E0458: typeof result is of type string and so will never equal undefined; use 'undefined' instead

The typeof operator will always return a string, so comparing to undefined will always yield false. (MDN)

let x = undefined;
if (typeof x === undefined) {
    // this will never run!
    alert("x is undefined!");
}

Instead, compare against the string "undefined":

let x = undefined;
if (typeof x === "undefined") {
    // this will run now :)
    alert("x is undefined!");
}