new agnostic

You can force an Object constructor to return an instance of the Object even when the new keyword hasn’t been used:

function User (name, passwordHash) {
  if (!(this instanceof User)) return new User(name, passwordHash) // magic line! = name
  this.passwordHash = passwordHash

Now, the behavior is the expected in both cases:

var x = User('baravelli', 'd8b74df393528d51cd19980ae0aa028e')
var y = new User('baravelli', 'd8b74df393528d51cd19980ae0aa028e')

x instanceof User // true
y instanceof User // true

This behavior is also supported by default by some native types in order to be backward compatibility with the ECMAScript specification, like: