Microsoft/TypeScript

Control Flow Analysis Performance Regression

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#62050 opened on Jul 12, 2025

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Domain: PerformanceHelp WantedPossible Improvement

Description

🔎 Search Terms

Control Flow

🕗 Version & Regression Information

⏯ Playground Link

No response

💻 Code

🙁 Actual behavior

I encountered a severe performance regression when type-checking a very large JavaScript file (~360,000 lines).

v4.9.5: Type-checking takes ~16,476ms.

v5.5.4: Type-checking time increases to ~64,196ms (4x slower).

Steps to Reproduce:

  1. Clone the attached project and run:
npm install
node checker.js
  1. Switch TypeScript versions (e.g., between v4.9.5 and v5.5.4) and re-test.

After testing, I believe this regression was introduced by PR #58013. When I revert this commit, the performance returns to v4.9.5 levels.

testLargeFile.zip

🙂 Expected behavior

The performance is the same as v4.9.5

Additional information about the issue

No response

Contributor guide

Tech stack
typescriptjavascript
Domain
performancebackend
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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
understanding of TypeScript compiler internalsperformance profiling experiencefamiliarity with control flow analysisNode.js environment
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
5
Research direction
Investigate the performance regression introduced by PR #58013, which modified control flow analysis. Start by profiling the type checking process using the attached test file. Focus on the control flow analysis code in src/compiler/checker.ts or binder.ts. Compare the behavior between v4.9.5 and v5.5.4 to identify the exact cause. Reverting the PR restored performance, so understanding its changes is key.