Microsoft/TypeScript

Destructuring assignment with computed property name does not help narrow types as intended.

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#62,401 opened on Sep 5, 2025

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 (4 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
Domain: check: Control FlowHelp WantedPossible Improvement

Description

🔎 Search Terms

"destructuring assignment", "computed property name", "type narrowing"

🕗 Version & Regression Information

Happen to all existing version in typescript playground.

⏯ Playground Link

https://tsplay.dev/w1eElw

💻 Code

declare let obj: Record<string, string>

declare let cond: boolean;
declare let key: string;
let result: string | null = null;

if (cond) {
    ({ [key]: result } = obj);
    result;
//  ^?
}

if (cond) {
    ({ 'key': result } = obj);
    result;
//  ^?
}

🙁 Actual behavior

Type narrowing did not happen to the assignee variable result after the assignment ({ [key]: result } = obj);.

🙂 Expected behavior

Expecting the assignment ({ [key]: result } = obj); will narrow the assignee variable result to non-null.

Additional information about the issue

No response

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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of TypeScript type narrowingKnowledge of TypeScript compiler's checkerFamiliarity with destructuring assignment in TypeScriptAbility to navigate TypeScript source code
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the TypeScript type narrowing logic for destructuring assignments with computed property names. Review the relevant code in the checker.ts file, specifically the part that handles destructuring. Use the provided playground link (https://tsplay.dev/w1eElw) to reproduce the issue and test potential fixes. Ensure that the narrowing happens correctly for computed property names like `[key]: result`.
Destructuring assignment with computed property name does not help narrow types as intended. · Microsoft/TypeScript#62401 | Good First Issue