[HOLD for payment 2024-10-10] [$1000] Enable the noUncheckedIndexedAccess TS compiler option

[roryabraham]

Slack proposal: https://expensify.slack.com/archives/C01GTK53T8Q/p1717468504413819

Problem

We currently have crash occurring on production. The problematic code can be summarized with a minimal example:

type MyType = {
    something: string;
};

const myArr: MyType[] = [];

// myItem is inferred to have type MyType, when really it is undefined 
const myItem = myArr[42];

// No compiler error, but the app will crash
console.log(myItem.something);

As you can see, we unsafely indexed an array, and then assumed the result was defined. Then trying to access a property of undefined, we experience a crash.

Solution

Enable the noUncheckedArrayAccess TypeScript config. With that config enabled, the type of myItem is correctly inferred to MyType | undefined, and we get a compiler error when trying to access myItem.something, preventing the crash at compile time.

Issue Owner

Current Issue Owner: @adelekennedy

[melvin-bot]

Triggered auto assignment to Contributor-plus team member for initial proposal review - @eh2077 (External)

[melvin-bot]

Triggered auto assignment to @adelekennedy (NewFeature), see https://stackoverflowteams.com/c/expensify/questions/14418#:~:text=BugZero%20process%20steps%20for%20feature%20requests for more details. Please add this Feature request to a GH project, as outlined in the SO.

[roryabraham]

It's worth noting that when enabling this rule we get more than 1000 errors across more than 200 files, so in order for me to accept a proposal it would need to include a rollout plan so we can incrementally adopt this new rule across a subset of files at a time.