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Narrowing down the type of a parameter isn't recognized in closure #34669
Description
TypeScript Version: v3.7-Beta
Search terms : narrow narrowing down type parameter closure
Code
function foo(arg: string | undefined) {
arg = arg ?? "default";
arg.length // no error
const bar = () => {
arg.length; // error
}
}Expected behavior:
arg should be of type string within the body of the closure.
Actual behavior:
arg is possibly 'undefined' within the body of the closure.
Playground Link: Playground link
Activity
MartinJohns commentedon Oct 23, 2019
Duplicate of #9998.
fatcerberus commentedon Oct 24, 2019
Is this actually a duplicate of #9998? That's a design limitation wherein narrowings are optimistically not reset when calling a function because it would be impractical to follow the control flow into the call. Here, the narrowing is being reset at a closure boundary, which is normal (i.e. not just a design limitation - it's deliberate).
Buntelrus commentedon Nov 20, 2020
What's the conclusion of this?
MartinJohns commentedon Nov 20, 2020
The conclusion is that it's a duplicate of #9998.
Buntelrus commentedon Nov 20, 2020
I don't get it. What can I do to solve this problem?