Microsoft/TypeScript
Narrow subtype-reduction-prone unions to their narrowest constituent
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#47,731 创建于 2022年2月3日
BugDomain: check: Control FlowHelp Wanted
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描述
Bug Report
Type narrowing doesn't seem to take into account an assignment I made to ensure the variable has the right type.
Maybe related? https://github.com/microsoft/TypeScript/issues/43584#issuecomment-816932485
🔎 Search Terms
type narrowing assignment
🕗 Version & Regression Information
4.5.4
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about "type narrowing"
⏯ Playground Link
💻 Code
declare let a: object | any[] | undefined
if (a === undefined) {
a = [] // I expect this line to have TS understand that a is any[] from now on
} else if (!Array.isArray(a)) {
throw new Error()
}
[...a] // complains here because a is object | any[]
🙁 Actual behavior
a has type object | any[]
🙂 Expected behavior
a to have type any[]