Fixed an issue causing spurious "used before being assigned" errors in for of/in loops by Andarist · Pull Request #61376 · microsoft/TypeScript

Mateusz Burzyński (Andarist) · 2025-03-07T22:08:19Z

…n for of/in loops

Ryan Cavanaugh (RyanCavanaugh) · 2026-01-08T21:15:02Z

TypeScript Bot (@typescript-bot) test this

Ryan Cavanaugh (RyanCavanaugh) · 2026-01-08T21:15:17Z

TypeScript Bot (@typescript-bot) test it

Ryan Cavanaugh (RyanCavanaugh)

I'm not 100.000% convinced that there's always a .parent.parent but it seems like this would have crashed a test if that weren't the case. Running RWC suite just to be sure

Jake Bailey (jakebailey) · 2026-01-08T21:20:29Z

TypeScript Bot (@typescript-bot) test it

TypeScript Bot (typescript-bot) · 2026-01-08T21:20:34Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`test top400`	✅ Started	✅ Results
`user test this`	✅ Started	✅ Results
`run dt`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

TypeScript Bot (typescript-bot) · 2026-01-08T21:43:46Z

Hey Jake Bailey (@jakebailey), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

TypeScript Bot (typescript-bot) · 2026-01-08T21:51:31Z

Jake Bailey (@jakebailey) Here are the results of running the user tests with tsc comparing main and refs/pull/61376/merge:

There were infrastructure failures potentially unrelated to your change:

1 instance of "Package install failed"
1 instance of "Git clone failed"

Otherwise...

Everything looks good!

