Microsoft/TypeScript

Using declarations like a statement leads to strange bugs.

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#62896 opened on Dec 14, 2025

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 (7 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: check: Control FlowFix AvailableHelp Wanted

Description

🔎 Search Terms

function declaration statement

🕗 Version & Regression Information

v 5.9.3

⏯ Playground Link

https://www.typescriptlang.org/play/?target=99#code/JYMwBAFALgTgrgUwJRgG4EMZgB5gLxgAMA3ALABQAxgPYB2AztQDYIB0T1A5hNksWAHoB1ANYUKIalggsoYYPiL8FAHjABmZQGotKDFgBWi4GSp1GLdlwgGUQhDBhT5tMDQC2AB2AsAXGCNgejA4egQAEzAAIwRJGARohGBaTjB0enpgTloAGjBaajAHJyxksHhaKGB3BLA68XJQSFhEFBA4WkoqujAQCBQAbwBfCX6hAqLHZzKPbxYwKpq8qLg5YunXCsWEfwBJV3pYYC6wd2pwhJoLvPbO7oY3dFc6JgBPRLALyiZMCLS5KDUTxgFioBBMMAbTIXNLRDiUERgIA

💻 Code

if (true) var x = 0;
console.log(x); //ok

for (let i = 0; i < 3; i++) var j = i;
console.log(j) //error in compile: j is used before being assign, no error in runtime    

if (true) function f() {}
f()//no error in compile time, but error in runtime: In strict mode code, functions can only be declared at top level or inside a block

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

The questions above was raised only in academic purpose. I understand that there are no real use cases to apply code above.

Contributor guide

Tech stack
typescript
Domain
developer experience
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.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
JavaScript scopingTypeScript basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the TypeScript compiler's handling of var and function declarations inside blocks. Look at the binder and checker to understand why function declarations in 'if' blocks are not flagged. Compare with ECMAScript specification for Annex B. Consider whether a diagnostic should be added for function declarations in non top level blocks in strict mode.