jest-runtime, jest-transform: share cacheFS between runtime and transformer

[ahnpnl]

🚀 Feature Proposal

Currently, when ts-jest/jest-preset-angular performs code compilation + type checking with TypeScript LanguageService/Program, TypeScript will attempt to do module resolution itself to gather enough information for the process.

The problem is when doing module resolution, TypeScript will also perform fs.readFileSync to get file content. So we end up in the situation that some files which are already read by TypeScript will be read again by Jest, which causes extra I/O on disk.

Therefore, I'd like to propose a feature request that jest-runtime shares cacheFS with jest-transform. The idea is that custom transformer can get/set this cacheFS so that already-read files shouldn't be read again.

The prototype implementation is: jest-runtime passes cacheFS to ScriptTransformer, ScriptTransformer passes that cacheFS to process.

Motivation

Improve performance by reducing I/O.

Example

This will help ts-jest/jest-preset-angular to speed up test run with isolatedModules: false

Pitch

The implementation will require changes in jest-runtime and jest-transform so it is core of Jest.

cc @thymikee @SimenB

[SimenB]

That sounds like a good idea to me. Wanna send a PR?

[ahnpnl]

Sure 👌

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