This is just a sample code to demonstrate how we can have a custom barrel import transformer to gain performance improvements when running Jest test in a project with Barrel imports.
Note 1 - transformParameters.ts can be updated to configure condition which the transform uses in this file
Note 2 - printLog when declared in a file contains console log statement and can be used easily commented to not have logs in test output. Just something to help follow the transform flow.
Note 3 - When running the transform make sure to clear the jest cache - npx jest --clearCache
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The test the sample start to execute when running script and observe the code generate in jest cache directory. Command to run - npm test
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The barrel-import-transform steps runs which dumps all the exports from file pattern specified in transformParameters.ts
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Then test starts to execute and ts/tsx files are passed to the barrelImportTransformerProcessor.ts which transforms the file to convert the barrel import into specific import by looking at the exports contained in the importLookupData.json file generated in step 2.
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The import is only transformed when shouldTransformImportDeclarationCriteria in transformParameters.ts returns true for a import statement currently processed by transformer.
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The change in the import statement might effect the jest.mock setup for test files hence the barrelImportTransformer.ts file contains code in updateJestMockStatements function to automatically transform the jest.mock statements as well. This doesnt work 100% of the time and will need manual fix to the test files.
For e.g, a spec file with import statement and mock setup like - import { a } from './src'; jest.mock('./src', moduleFactoryFn); will be auto converted to below. This should be ok for the most part but if it doesnt work then test should be modified to correct the mock declarations - import { a } from './src/a'; jest.mock('./src/a', moduleFactoryFn);
