Description
π Search Terms
generic truthy narrowing generic index
π Version & Regression Information
This is the behavior in every version I tried, and I reviewed the FAQ for entries about narrowing truthy generic types
β― Playground Link
π» Code
const record: Record<string, unknown> = {};
function foo<T extends string | false | null>(val: T) {
if (val) {
val;
//^?
bar(val);
record[val] = 5;
} else {
val
//^?
}
}
function foo2(val: string | false | null) {
if (val) {
val;
//^?
bar(val);
record[val] = 5;
} else {
val
//^?
}
}
function bar(val: string) {
return val;
}
π Actual behavior
Error on line 8, cannot use val to index object type with string key.
Val appears to be narrowed to NonNullable<T>
yet I can use it as string when calling functions?
Note: foo2
does not use generics and has more expected bahavior.
π Expected behavior
type Falsy = false | null | undefined | 0 | "";
(might remove 0 | ""
from this because that gets into negated types... false seems like a common enough one to matter though)
On line 5 inside of the if check, I expect val to be narrowed to Exclude<T, Falsy>
but instead, is incorrectly (imo) narrowed to NonNullable<T>
which does not exclude false.
What is exceptionally strange, is that the type narrowing behavior seems to differ depending on how val
is used. I can pass it to the function that takes a string parameter, but when I try to use it to index an object on line 8, I get an error.
Additional information about the issue
This issue was discussed in detail by others more experienced than me in this typescript discord thread:
https://discord.com/channels/508357248330760243/1275942751724372110
Invite to server if you aren't a member:
https://discord.com/invite/typescript