In this code
const arr = ['hello', 'world'];
const x = arr[2];
const y = arr[0.5];
arr is a string[], and 10 and 0.5 are numbers, so TS implies that 10 and 0.5 can index arr, therefore inferring that x and y are strings too, while at runtime they'll be undefined.
Is there a configuration option I can write in my tsconfig.json that basically tells typescript "if you're not sure that arr: T[] has an element at i, do not infer that arr[i] is a T"?
TypeScript 4.1 introduced support for checked indexed accesses, which can be enabled via the --noUncheckedIndexedAccess compiler option. If you turn that on, you get this behavior:
const arr = ['hello', 'world'];
const x = arr[10] // number | undefined
console.log(x.toUpperCase()) // compiler error
// ~ <-- object is possibly undefined
console.log(x?.toUpperCase()) // okay
which is what you're looking for. This option is not included in the --strict suite of type checking features, because it can make the compiler complain even for code which is generally considered safe:
for (let i = 0; i < arr.length; i++) {
console.log(arr[i].toUpperCase()) // error!
// -------> ~~~~~~
// arr[i] is possibly undefined
}
So you'll have to decide if your desired behavior is worth the possible false warnings from the type checker, and if so, turn on --noUncheckedIndexedAccess explicitly.