'noUncheckedIndexAccess' Typescript compiler option

noUncheckedIndexAccess.ts

function upper(arr: string[]) {
  for (let i = 0; i < arr.length; i += 1) {
    let str = arr[i];
    console.log(str.upperCase())
  }
}

upper(['foo', 'bar', 'baz']);

// In Typescript when index into an array you get the element type of the array.
// but technically that isnt really safe, because Javascript has funky ways of creating
// arrays with holes in it.

// this Typescript will catch up
upper(['foo', , 'baz']); // type 'undefined' is not assignable to type 'string'

// but if instead we do this:
let a = [];
a[42] = 'hello';
upper(a)

// at this point we have created an array with 41 holes and the 42nd element is present
// and the length is 42 and thats awfully hard for a typechecker to comprehend.
// it is problem that is not solvable meaningfully in a typechecker.


// { "noUncheckedIndexAccess": true } this basically reflects the reality of the world
// that whenever you fish something out of an array at a computed index you might 
// get undefined because you might actually access an element that isnt there and it is
// up to you now to ensure that you handle that, for example by giving a default value

function upper(arr: string[]) {
  for (let i = 0; i < arr.length; i += 1) {
    let str = arr[i] ?? '';
    console.log(str.upperCase())
  }
}