TypeScript Bot (typescript-bot) · 2026-01-08T22:15:28Z

Jake Bailey (@jakebailey)
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - node (v18.15.0, x64)}
_Errors	₁	₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_62,370	_62,370	_~	_~	_~	_{p=1.000 n=6}
_Types	_50,387	_50,387	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{193,769k (± 1.01%)}	_{194,377k (± 1.05%)}	_~	_192,468k	_196,380k	_{p=0.810 n=6}
_{Parse Time}	_{1.29s (± 0.40%)}	_{1.30s (± 0.63%)}	_{+0.01s (+ 0.77%)}	_1.29s	_1.31s	_{p=0.050 n=6}
_{Bind Time}	_0.76s	_0.76s	_~	_~	_~	_{p=1.000 n=6}
_{Check Time}	_{9.85s (± 0.37%)}	_{9.85s (± 0.21%)}	_~	_9.82s	_9.88s	_{p=0.871 n=6}
_{Emit Time}	_{2.74s (± 1.17%)}	_{2.74s (± 0.48%)}	_~	_2.72s	_2.75s	_{p=0.935 n=6}
_{Total Time}	_{14.65s (± 0.35%)}	_{14.66s (± 0.21%)}	_~	_14.62s	_14.70s	_{p=0.747 n=6}
_{angular-1 - node (v18.15.0, x64)}
_Errors	₂	₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_955,823	_955,823	_~	_~	_~	_{p=1.000 n=6}
_Types	_415,853	_415,853	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{1,253,870k (± 0.00%)}	_{1,253,875k (± 0.00%)}	_~	_1,253,820k	_1,253,933k	_{p=0.810 n=6}
_{Parse Time}	_{6.52s (± 0.56%)}	_{6.57s (± 0.66%)}	_{+0.06s (+ 0.87%)}	_6.51s	_6.62s	_{p=0.044 n=6}
_{Bind Time}	_{1.96s (± 0.21%)}	_{1.96s (± 0.21%)}	_~	_1.95s	_1.96s	_{p=1.000 n=6}
_{Check Time}	_{32.40s (± 0.23%)}	_{32.40s (± 0.40%)}	_~	_32.24s	_32.56s	_{p=0.936 n=6}
_{Emit Time}	_{14.99s (± 0.19%)}	_{14.98s (± 0.23%)}	_~	_14.95s	_15.03s	_{p=0.935 n=6}
_{Total Time}	_{55.86s (± 0.14%)}	_{55.92s (± 0.20%)}	_~	_55.75s	_56.06s	_{p=0.296 n=6}
_{mui-docs - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_2,704,424	_2,704,424	_~	_~	_~	_{p=1.000 n=6}
_Types	_935,546	_935,546	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,038,338k (± 0.00%)}	_{3,038,327k (± 0.00%)}	_~	_3,038,235k	_3,038,492k	_{p=0.575 n=6}
_{Parse Time}	_{8.67s (± 0.09%)}	_{8.67s (± 0.10%)}	_~	_8.66s	_8.68s	_{p=0.798 n=6}
_{Bind Time}	_{2.31s (± 0.22%)}	_{2.31s (± 0.36%)}	_~	_2.29s	_2.31s	_{p=0.923 n=6}
_{Check Time}	_{93.25s (± 0.83%)}	_{93.39s (± 0.25%)}	_~	_93.04s	_93.62s	_{p=0.688 n=6}
_{Emit Time}	_{0.30s (± 1.70%)}	_{0.31s (± 2.04%)}	_~	_0.30s	_0.32s	_{p=0.091 n=6}
_{Total Time}	_{104.53s (± 0.74%)}	_{104.68s (± 0.23%)}	_~	_104.33s	_104.91s	_{p=0.689 n=6}
_{self-build-src - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,252,021	_1,252,021	_~	_~	_~	_{p=1.000 n=6}
_Types	_259,853	_259,853	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,997,671k (± 9.90%)}	_{3,119,560k (± 0.01%)}	_~	_3,119,080k	_3,120,076k	_{p=0.093 n=6}
_{Parse Time}	_{8.20s (± 1.13%)}	_{8.24s (± 0.62%)}	_~	_8.15s	_8.30s	_{p=0.748 n=6}
_{Bind Time}	_{2.78s (± 1.60%)}	_{2.75s (± 0.85%)}	_~	_2.73s	_2.78s	_{p=0.469 n=6}
_{Check Time}	_{53.20s (± 0.46%)}	_{53.27s (± 0.38%)}	_~	_52.98s	_53.61s	_{p=0.689 n=6}
_{Emit Time}	_{4.29s (± 1.54%)}	_{4.28s (± 2.07%)}	_~	_4.14s	_4.40s	_{p=0.810 n=6}
_{Total Time}	_{68.47s (± 0.50%)}	_{68.55s (± 0.26%)}	_~	_68.26s	_68.77s	_{p=1.000 n=6}
_{self-build-src-public-api - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_1,252,021	_1,252,021	_~	_~	_~	_{p=1.000 n=6}
_Types	_259,853	_259,853	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{2,454,375k (± 0.02%)}	_{2,454,087k (± 0.03%)}	_~	_2,453,424k	_2,455,372k	_{p=0.230 n=6}
_{Parse Time}	_{5.38s (± 0.74%)}	_{5.34s (± 0.48%)}	_~	_5.31s	_5.38s	_{p=0.092 n=6}
_{Bind Time}	_{1.89s (± 0.54%)}	_{1.89s (± 0.99%)}	_~	_1.87s	_1.92s	_{p=0.317 n=6}
_{Check Time}	_{35.65s (± 0.24%)}	_{35.59s (± 0.13%)}	_~	_35.53s	_35.64s	_{p=0.148 n=6}
_{Emit Time}	_{3.04s (± 1.94%)}	_{3.06s (± 1.20%)}	_~	_3.00s	_3.10s	_{p=0.520 n=6}
_{Total Time}	_{45.96s (± 0.27%)}	_{45.89s (± 0.18%)}	_~	_45.75s	_45.98s	_{p=0.378 n=6}
