Enabling the setting noUncheckedIndexedAccess to true in Typescript signifies that the possibility of array access going out of bounds is always considered. As a result, your array type will be interpreted as:
((number | undefined)[] | undefined)[]
When accessing arr[x], it may return a value that is potentially undefined (xxx | undefined). Due to the intricacies in tracking changes to elements within arrays, Typescript is unable to definitively determine whether the array arr has been modified between the checks of if (arr[x]) and the actual access of arr[x]. Therefore, it cannot confidently assert that arr[x] is not undefined.
In cases where a constant or literal value like arr[0] is passed in, Typescript assumes that this value remains unchanged after if (arr[0]), which may not always hold true. An example where this oversight can lead to runtime errors is shown below:
const arr: number[][] = [[], [], []];
if (arr[0]) {
arr.length = 0;
arr[0][0] = 42; // Cannot set properties of undefined (setting '0')
}
Regardless of whether Typescript assumes the presence of undefined or not, its judgement is not foolproof. Hence, it tends to provide a more conservative estimate, leading to the current behavior.
If you are confident that your array accesses will not exceed bounds, the use of ! (arr[x]![y]) can be employed to bypass compile-time linting.