Handle ERR_REQUIRE_ESM when reading configuration

[azz]

Summary

#9431 added support for jest.config.mjs files.

This PR attempts to support jest.config.js files that are ES modules, when a project's package.json has "type": "module".

Current behaviour (Jest 25.1.0):

• If config file ends with .mjs, do await import(configPath).
• Otherwise, do require(configPath).

Proposed behaviour:

• Do require(configPath)
• If an ERR_REQUIRE_ESM is thrown, do await import(configPath).

This will leave the module detection logic to Node.js, in lieu of an API to query it (nodejs/node#49446).

It will be compatible with older versions of Node (8), and should be somewhat resilient to changes in the modules spec.

Test plan

I'm opening this PR as a discussion, so I haven't added any tests just yet.

There's already some discussion here.

[codecov-io]

Codecov Report

Merging #9573 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9573      +/-   ##
==========================================
- Coverage   65.04%   65.03%   -0.02%     
==========================================
  Files         286      286              
  Lines       12140    12139       -1     
  Branches     3008     3008              
==========================================
- Hits         7897     7895       -2     
  Misses       3605     3605              
- Partials      638      639       +1

Impacted Files	Coverage Δ
packages/jest-config/src/importEsm.ts	0% <ø> (ø)
...ges/jest-config/src/readConfigFileAndSetRootDir.ts	0% <0%> (ø)	⬆️
packages/expect/src/utils.ts	94.96% <0%> (-1.26%)	⬇️

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[SimenB]

I think I read somewhere that this will print a warning to the console. Does it? If not this looks ideal 🙂

[azz]

In my test script on Node 13.8 I get the warning:

(node:1) ExperimentalWarning: The ESM module loader is experimental.

But doesn't that happen with the code on master too? It will get logged the first time you do import() on an ES module.

[azz]

test.cjs:

async function test() {
  let mod;
  try {
    mod = require('./config.js');
    console.log('require worked');
  } catch (e) {
    if (e.code === 'ERR_REQUIRE_ESM') {
      console.log('falling back to import()');
      mod = (await import('./config.js')).default;
    }
 }
 console.log(mod);
}

test();

config.js:

export default { foo: 'bar' };

package.json:

{"type":"module"}

$ docker run --rm -it -v "$(pwd):/tmp" -w /tmp node:13 node test
falling back to import()
(node:1) ExperimentalWarning: The ESM module loader is experimental.
{ foo: 'bar' }

[SimenB]

Perfect, thanks for checking! 🙂 Just a test and we can merge this then 👍

[SimenB]

thank you!

[github-actions]

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