Microsoft/TypeScript

ThisType is invalid when using object spread on generic object

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#52,207 opened on Jan 12, 2023

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 (5 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: This-TypingHelp Wanted

Description

Bug Report

🔎 Search Terms

ThisType Object Spread Generic

🕗 Version & Regression Information

  • I was unable to test this on prior versions because I was not using this feature previously

⏯ Playground Link

Playground link with relevant code

💻 Code

function createObject<D extends {}>(def: D & ThisType<D>) { /* some code */ }

const namableObject = { name: '' };
// ThisType will be wrongly typed when using object spread
const spreadObject = createObject({
  ...namableObject,
  get thisName() {
    return this.name; // <-- Property 'name' does not exists on type '{}'
  },
});
// Correctly typed otherwise
const fullObject = createObject({
  name: '',
  get thisName() {
    return this.name;
  },
});

🙁 Actual behavior

The generic type is broken when the given object value contains a spread object.

🙂 Expected behavior

The generic type should contain the spreaded object properties as those are finally included into the result object.

Comment

This may be related to #26412 and other issues like this one but I didn't found a workarround to get a ThisType correctly typed.

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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript type systemThisType utility typeobject spreadgeneric constraints
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate how TypeScript resolves `ThisType` in object literals containing spread elements. Focus on the checker's handling of `ThisType` and object spreads in `src/compiler/checker.ts`. Examine the relationship with issue #26412 and related issues. The playground link provides a reproduction; attempt to narrow down the code path where spread properties are not merged into the `ThisType` context.