microsoft / microsoft/TypeScript
Inferring "this" from arrow function is {}
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Description
TypeScript Version: Version 3.6.0-dev.20190601
Search Terms: This, Arrow, Functions, Infer, Inference, Empty, Object, Incorrect, Any
Code
type FunctionThisType<T extends (...args: any[]) => any> =
T extends (this: infer R, ...args: any[]) => any ? R : any
let fn = () => {}
let value: FunctionThisType<typeof fn> = "wrong"
Expected behavior:
error TS2322: Type '"wrong"' is not assignable to type 'typeof globalThis'.
Found 1 error.
Actual behavior:
No errors found.
Playground Link: Playground
Related Issues: None found.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked TypeScript Playground and reproduce the conditional-type inference case for an arrow function's this type. Trace how FunctionThisType<typeof fn> is inferred, then verify that the example reports the expected TS2322 error instead of accepting the string assignment.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100