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Annotation of boolean type ignore in favor of narrowed type #31734
Description
Activity
Union types are narrowed on assignment.
booleanis a union typetrue | falsebehind the scenesAh yeah I could reproduce the error with a different union type
type U = "a" | "b"; let u: U = "a"; if (u == "b") { console.log(); }Seems like an explicit annotation should override narrowing though? Explicit seems like it should always win over any type of inference.
Seems like an explicit annotation should override narrowing though?
Intuitively, there should really not be a difference between
let u: U = "a"; /* some statement */
and
let u: U; u = "a"; /* some statement */
Huh, intuitively I think of the things on the left of an assignment as happening after the things on the right. Maybe this is my C++ background coming through (https://godbolt.org/z/VvVzbE). I guess for javascript both ways are equally valid though.
Cross-linking to #8513, of which this issue is arguably a duplicate
This behavior is unintuitive to me, as well. In particular, with aliased object types, adding a widening type annotation can actually result in narrowing:
type Open = { isOpen: true }; type Closed = { isOpen: false }; type Status = Open | Closed; const openBare = { isOpen: true }; const openAnnotated: Status = { isOpen: true }; type Bare = typeof openBare; // { isOpen: boolean } type Annotated = typeof openAnnotated; // { isOpen: true }
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on Oct 21, 2025
TypeScript Version: 3.5.1
Search Terms:
Boolean explicit annotation ignore
Code
Expected behavior:
4th example above shouldn't error.
Actual behavior:
Error: This condition will always return 'false' since the types 'true' and 'false' have no overlap.
Playground Link:
https://www.typescriptlang.org/play/#src=%2F%2F%20Errors%20as%20expected%0D%0Alet%20bothInferred%20%3D%20true%3B%0D%0Aif%20(bothInferred%20%3D%3D%20false)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20Doesn't%20error%20as%20expected%20%0D%0Alet%20bothExplicit%3A%20boolean%20%3D%20true%20as%20boolean%3B%0D%0Aif%20(bothExplicit%20%3D%3D%20false)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20Doesn't%20error%20as%20expected%0D%0Alet%20leftInferredRightExplicit%20%3D%20true%20as%20boolean%3B%0D%0Aif%20(leftInferredRightExplicit%20%3D%3D%20false)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20Why%20error%3F%0D%0Alet%20rigthInferredLeftExplicit%3A%20boolean%20%3D%20true%3B%0D%0Aif%20(rigthInferredLeftExplicit%20%3D%3D%20false)%20%7B%20%7D%0D%0A
Related Issues:
Tried similar patterns with other types, could not find an error.