Jest: Add Babel 7 support to babel-preset-jest

[edmorley]

Hi :-)

Do you want to request a _feature_ or report a _bug_?
Feature

What is the current behavior?
Attempting to use babel-preset-jest with Babel 7 beta 46 roughly like so:

const babelConfig = {
  // ...
  presets: [require.resolve('babel-preset-jest')],
  plugins: [
    // ...
  ]
}

(sorry for not a full example; it's deeply intertwined with Neutrino and the cause is clear from code-inspection below)

...results in:

    Plugin/Preset files are not allowed to export objects, only functions. In C:\Users\Ed\src\neutrino-dev\node_modules\babel-preset-jest\index.js

      at createDescriptor (../neutrino-dev/node_modules/@babel/core/lib/config/config-descriptors.js:181:11)
      at ../neutrino-dev/node_modules/@babel/core/lib/config/config-descriptors.js:106:12
          at Array.map (<anonymous>)
      at createDescriptors (../neutrino-dev/node_modules/@babel/core/lib/config/config-descriptors.js:105:27)
      at createPresetDescriptors (../neutrino-dev/node_modules/@babel/core/lib/config/config-descriptors.js:97:10)
      at ../neutrino-dev/node_modules/@babel/core/lib/config/config-descriptors.js:83:14
      at cachedFunction (../neutrino-dev/node_modules/@babel/core/lib/config/caching.js:42:17)
      at presets (../neutrino-dev/node_modules/@babel/core/lib/config/config-descriptors.js:28:68)
      at mergeChainOpts (../neutrino-dev/node_modules/@babel/core/lib/config/config-chain.js:371:68)
      at ../neutrino-dev/node_modules/@babel/core/lib/config/config-chain.js:324:7

This is because as of babel/babel#6494, presets must export something like:

exports.default = function () {
  return {
    plugins: [
      // ...
    ]
  };
};

...whereas babel-preset-jest exports an object:
https://github.com/facebook/jest/blob/da60c7811bb808c474e7ef887884e5d00cffa115/packages/babel-preset-jest/index.js#L8-L14

babel-jest appears to work around this by importing the contents of the preset and .concat()ing it directly:
https://github.com/facebook/jest/blob/cf2b9e80a0b099c2f861660fd9cf589848106fa1/packages/babel-jest/src/index.js#L77

What is the expected behavior?

That the .concat() workaround not be required when using babel-preset-jest directly (which is required for use-cases like those in #1468).

Please provide your exact Jest configuration
The config we're using under Babel 6 (the Babel 7 branch is a local WIP; but master should give the context as to use case) is here:
https://github.com/mozilla-neutrino/neutrino-dev/blob/c725322e0e7fe62783cc3b6d78b46be1793c51a7/packages/jest/src/index.js
https://github.com/mozilla-neutrino/neutrino-dev/blob/c725322e0e7fe62783cc3b6d78b46be1793c51a7/packages/jest/src/transformer.js

Run npx envinfo --preset jest in your project directory and paste the
results here

I use yarn rather than npm so don't have npx installed. Perhaps it would be good to update the template with the yarn equivalents?

[SimenB]

PR welcome! A function should work fine for Babel 6 as well.

As a workaround you can create your own file which does require and wraps it in a function, then do require.resolve on that one instead.