snippet.js

function Klass(name) {
  if (!(this instanceof Klass)) {
    return new Klass(name);
  }
  this.name = name;
}

Klass.prototype.greeting = function() {
  alert('Hi, My name is ' + this.name + '!');
};

var bob = Klass('Bob');
var alice = new Klass('Alice');

console.log(bob.greeting());  // Hi, My name is Bob!
console.log(alice.greeting());  // Hi, My name is Alice!