Microsoft/TypeScript

Inconsistency between element access and object destruction

Open

#59,571 opened on Aug 9, 2024

View on GitHub
 (1 comment) (0 reactions) (0 assignees)TypeScript (6,726 forks)batch import
Domain: check: Type InferenceHelp WantedPossible Improvement

Repository metrics

Stars
 (48,455 stars)
PR merge metrics
 (Avg merge 6d 17h) (9 merged PRs in 30d)

Description

🔎 Search Terms

destruct object

🕗 Version & Regression Information

Before v4.4.4, index expression in element access was also not inferred as const. So this issue starts from v4.4.4. Reproducible in Nightly build.

⏯ Playground Link

https://www.typescriptlang.org/play/#code/DYUwLgBAdhC8EEYIEMDOEDGB7KqwChtdIs4IBvFBALggCIE6V0i8IBffQnNgDzKwBtAAbIAJOSjthAXXwB6eRGUQAegH5uxChBHjJ0mbQCeHAfiA

💻 Code

let n = 1 as const
const o = { a1: "abcd" }

const x = o[`a${n}`]
//    ^? : string
const { [`a${n}`]: y } = o
//           ^? Errors: Type '{ a1: string; }' has no matching index signature for type 'string'.(2537)

🙁 Actual behavior

`a${n}` in object destruction not inferred as const ("a1")

🙂 Expected behavior

`a${n}` in object destruction should be inferred as "a1", just like the one in element access.

Additional information about the issue

A possible workaround is to add as const after `a${n}`. I think the main problem is the inconsistency between element access and object destruction, which is confusing.

Contributor guide

Research direction
Investigate why template literal types are not inferred as const in destructuring patterns, unlike in element access. Check the TypeScript compiler's handling of computed property names in destructuring and compare with bracket notation access.
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.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
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
TypeScriptTemplate literal types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30