TS does not automatically understand object with accessor type if not extracted to a variable

[aghArdeshir]

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])

[IllusionMH]

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]

I think this PR #45974 handles this case. Playground

[aghArdeshir]

Yes @a-tarasyuk . That would cover this case. Nice 👍

[aghArdeshir]

Yes @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]

@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 if and usage but not with property access.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.