This shows how to use "Defensive Programming" (and specifically the "exhaustive type checking" features of TypeScript) to make sure that all code handles all future possibilities.

exhaustiveTrafficSignals.ts

type ITrafficLight = "red" | "redBlinking" | "green" | "yellow" | "yellowBlinking";

function respondToTrafficSignal(signal: ITrafficLight): "stop" | "go" | "pause" {
    if(signal === "red"){
        return "stop";
    } else if(signal === "yellow"){
        return "pause";
    } else if(signal === "green") {
        return "go";
    } else {
        return neverGonnaGetHere(signal) // The compiler catches the logical error: Type '"redBlinking"' is not assignable to type 'never'.
    }
}

function neverGonnaGetHere(hopefullyNotPossibleValue: never): never {
    throw new Error(`Did not expect this value: ${hopefullyNotPossibleValue}`)
}