures/example.js',
    plugins: [customPluginBeingTested],
  })
    .then((bundle) => bundle.generate({ format: 'cjs' }))
    .then(({ output }) => output[0].code);

  const module = jest.loadModuleFromSource(`${__dirname}/fixtures/example.out.js`, code);
  expect(module.exports.template()).toEqual(expect.objectContaining({ nodeName: 'DIV' }));
});

Pitch

There doesn't appear to be much public API surface that allows more complex interactions with the runtime or the module registry. It'd be nice to support additional use-cases in testing dynamically generated code that don't seem to currently be supported.

---

Happy to take a stab at the implementation if y'all think this is a feature worth of the jest platform.

[jeysal]

We usually write to temp dirs when testing things like these. Admittedly though, this may be a significant slowdown on Win/Mac.
I'm not particularly excited about adding a new API for this, but what would be great IMO would be exposing a modified 'module' API that fixes the testEnvironment problem, if at all possible.

[skeggse]

A modified module would totally work for me; would we be overriding the _compile method, or adding a new one?

[jeysal]

Override - I view it similarly to how we have a custom require (which is also normally part of the Node module system) with some Jest-specific behavior included.

[skeggse]

@jeysal I'd love your feedback on a PoC PR for this: #10072

[jeysal]

Thanks a lot for offering to implement this ❤️ I've left a comment over there

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