Transform Async

[Cretezy]

Do you want to request a feature or report a bug?

Feature.

What is the current behavior?

Transforms are synchronous, basically process returns a string of the transformed source.

What is the expected behavior?

It would be useful to also support Promises, for asynchronous transformation.

[cpojer]

Unfortunately Jest expects transformers to be synchronous and we cannot change it at this time because we transform just-in-time when user code calls require.

[Cretezy]

Make sense, thank you!

[jpvincent]

Hi
I understand the rationales for the "No", but let me add that here we would really need the transform to support asynchronous methods, like Webpack does with its loader. We made a custom loader (for translations files in resx format) that works for Webpack and that we could reuse almost as-is if the jest transform option was asynchronous.
Currently the workaround has been to pre-compile all our translation files before running Jest, but it's not optimized at all.

Thanks anyway for the transform option to exist at all :)

[ksjogo]

@jpvincent
I ran into the same thing with a resx transformer.
This seems to work:

const xml2js = require('xml2js');
const util = require('./util');
const deasync = require('deasync');

/** @type { jest.Transformer } */
const transformer = {
  process(src) {
    let result = null;
    xml2js.parseString(src, (err, parsedXml) => {
      if (err) throw err;
      result = util.transformResx(parsedXml);
    });
    while (result === undefined) {
      deasync.runLoopOnce();
    }
    return result;
  }
};

module.exports = transformer;

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