_{self-compiler - node (v18.15.0, x64)}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_264,678	_264,678	_~	_~	_~	_{p=1.000 n=6}
_Types	_104,066	_104,066	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{443,035k (± 0.02%)}	_{442,968k (± 0.01%)}	_~	_442,918k	_443,025k	_{p=0.230 n=6}
_{Parse Time}	_{3.52s (± 0.54%)}	_{3.52s (± 0.43%)}	_~	_3.50s	_3.54s	_{p=0.934 n=6}
_{Bind Time}	_{1.38s (± 1.62%)}	_{1.38s (± 1.06%)}	_~	_1.37s	_1.40s	_{p=0.744 n=6}
_{Check Time}	_{19.14s (± 0.33%)}	_{19.12s (± 0.47%)}	_~	_19.04s	_19.29s	_{p=0.521 n=6}
_{Emit Time}	_{1.53s (± 0.92%)}	_{1.54s (± 0.76%)}	_~	_1.53s	_1.56s	_{p=0.188 n=6}
_{Total Time}	_{25.56s (± 0.38%)}	_{25.56s (± 0.36%)}	_~	_25.46s	_25.73s	_{p=0.872 n=6}
_{ts-pre-modules - node (v18.15.0, x64)}
_Errors	₇₂	₇₂	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_225,493	_225,493	_~	_~	_~	_{p=1.000 n=6}
_Types	_94,373	_94,373	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{369,906k (± 0.04%)}	_{369,836k (± 0.01%)}	_~	_369,810k	_369,876k	_{p=0.298 n=6}
_{Parse Time}	_{2.84s (± 0.98%)}	_{2.83s (± 0.70%)}	_~	_2.80s	_2.86s	_{p=0.624 n=6}
_{Bind Time}	_{1.65s (± 1.19%)}	_{1.66s (± 1.65%)}	_~	_1.62s	_1.69s	_{p=1.000 n=6}
_{Check Time}	_{16.60s (± 0.39%)}	_{16.66s (± 0.23%)}	_~	_16.61s	_16.71s	_{p=0.062 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_0.00s	_~	_~	_~	_{p=0.405 n=6}
_{Total Time}	_{21.09s (± 0.25%)}	_{21.15s (± 0.23%)}	_~	_21.09s	_21.24s	_{p=0.075 n=6}
_{vscode - node (v18.15.0, x64)}
_Errors	₁₁	₁₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_4,087,957	_4,087,957	_~	_~	_~	_{p=1.000 n=6}
_Types	_1,291,489	_1,291,489	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{3,880,562k (± 0.00%)}	_{3,880,640k (± 0.00%)}	_~	_3,880,487k	_3,880,789k	_{p=0.298 n=6}
_{Parse Time}	_{15.73s (± 0.64%)}	_{15.79s (± 0.52%)}	_~	_15.69s	_15.89s	_{p=0.423 n=6}
_{Bind Time}	_{5.47s (± 2.81%)}	_{5.45s (± 2.27%)}	_~	_5.32s	_5.60s	_{p=0.687 n=6}
_{Check Time}	_{112.05s (± 1.00%)}	_{119.05s (± 4.44%)}	_{🔻+7.00s (+ 6.25%)}	_112.68s	_125.22s	_{p=0.008 n=6}
_{Emit Time}	_{40.29s (± 6.96%)}	_{47.75s (±14.25%)}	_{🔻+7.46s (+18.52%)}	_39.11s	_54.06s	_{p=0.045 n=6}
_{Total Time}	_{173.54s (± 1.79%)}	_{188.03s (± 6.17%)}	_{🔻+14.49s (+ 8.35%)}	_175.99s	_200.41s	_{p=0.013 n=6}
_{webpack - node (v18.15.0, x64)}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_383,794	_383,794	_~	_~	_~	_{p=1.000 n=6}
_Types	_168,221	_168,221	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{542,286k (± 0.03%)}	_{542,292k (± 0.03%)}	_~	_542,175k	_542,553k	_{p=0.936 n=6}
_{Parse Time}	_{5.82s (± 0.44%)}	_{5.82s (± 0.47%)}	_~	_5.78s	_5.85s	_{p=0.936 n=6}
_{Bind Time}	_{2.49s (± 1.67%)}	_{2.49s (± 2.01%)}	_~	_2.42s	_2.55s	_{p=0.808 n=6}
_{Check Time}	_{29.67s (± 0.82%)}	_{29.24s (± 1.09%)}	_~	_29.01s	_29.86s	_{p=0.092 n=6}
_{Emit Time}	_0.00s	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=0.405 n=6}
_{Total Time}	_{37.98s (± 0.68%)}	_{37.55s (± 0.92%)}	_~	_37.28s	_38.22s	_{p=0.093 n=6}
_{xstate-main - node (v18.15.0, x64)}
_Errors	₃₀	₃₀	_~	_~	_~	_{p=1.000 n=6}
_Symbols	_696,322	_696,322	_~	_~	_~	_{p=1.000 n=6}
_Types	_212,496	_212,496	_~	_~	_~	_{p=1.000 n=6}
_{Memory used}	_{591,025k (± 0.03%)}	_{591,002k (± 0.02%)}	_~	_590,862k	_591,185k	_{p=0.936 n=6}
_{Parse Time}	_{4.16s (± 0.30%)}	_{4.15s (± 0.34%)}	_~	_4.13s	_4.17s	_{p=0.243 n=6}
_{Bind Time}	_{1.41s (± 0.87%)}	_{1.42s (± 0.97%)}	_{+0.02s (+ 1.42%)}	_1.41s	_1.44s	_{p=0.042 n=6}
_{Check Time}	_{21.25s (± 1.83%)}	_{21.45s (± 1.92%)}	_~	_20.90s	_21.80s	_{p=0.335 n=6}
_{Emit Time}	_{0.00s (±244.70%)}	_{0.00s (±244.70%)}	_~	_0.00s	_0.01s	_{p=1.000 n=6}
_{Total Time}	_{26.82s (± 1.47%)}	_{27.02s (± 1.53%)}	_~	_26.46s	_27.34s	_{p=0.378 n=6}

