adding-types.ts

function Greeter(greeting: string) {
  this.greeting = greeting;
}

Greeter.prototype.greet = function() {
  return "Hello, " + this.greeting;
};

let greeter = new Greeter("world");

let button = document.createElement("button");
button.textContent = "Say Hello";
button.onclick = function() {
  alert(greeter.greet());
};

document.body.appendChild(button);