microsoft / microsoft/TypeScript
No error when using arrow function as right operand of `instanceof`
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Description
🔎 Search Terms
unsound, instanceof, instance, object, types, error
🕗 Version & Regression Information
- This is unsound error
- All versions in playground have this bug
⏯ Playground Link
💻 Code
console.log(TypeError instanceof (() => 3));
🙁 Actual behavior
TypeScript successfully transpiles the code
🙂 Expected behavior
Throws error because the right-side operand has not prototype
Additional information about the issue
No response
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
Reproduce the issue using the provided TypeScript Playground link and the instanceof example with an arrow function as the right operand. Trace how this expression is checked and emitted, then verify that the completed change reports the missing-prototype error while preserving valid instanceof cases.
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