This is the non-defensive implementation. To see a way to prevent possible errors, check out the defensive implementation: https://gist.github.com/TheCubicleBuddha/7a8854f244697b9894e41d44e1fc6967

simpleTrafficSignals.ts

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

// Can you see where the function below might go wrong in the future?
//      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";
    }
}