type | undefined not assignable to type regardless of null check from Record with noUncheckedIndexedAccess

[DerHerrGammler]

Bug Report

🔎 Search Terms

• null check
• record
• noUncheckedIndexedAccess

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about noUncheckedIndexedAccess option and null check

⏯ Playground Link

Playground link with relevant code

💻 Code

const arr: string[] = []

function addDefinitions(defs: Record<string, string>): void {
    Object.keys(defs).forEach((type: string): void => {;
        if (defs[type] != undefined) {
            arr.push(defs[type]); // ❌ ts error
        }
    });
}

🙁 Actual behavior

In my code i get a Argument of type 'Definition | undefined' is not assignable to parameter of type 'Definition'. error. But as you see i check the object value with if (defs[type] != undefined). But at this.addDefinition(type, defs[type]); the error is thrown anyway.

The active option noUncheckedIndexedAccess will produces this error. But as you see there is a null check so that this error should not be thrown.

🙂 Expected behavior

Because of the null check this error should not be shown.

[RyanCavanaugh]

For performance/correctness reasons we don't narrow when the key is a variable (we can't tell if it's been mutated or not). This limitation was called out in the PR text: #39560. noUncheckedIndexedAccess is extremely conservative in this regard.

[fatcerberus]

Workaround: Assign defs[type] to a temp variable and narrow that.

[DerHerrGammler]

So in those cases best pratice is something like first assign to a variable and checking the variable against null
Like this:

const arr: string[] = []

function addDefinitions(defs: Record<string, string>): void {
    Object.keys(defs).forEach((type: string): void => {;
        const def: string | undefined = defs[type] // <<< assign to const
        if (def != undefined) {
            arr.push(def)
        }
    });
}

Or is there any better solution for this?

[DerHerrGammler]

I just found out, that i can use Object.entries like the following and there is no error thrown:

const arr: string[] = []

function addDefinitions(defs: Record<string, string>): void {
    Object.entries(defs).forEach(([, def]): void => {;
        arr.push(def)
    });
}

thanks you all for the quick help