Microsoft/TypeScript

Spread operator with wrong optional property raises error on incorrect source line

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#51,376 opened on Nov 1, 2022

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 (1 comment) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: Related Error SpansHelp Wanted

Description

Bug Report

🔎 Search Terms

spread error wrong line

🕗 Version & Regression Information

  • This is the behavior in every version I tried, [not applied: and I reviewed the FAQ for entries about]. I tried on v3.3.3 and also on nightly.

⏯ Playground Link

Playground link with relevant code

💻 Code

type Item = {
    attribute: string;
};

const result: Partial<Record<'attribute', string | null>> = {
    attribute: "",
}

const item: Item ={
    attribute: "",
    ...result,
}

🙁 Actual behavior

This code is, of course, an error. The surprising thing is where the error is raised. In this case the line reading attribute: "", will be highlighted with Type 'string | null' is not assignable to type 'string'.

In this simple reproduction the error is clear, but when I discovered it the types were more complicated and there were a dozen or more attributes in between the scary red line and the offending spread.

🙂 Expected behavior

The diagnostic error would be much more helpful if the spread containing the invalid property was highlighted instead of the property which actually has correct types.

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.
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.
needs maintainer response
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript fundamentalsfamiliarity with spread operator
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
This bug involves an incorrect error location when spreading an object with an optional property. The error should be reported on the spread expression, not on the property that appears earlier. To fix, investigate the TypeScript checker's handling of object literal spread in `src/compiler/checker.ts`. Look for where error diagnostics are emitted for spread assignments and how error locations are determined. Check related issues or PRs for similar error location fixes.