chore: migrate @jest/transform to TypeScript

[SimenB]

Summary

diff --git c/packages/jest-transform/build/ScriptTransformer.js w/packages/jest-transform/build/ScriptTransformer.js
index c572879d3..9cc6180d3 100644
--- c/packages/jest-transform/build/ScriptTransformer.js
+++ w/packages/jest-transform/build/ScriptTransformer.js
@@ -115,10 +115,6 @@ function _slash() {
   return data;
 }
 
-var _package = require('../package.json');
-
-var _shouldInstrument = _interopRequireDefault(require('./shouldInstrument'));
-
 function _writeFileAtomic() {
   const data = _interopRequireDefault(require('write-file-atomic'));
 
@@ -139,6 +135,8 @@ function _realpathNative() {
   return data;
 }
 
+var _shouldInstrument = _interopRequireDefault(require('./shouldInstrument'));
+
 var _enhanceUnexpectedTokenMessage = _interopRequireDefault(
   require('./enhanceUnexpectedTokenMessage')
 );
@@ -152,11 +150,14 @@ function _interopRequireDefault(obj) {
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
- *
- *
  */
-// This data structure is used to avoid recalculating some data every time that
+// @ts-ignore: should just be `require.resolve`, but the tests mess that up
+// Use `require` to avoid TS rootDir
+const _require = require('../package.json'),
+  VERSION = _require.version; // This data structure is used to avoid recalculating some data every time that
 // we need to transform a file. Since ScriptTransformer is instantiated for each
+// file we need to keep this object in the local scope of this module.
+
 const projectCaches = new WeakMap(); // To reset the cache for specific changesets (rather than package version).
 
 const CACHE_VERSION = '1';
@@ -208,10 +209,11 @@ class ScriptTransformer {
   }
 
   _getFileCachePath(filename, content, instrument) {
+    // @ts-ignore: not properly exported (needs ESM)
     const baseCacheDir = _jestHasteMap().default.getCacheFilePath(
       this._config.cacheDirectory,
       'jest-transform-cache-' + this._config.name,
-      _package.version
+      VERSION
     );
 
     const cacheKey = this._getCacheKey(content, filename, instrument); // Create sub folders based on the cacheKey to avoid creating one
@@ -245,7 +247,7 @@ class ScriptTransformer {
   }
 
   _getTransformer(filename) {
-    let transform;
+    let transform = null;
 
     if (!this._config.transform || !this._config.transform.length) {
       return null;
@@ -258,7 +260,7 @@ class ScriptTransformer {
 
       if (transformer != null) {
         return transformer;
-      } // $FlowFixMe
+      }
 
       transform = require(transformPath);
 
@@ -282,6 +284,7 @@ class ScriptTransformer {
     const result = (0, _core().transformSync)(content, {
       auxiliaryCommentBefore: ' istanbul ignore next ',
       babelrc: false,
+      // @ts-ignore: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/33118
       caller: {
         name: '@jest/transform',
         supportsStaticESM: false
@@ -301,7 +304,16 @@ class ScriptTransformer {
         ]
       ]
     });
-    return result ? result.code : content;
+
+    if (result) {
+      const code = result.code;
+
+      if (code) {
+        return code;
+      }
+    }
+
+    return content;
   }
 
   _getRealPath(filepath) {
diff --git c/packages/jest-transform/build/shouldInstrument.js w/packages/jest-transform/build/shouldInstrument.js
index 7eca95eee..0f446d28a 100644
--- c/packages/jest-transform/build/shouldInstrument.js
+++ w/packages/jest-transform/build/shouldInstrument.js
@@ -54,8 +54,6 @@ function _interopRequireDefault(obj) {
  *
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
- *
- *
  */
 const MOCKS_PATTERN = new RegExp(
   (0, _jestRegexUtil().escapePathForRegex)(
@@ -78,7 +76,7 @@ function shouldInstrument(filename, options, config) {
 
   if (
     !config.testPathIgnorePatterns ||
-    !config.testPathIgnorePatterns.some(pattern => filename.match(pattern))
+    !config.testPathIgnorePatterns.some(pattern => !!filename.match(pattern))
   ) {
     if (
       config.testRegex &&
@@ -124,7 +122,7 @@ function shouldInstrument(filename, options, config) {
 
   if (
     config.coveragePathIgnorePatterns &&
-    config.coveragePathIgnorePatterns.some(pattern => filename.match(pattern))
+    config.coveragePathIgnorePatterns.some(pattern => !!filename.match(pattern))
   ) {
     return false;
   }

Test plan

Green CI

[codecov-io]

Codecov Report

Merging #7918 into master will decrease coverage by 0.07%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##           master    #7918      +/-   ##
==========================================
- Coverage   65.49%   65.42%   -0.08%     
==========================================
  Files         255      255              
  Lines        9930     9949      +19     
  Branches      965     1038      +73     
==========================================
+ Hits         6504     6509       +5     
- Misses       3192     3193       +1     
- Partials      234      247      +13

Impacted Files	Coverage Δ
...est-transform/src/enhanceUnexpectedTokenMessage.ts	33.33% <ø> (ø)
packages/jest-transform/src/shouldInstrument.ts	95.34% <100%> (ø)
packages/babel-jest/src/index.ts	50% <100%> (-1.52%)	⬇️
packages/jest-transform/src/ScriptTransformer.ts	81.28% <84.21%> (ø)

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[SimenB]

quite happy with how this turned out. By using an interface, we can narrow down the type, and it'll tell us here if we break Transform's contract, and not just a local one. It also narrows down options of the factory to babel's options instead of any

And importers of babel-jest directly will see that createTransformer is always there - it's not possibly undefined. Pretty nice!

[SimenB]

setting the type here allows us to not type all of the args explicitly

[SimenB]

this was the solution to the FIXME below 🙂

[rubennorte]

This broke all usages of createTransformer (vs using the transformer that's already created by Jest) because the object createTransformer returns also has a createTransformer property, which is checked in @babel/transform to use that instead of the passed one: https://github.com/facebook/jest/blob/master/packages/jest-transform/src/ScriptTransformer.ts#L157

[SimenB]

Oh, oops! 😛 Good catch!

[thymikee]

👌

[thymikee]

Can it be really undefined?

[SimenB]

Yes, due to this._cache.transformedFiles.get(scriptCacheKey) below. We cannot return undefined from this method though.

But your comment made me look, and we don't actually return strings either, so was able to make it a bit stricter

[github-actions]

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