Microsoft/TypeScript
View on GitHubTypeScript can't infer correct type from while loop if while condition is given a promise
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#55,300 opened on Aug 8, 2023
Domain: check: Control FlowHelp WantedPossible Improvement
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Description
🔎 Search Terms
promise in while loop condition
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about Bugs that aren't bugs
⏯ Playground Link
💻 Code
function sleep(ms: number): Promise<true> {
return new Promise<true>(a => setTimeout(() => a(true), ms))
}
async function example1(): Promise<string> {
let value: string | undefined
while (true) {
await sleep(5000)
value = '5'
break
}
return value
}
async function example2(): Promise<string> {
let value: string | undefined
while (await sleep(5000)) {
value = '5'
break
}
return value
}
🙁 Actual behavior
In example2, TypeScript can't infer the type of value and that it should be now a string and not undefined simply because the Promise that returns true in the while loop has been moved to the while condition instead of the body of the while loop.
🙂 Expected behavior
The expected behaviour should be that TypeScript should be able to infer that it is still a string at the return statement just like in example1