Microsoft/TypeScript

Aliased optional chain calls don't narrow like their inline equivalents

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#59,145 opened on Jul 5, 2024

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 (0 comments) (2 reactions) (0 assignees)TypeScript (6,726 forks)batch import
Domain: check: Control FlowHelp WantedPossible Improvement

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Description

🔎 Search Terms

aliased conditions control flow analysis cfa optional chain call

🕗 Version & Regression Information

  • This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.0-dev.20240704#code/GYVwdgxgLglg9mABAJwKYGc4BsBuqCMAFDACYBci6UyMYA5ogD6LgmrC2okCUiA3gChEiCAiqIY6AEKssXRAF4JJAPwA6WhCwg26QgCIA+gCNZXfdwDcQicETFpZnvxvDRYcVWQUvtBktJrYQBfAVCBUEhYBBQMbDwAJmJySmo-JhYwNg4wLl5BYRg7ZPVNbV0DEycLfNcRMShU7yb0gJIgxFDgoA

💻 Code

function resolve1(id: string | undefined) {
  const isBundled = id?.includes("_bundled");
  if (isBundled) {
    const str: string = id; // error
  }
}

function resolve2(id: string | undefined) {
  if (id?.includes("_bundled")) {
    const str: string = id; // ok
  }
}

🙁 Actual behavior

resolve1 errors within the if block

🙂 Expected behavior

I'd expect it to be OK. Just like resolve2 is OK with it here - the condition is exactly the same and TS supports Control Flow Analysis of Aliased Conditions and Discriminant since 4.4

Additional information about the issue

No response

Contributor guide

Research direction
Investigate how control flow analysis handles aliased conditions from optional chain calls. Look at the implementation of `narrowTypeByOptionalChain` or similar functions in the TypeScript checker.
Tech stack
typescript
Domain
backend
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.
3-5 days
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
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30