Checking for existence of property in if block when --noUncheckedIndexedAccess, does not work when properties are in an array

[LowArmour]

Bug Report

When --noUncheckedIndexedAccess is activated, this blog from Microsoft Devs states that you can check if the property exists either by using "!" or within an "if" block: https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#no-unchecked-indexed-access

Checking if the property exists by using an if block does not work if the property is an element of an array over which you iterate.

This is relevant when you cannot use the "!" operator, for example the following eslint rule is set to "error": https://typescript-eslint.io/rules/no-non-null-assertion/.

🔎 Search Terms

--noUncheckedIndexedAccess bugs

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

type B = {
    [key: string]: number
}

const obj: B = {
    a: 1,
}
const propA = 'a'
if (obj[propA]) {
    const sum = 1 + obj[propA]  //does not show the error
    console.log(sum)
}
const props = ['a', 'b', 'c', 'd', 'e']
for (const prop of props) {
    if (obj[prop]) {
        const sum = 1 + obj[prop] //does show the error: Object is possibly 'undefined'
        console.log(sum)
    }
}

🙁 Actual behavior

Having --noUncheckedIndexedAccess flag activated and using if blocks to check if an object has a property, when such property is an element of an array over which you iterate does not work.

🙂 Expected behavior

The above should work also when the property is an element of an array over which you iterate.

[MartinJohns]

Duplicate of #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal. Just store it in a local const variable.

[LowArmour]

Now I understand. Thanks for help!

[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.