Object JavaScript sample JavaScript with a bug

SamplejavaScript.js

function Greeter(greeting) {
    this.greeting = greeting;
}
Greeter.prototype.greet = function() {
    return "Hello " + this.greeting;
};
var greeter = new Greeter( {
    message: "world"
});
var button = document.createElement('button');
button.innerText= "Say Hello";
button.onClick = function() {
    alert(greeter.greet());
};
document.body.appendChild(button);