System info unknown

Hosts

node (v18.15.0, x64)

Scenarios

Compiler-Unions - node (v18.15.0, x64)
angular-1 - node (v18.15.0, x64)
mui-docs - node (v18.15.0, x64)
self-build-src - node (v18.15.0, x64)
self-build-src-public-api - node (v18.15.0, x64)
self-compiler - node (v18.15.0, x64)
ts-pre-modules - node (v18.15.0, x64)
vscode - node (v18.15.0, x64)
webpack - node (v18.15.0, x64)
xstate-main - node (v18.15.0, x64)

_Benchmark	_Name	_Iterations
_Current	_pr	₆
_Baseline	_baseline	₆

Developer Information:

Download Benchmarks

TypeScript Bot (typescript-bot) · 2026-01-08T22:48:10Z

Jake Bailey (@jakebailey) Here are the results of running the top 400 repos with tsc comparing main and refs/pull/61376/merge:

Everything looks good!

Port of microsoft/TypeScript#61376. Adds check for isForInOrOfStatement to prevent false positive TS2454 errors for variables declared in for-in/for-of loops when referenced in closures within the loop body. Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

…n for of/in loops (microsoft#61376) Co-authored-by: Ryan Cavanaugh <RyanCavanaugh@users.noreply.github.com>

Added test from upstream PR microsoft/TypeScript#61376 to verify the fix for spurious "used before being assigned" errors (TS2454) in for-in/of loops. Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>

Fixed an issue causing spurious "used before being assigned" errors i…

4ec5069

…n for of/in loops

github-project-automation Bot added this to PR Backlog Mar 7, 2025

github-project-automation Bot moved this to Not started in PR Backlog Mar 7, 2025

TypeScript Bot (typescript-bot) added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Mar 7, 2025

Merge branch 'main' into fix/used-before-assigned-for-of-in

0bd795e

TypeScript Bot (typescript-bot) added For Backlog Bug PRs that fix a backlog bug and removed For Uncommitted Bug PR for untriaged, rejected, closed or missing bug labels Jan 8, 2026

Ryan Cavanaugh (RyanCavanaugh) approved these changes Jan 8, 2026

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github-project-automation Bot moved this from Not started to Needs merge in PR Backlog Jan 8, 2026

Ryan Cavanaugh (RyanCavanaugh) added this pull request to the merge queue Jan 9, 2026

Ryan Cavanaugh (RyanCavanaugh) removed this pull request from the merge queue due to a manual request Jan 9, 2026

Ryan Cavanaugh (RyanCavanaugh) added this pull request to the merge queue Jan 9, 2026

Merged via the queue into microsoft:main with commit 632479f Jan 9, 2026
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github-project-automation Bot moved this from Needs merge to Done in PR Backlog Jan 9, 2026

Copilot AI mentioned this pull request Jan 9, 2026

Port PR #61376: Fix spurious "used before being assigned" errors in for of/in loops microsoft/typescript-go#2462

Merged

Microsoft (microsoft) locked as resolved and limited conversation to collaborators Jul 9, 2026

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Fixed an issue causing spurious "used before being assigned" errors in for of/in loops - #61376

Fixed an issue causing spurious "used before being assigned" errors in for of/in loops#61376
Ryan Cavanaugh (RyanCavanaugh) merged 2 commits into
microsoft:mainfrom
Andarist:fix/used-before-assigned-for-of-in

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