Support transforming resolves (to allow for .ts-based snapshotResolver)

🚀 Feature Proposal

Currently resolver files used for properties such as snapshotResolver don’t get transformed, meaning that you can only use language features that can be transformed by Babel.

For example, TypeScript resolvers won’t work:

package.json:

...
  "jest": {
    "snapshotResolver": "./test/snapshotResolver.ts",
    "moduleFileExtensions": [
      "ts",
      "tsx",
      "js",
      "json",
      "jsx",
      "node"
    ],
    "moduleNameMapper": {
      "^@src/(.*)$": "<rootDir>/src/$1",
      "^@test/(.*)$": "<rootDir>/test/$1"
    },
    "transform": {
      ".(ts|tsx)": "ts-jest"
    }
  },
...

// snapshotResolver.ts
module.exports = {
  /** resolves from test to snapshot path */
  resolveSnapshotPath: function (testPath: string, snapshotExtension: string) {
    return testPath.replace('src/', '__snapshots__/') + snapshotExtension;
  },
  /** resolves from snapshot to test path */
  resolveTestPath: function (snapshotFilePath: string, snapshotExtension: string) {
    return snapshotFilePath
      .replace('__snapshots__/', 'src/')
      .slice(0, -snapshotExtension.length);
  },
  testPathForConsistencyCheck: ''
};

Output:

  ● Test suite failed to run

    Jest encountered an unexpected token

    This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.

    By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

    Here's what you can do:
     • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
     • If you need a custom transformation specify a "transform" option in your config.
     • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

    You'll find more details and examples of these config options in the docs:
    https://jestjs.io/docs/en/configuration.html

    Details:

    C:\Users\G-Rath\workspace\projects-personal\strongly-typed-event-emitter\test\snapshotResolver.ts:3
      resolveSnapshotPath: (testPath: string, snapshotExtension: string) => {
                                    ^

    SyntaxError: Unexpected token :

      at ScriptTransformer._transformAndBuildScript (node_modules/@jest/transform/build/ScriptTransformer.js:471:17)
      at ScriptTransformer.transform (node_modules/@jest/transform/build/ScriptTransformer.js:513:25)

Based off discussion with @SimenB

[SimenB]

I’ll fix that

[SimenB]

This should be pretty doable 🙂 We recently refactored the logic for transpilation of this in #8756, so the diff should be pretty small. See #8751 for how test envs can be transpiled. Copying that approach for snapshot resolvers (and resolvers if you want) should be pretty straight forward