Type Assertions

The following two lines look reasonable, but the compiler reports two errors on the second one.

const input = document.getElementById("username");
input.value = "Alice";

error TS18047: 'input' is possibly 'null'.
error TS2339: Property 'value' does not exist on type 'HTMLElement'.

The return type of getElementById is HTMLElement | null. The compiler cannot read your HTML page, so it has no way of telling that username is an input field. It therefore rejects value, a property that only input elements have.