Showing how introducing a new type in the ITrafficSignal union will cause a runtime bug. And in this case, a deadly mistake.

theFutureIntroducedABugInTrafficSignals.ts

type ITrafficLight =  "red"
                    | "redBlinking" // <-- new case
                    | "green" 
                    | "yellow" 
                    | "yellowBlinking"; // <-- new case

// Can you spot the bug?
//      You can skip ahead to the answer here: https://gist.github.com/TheCubicleBuddha/7a8854f244697b9894e41d44e1fc6967

function respondToTrafficSignal(signal: ITrafficLight): "stop" | "go" | "pause" {
    if(signal === "red"){
        return "stop";
    } else if(signal === "yellow"){
        return "pause";
    } else {
        return "go";
    }
}