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TS does not automatically understand object with accessor type if not extracted to a variable #46111
Description
Bug Report
🔎 Search Terms
extract to variable accessor, accessor automatically infer
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I searched the FAQ page for
].
⏯ Playground Link
Playground link with relevant code
💻 Code
type T_Object = {
theKey: string;
};
type T_MyType = T_Object | string;
function isItObject(
input: T_MyType
): input is T_Object {
return !!(input as T_Object).theKey;
}
const myVariable: { [key: string]: T_MyType } =
//@ts-ignore
somehowGetSomething();
function myFunctionWithError
(key: string) {
if (isItObject(myVariable[key])) {
console.log('it is object');
return myVariable[key].theKey;
}
return myVariable[key];
}
function myValidFunction
(key: string) {
const extractedVariable = myVariable[key];
if (isItObject(extractedVariable)) {
console.log('it is object');
return extractedVariable.theKey;
}
return myVariable[key];
}🙁 Actual behavior
There are two functions at the bottom of the code. The only difference is that in second case, I extracted myVariable[key] to a variable and used that in if condition and return value. In first function it gives me error, but in the second one (extracted variable) it doesn't.
🙂 Expected behavior
I expected TypeScript to automatically infer myVariable[key] is an object inside the if condition with isItObject(myVariable[key])
Activity
IllusionMH commentedon Sep 28, 2021
Looks like duplicate of #44899 which considered duplicate of #10530 (that accumulated a lot of different cases in comments e.g. #10530 (comment))
a-tarasyuk commentedon Sep 28, 2021
I think this PR #45974 handles this case. Playground
aghArdeshir commentedon Sep 28, 2021
Yes Oleksandr Tarasiuk (@a-tarasyuk) . That would cover this case. Nice 👍
aghArdeshir commentedon Sep 28, 2021
Yes Andrii Dieiev (@IllusionMH) those two iseues seem like to be a better way of expressing what I wanted. Thanks. 👍
I think this issue can/should be closed.
IllusionMH commentedon Sep 28, 2021
Oleksandr Tarasiuk (@a-tarasyuk) great work as always!
However is it safe to do this case if
myVariable[key]might not return same value between calls?With intermediate constant you can be sure that constant isn't changed between
ifand usage but not with property access.typescript-bot commentedon Oct 1, 2021
This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.