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Adding boolean type assertion changes type narrowing #60190
Description
🔎 Search Terms
Type Narrowing
Boolean
Undefined
🕗 Version & Regression Information
Version: 5.6.3
Regression: Unknown
⏯ Playground Link
💻 Code
function testOne(foo: string | undefined) {
const thing = !!foo;
if(thing) {
// Foo type is String
return foo.charAt(0);
}
return 'undefined'
}
function testTwo(foo: string | undefined) {
const thing: boolean = !!foo;
if(thing) {
// Foo type is string | undefined
return foo.charAt(0);
}
return 'undefined'
}
🙁 Actual behavior
foo is typed as string | undefined if thing has explicit type of boolean
🙂 Expected behavior
foo should be typed as string if thing has type of boolean just like if thing's type is inferred
Additional information about the issue
No response
Activity
[-]Adding boolean type assertion changes type narrowing for undefined[/-][+]Adding boolean type assertion changes type narrowing [/+]RyanCavanaugh commentedon Oct 10, 2024
This was an intentional boundary in #44730
jauwesome commentedon Oct 10, 2024
Ryan Cavanaugh (@RyanCavanaugh) Thanks for the quick response! Is there a specific reason why this choice was made? I don't see that listed. Regardless we can close the ticket.
RyanCavanaugh commentedon Oct 10, 2024
IIRC it's possible to get into a type resolution circularity with this inference in a way that wouldn't really be solvable, so you need some "out" in cases where you're not using it for narrowing purposes
jauwesome commentedon Oct 11, 2024
Good to know! That makes sense. Thanks again for your help!