Microsoft/TypeScript

`using` transform throws when a for body binding shadows the for head binding

Open

#61,714 opened on May 16, 2025

View on GitHub
 (6 comments) (1 reaction) (1 assignee)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: API: TransformsFix AvailableHelp Wanted

Description

🔎 Search Terms

explicit resource management, using, await using, loop initializer, Symbol.dispose, Symbol.asyncDispose, iteration, shadows

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250516#code/MYGwhgzhAEBiD29oG8C+BYAUAM3gJ2gAoBXCASwDsBzaXJebaAbQF0BKFLaaYeCiAC61E0ALzQKAUwDucRITYBuLKiA

💻 Code

class Foo {}
for (using foo of []) {
  const foo = new Foo();
}

🙁 Actual behavior

The transformed output throws "redeclaration of const foo "

🙂 Expected behavior

The transformed output should not throw.

Additional information about the issue

Babel is also affected by this issue: https://github.com/babel/babel/issues/17318. We plan to wrap the for body within a new block when it contains shadowed binding.

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript compiler architectureDownlevel emit transformersScope analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
The issue is caused by the downlevel emit transformer not handling shadowed bindings in for of loops with `using`. The fix should wrap the for body in an additional block when a variable in the body shadows the loop variable. See the related Babel issue (https://github.com/babel/babel/issues/17318) for a similar fix. The relevant code is likely in the downlevel emit transformer for 'using' declarations in the TypeScript compiler source (e.g., in src/services/codefixes/downlevelEmit.ts or the transformer directory).