api-mock-demo-2.js

server.post("/register", (request, response) => {
    const userName = request.body.userName;

    // we can use request data to trigger positive
    // or negative response, whatever we need
    if (userName === "invalid") {
        response.send(400, {
            message: "Invalid user name."
        });
    } else {
        response.send(200);
    }
});