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wrong type inference in Promise #31266
Description
TypeScript Version: 3.4.3
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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 | undefinedExpected behavior:
Actual behavior:
Playground Link:
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Activity
GongT commentedon May 6, 2019
No, that is not ok: foo.length -> [Object is possibly 'undefined']
lll000111 commentedon May 6, 2019
初绎 (@GongT) You mean the 3rd line? There is no error in the playground, with all optional checks enabled.
j-oliveras commentedon May 6, 2019
Most likely, related to #9998.
weswigham commentedon May 8, 2019
Yep, see #9998 - we cutoff control flow analysis on function boundaries (mostly), so
fooonly has it's declared type within the Promise callback. If you capture the narrowed value with aconstbefore the callback is made, that'll capture the narrowed value.fatcerberus commentedon May 9, 2019
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
Promisewe 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 commentedon May 9, 2019
ok,It seems that it is the correct solution