tsc --build --watch emitting unchanged files to disk

[frags51]

Bug Report

Similar to #29927

🔎 Search Terms

tsc --watch writing to disk too often

🕗 Version & Regression Information

Occurring in TS 3.9.5 (and also tried on TS 4.5, although we did get TS2571 errors in our code while using TS4.5.
We are using TS in a React-Native project. This issue is causing longer times to hot reload in RN.

tsconfig:

{
  "compilerOptions": {
    "module": "commonjs",
    "target": "es6",
    "jsx": "react",
    "moduleResolution": "node",
    "importHelpers": true,
    "experimentalDecorators": true,
    "resolveJsonModule": true,
    "sourceMap": false,
    "strictNullChecks": true,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "forceConsistentCasingInFileNames": true,
    "noImplicitAny": true,
    "preserveConstEnums": true,
    "noErrorTruncation": true,
    "strict": true,
    "declaration": true,
    "allowJs": true,
    "skipLibCheck": true,
    "esModuleInterop": true
  },
  "compileOnSave": true
}

🙁 Actual behavior

On changing a single TS file, multiple JS files are written to disk. Can confirm that they need to be actually changed.

🙂 Expected behavior

Only the single (or files that actually need to be changed) should be written to disk.

[RyanCavanaugh]

This is not necessarily unexpected behavior for --build mode since it is a timestamp-based incremental build system. The content and structure of the files matters here and we'd need a complete repro to determine if there's a bug or not.

[DanielRosenwasser]

For what it's worth, I've proposed a possible mitigation in #46677. But it's hard to say without a full repro.

[frags51]

Project structure is as follows:
We have used Lerna to create a monorepo for our React Native app.

packages
 | - A (Root package, depends on B, C. react-native start is run from this package)
   | src/.ts(x) (Files from B, C are imported as import Something from 'B/build/Something' and not 'B/Something'
   | app.js, index.js, tsconfig (at root level)
   | resources/images
   | build/.js (Emitted by tsc)
 | - B
   | src/.ts
   | tsconfig (at root level)
   | build/.js (Emitted by tsc)
 | - C
   | src/.ts
   | tsconfig (at root level)
   | build/.js (Emitted by tsc)

tsc --build is needed so that changes in B, C also lead to a hot reload. If we remove --build still we see multiple JS files being emitted on change to TS file in A. (Although these two situations may be occurring due to different causes)

[frags51]

We got our scenario working by not using tsc at all (just using tsc --noEmit for compilation errors) and instead letting Metro/Babel transform ts files to js during dev. We also had to move to consuming TS files directly (earlier we were sending JS files to metro)

[DanielRosenwasser]

Sorry that you had to find mitigations for this. I believe that this is fixed now (since #46677 is fixed). If you still run into this in other projects, you can now try a nightly version of TypeScript and see if things are improved at all.