Fix issue with jest caching of block.json files (#16899)

* Generate cache key for block index files by concatenating the index with the block json file

* Move implementation to the test/unit folder

test/unit/jest.config.js

@@ -25,4 +25,7 @@ module.exports = {
 		'<rootDir>/.*/build-module/',
 		'<rootDir>/.+\.native\.js$',
 	],
+	transform: {
+		'^.+\\.[jt]sx?$': '<rootDir>/test/unit/scripts/babel-transformer.js',
+	},
 };

test/unit/scripts/babel-transformer.js

@@ -0,0 +1,40 @@
+const fs = require( 'fs' );
+const babelJest = require( 'babel-jest' );
+const babelJestTransformer = babelJest.createTransformer();
+
+module.exports = {
+	// This transformer extends the babel-jest transformer.
+	...babelJestTransformer,
+
+	/**
+	 * A cache key generator that extends babel-jest's cache key generator,
+	 * but adds some additional handling for invalidating the cache when a
+	 * change to a block.json file is made.
+	 *
+	 * @param {string} src      The source of the file being transformed.
+	 * @param {string} filename The filename of the file being transformed.
+	 * @param {...any} args     Any other args passed to the function.
+	 *
+	 * @return {string} The cache key for the file.
+	 */
+	getCacheKey( src, filename, ...args ) {
+		const isBlockIndex = /block-library[\/\\]src[\/\\].+[\/\\]index\.js/.test( filename );
+
+		if ( ! isBlockIndex ) {
+			return babelJestTransformer.getCacheKey( src, filename, ...args );
+		}
+
+		const blockJSONFilename = filename.replace( 'index.js', 'block.json' );
+
+		if ( ! fs.existsSync( blockJSONFilename ) ) {
+			return babelJestTransformer.getCacheKey( src, filename, ...args );
+		}
+
+		// If the file is a block index file and there's a block json file, generate the
+		// jest cache key for this file by concatenating the block index and block json
+		// src together. This will result in the cache key changing and the cache being
+		// invalidated for the index when any changes to the json are made.
+		const blockJSONSrc = fs.readFileSync( blockJSONFilename );
+		return babelJestTransformer.getCacheKey( `${ src }\n${ blockJSONSrc }`, filename, ...args );
+	},
+};