microsoft / microsoft/TypeScript
Should warn against invalid typeof expression
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Suggestion
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Description
TypeScript Version: 3.2.2
Search Terms: typeof undefined
Code
// This condition will always return 'false' since the types '"string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"' and '42' have no overlap.
typeof 42 === 42
typeof 42 === undefined
typeof 42 === null
Expected behavior:
typeof 42 === undefined
typeof 42 === null
Should warn as what typeof 42 === 42 warns.
Actual behavior:
No warnings
Related Issues:
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First steps
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Research direction
Start by reproducing the three comparisons in the linked TypeScript Playground using the reported TypeScript 3.2.2 version and typescript@next. Trace the type-checking path that reports the invalid typeof 42 === 42 comparison. Done means equivalent warnings are produced for comparisons with undefined and null, with appropriate regression coverage.
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
- Clearly specified
- Newbie friendliness
- 45/100