Typescript: wrong type inference in Promise

[dislido]

TypeScript Version: 3.4.3

Search Terms:

Code

let foo: string | undefined;
if (!foo) foo = 'hello';

foo.length; // ok

new Promise(resolve => resolve(foo.length));
//                              ^ should be string, but inferred as string | undefined

Expected behavior:

Actual behavior:

Playground Link:

Related Issues:

[GongT]

No, that is not ok: foo.length -> [Object is possibly 'undefined']

[lll000111]

@GongT You mean the 3rd line? There is no error in the playground, with all optional checks enabled.

[j-oliveras]

Most likely, related to #9998.

[weswigham]

Yep, see #9998 - we cutoff control flow analysis on function boundaries (mostly), so foo only has it's declared type within the Promise callback. If you capture the narrowed value with a const before the callback is made, that'll capture the narrowed value.

[fatcerberus]

we cutoff control flow analysis on function boundaries (mostly)

Which makes perfect sense, because in general the compiler can't know when a function will be called--or even how many times. In the specific case of Promise we know the function will be called only once, and immediately--but this isn't an assumption the compiler can make in the general case.

[dislido]

let foo: string | undefined;
if (!foo) foo = 'hello';

new Promise(resolve => resolve(foo!.length));

ok,It seems that it is the correct solution