Microsoft/TypeScript

Destructuring into an empty object vs an object with existing properties yields different results

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#60,386 opened on Oct 31, 2024

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 (3 comments) (1 reaction) (0 assignees)TypeScript (6,726 forks)batch import
BugDomain: check: Excess Property CheckingHelp Wanted

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Description

🔎 Search Terms

"destructuring", "record"

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I reviewed the FAQ.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.6.3#code/MYewdgzgLgBATgU1HAJgdQJZQBYGUpwZgDmEAXDAEpIioA80hJANDI0cQHwwC8MA3gF8A3AChRCAB4AHWrFCRYiZOizYAcgFcAtgCMEcclRr12LGGB3643Pv1EwYUggEMACnBDSDUAJ4UABmYHGAA6cOVaVRx8JlJgkSA

💻 Code

This raises a type error:

const recordWithStrings: Record<string, string> = {};

export const recordWithNumbers: Record<string, number> = {
  ...recordWithStrings,
};

and this doesn't:

const recordWithStrings: Record<string, string> = {};

export const recordWithNumbers: Record<string, number> = {
  extraProperty: 0,
  ...recordWithStrings,
};

Is there a sound reason for that?

🙁 Actual behavior

The 2nd example doesn't raise a type error.

🙂 Expected behavior

The 2nd example should raise a type error.

Additional information about the issue

No response

Contributor guide

Research direction
Investigate the TypeScript type checker's handling of spread expressions in object literals, particularly how it infers types when spreading a Record<string, string> into a Record<string, number>. Check the behavior difference when an extra property is present. Look at the relevant source code in the checker.ts file, especially the checkSpreadElement or related functions.
Tech stack
typescript
Domain
None
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
TypeScript
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65