function Employee(a) {
  this.name = a;
}

function init(){
  John = Employee("Johnson");
  alert(John.name);
}

You guessed it right. The code is missing the keyword new before the constructor name. Calling init() will give an error. One defensive programming action would be to add a check inside the constructor. It will match the calling object with the constructor class, and will call the constructor (with new this time), to create the object:

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

function init(){
  John = Employee("Johnson");
  alert(John.name);
}

Try calling init(). You will never get an error, because we create the object for you, even if the calling code doesn't.

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