Support ESM test files

[novemberborn]

Once avajs/ava#2345 has shipped it should be possible to configure AVA to load ts files as ES modules.

This means we need to import them, instead of using require.

We should also explore if there may be a way to configure this within the typescript configuration.

[cinderblock]

Is this why I'm getting this error?

  C:\Users\Cameron\git\http-terminator\test\http-terminator\http.ts:3
  import createTests from '../helpers/createTests';
  ^^^^^^

  SyntaxError: Cannot use import statement outside a module

  × test\http-terminator\http.ts exited with a non-zero exit code: 1

  Uncaught exception in test\http-terminator\https.ts

[novemberborn]

Yes, right now the files are loaded using require().

[sindresorhus]

@novemberborn Just so we're on the same page, this issue is about supporting projects that use AVA with @ava/typescript where package.json has "type": "module" and the tsconfig has "module": "es2020", meaning the TS output is native ESM. Correct?

[novemberborn]

@sindresorhus Yes.

[novemberborn]

Having looked at the logic here and in AVA itself, I think the way to do this is first to remove this check:

typescript/index.js

Lines 162 to 166 in 27650db

	for (const extension of extensionsToLoadAsModules) {
	if (ref.endsWith(`.${extension}`)) {
	throw new Error('@ava/typescript cannot yet load ESM files');
	}
	}

Then instead of immediately using the requireFn here:

typescript/index.js

Line 171 in 27650db

return requireFn(rewritten);

We currently always rewrite to js file, so if js is in extensionsToLoadAsModules then just use import().

We then need to document that, since TypeScript compiles to JS files, you need to configure either your package.json#type or AVA itself to load JS files as modules. Configuring the ts extension will cause AVA to disregard @ava/typescript, as per these lines: https://github.com/avajs/ava/blob/a87460acf570a1d6080f35f0ecf2ccb1cac476b9/lib/worker/base.js#L161-L169

[novemberborn]

I think the way to do this is first to remove this check:

typescript/index.js

Lines 162 to 166 in 27650db

	for (const extension of extensionsToLoadAsModules) {
	if (ref.endsWith(`.${extension}`)) {
	throw new Error('@ava/typescript cannot yet load ESM files');
	}
	}

avajs/ava#2882 reorders it so that the provider has first dibs.