Microsoft/TypeScript

`tsc --build --watch` produces stray `.js`/`.js.map` files when adding a file to upstream project

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#61,717 opened on May 17, 2025

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BugDomain: tsc -bHelp Wanted

Description

🔎 Search Terms

tsc build watch extra unwanted stray .js .js.map files projects

🕗 Version & Regression Information

  • This changed between versions 5.5 and 5.6

⏯ Playground Link

No response

💻 Code

Repo with the repro: https://github.com/echentw/tsc-watch-bug-repro

  • Instructions for repro-ing are under "Bug 1 repro instructions".

Description of the repro:

  • The setup:
    • The repo contains 2 TS projects: project1 and project2
    • The .js and .js.map files should be outputted to project1/dist/ and project2/dist/
    • project1 defines some math functions that project2 imports and uses
    • The git branches:
      • the main branch has the base code
      • the multiply-function branch has one additional commit on top of main, which:
        • adds a project1/src/multiply.ts file which exports a multiply function
        • and edits project2/src/main.ts to import and use that multiply function.
  • When you switch from main to the multiply-function branch while tsc --build --watch is running from within project2/, then the following unwanted files are created:
    • project1/src/multiply.js
    • project1/src/multiply.js.map

I checked that this doesn't happen with TS version 5.5, but it does happen with TS version 5.6 and onwards.

🙁 Actual behavior

project1/src/multiply.js and project1/src/multiply.js.map are created

🙂 Expected behavior

project1/src/multiply.js and project1/src/multiply.js.map should not be created

Additional information about the issue

Ideally, switching branches or modifying code while tsc --build --watch is running wouldn't produce stray .js/.js.map files. They are unpleasant to deal with and can be accidentally committed.

No response

Contributor guide

Tech stack
typescript
Domain
build system
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScripttsc build watchproject references
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Start by examining the reproduction repository at https://github.com/echentw/tsc watch bug repro to understand the setup. The bug is a regression between TS 5.5 and 5.6, so look at the changelog or git history for changes in watch mode or project references. Files of interest include the TypeScript compiler source related to ` build watch`, particularly `src/services/watch.ts` and `src/services/project.ts`. The issue occurs when switching branches while watch is running, leading to stray .js/.map in source directories. Investigate how the compiler handles file additions in upstream projects during watch mode.