[qqqq2010]

It's ensuring a default value basically. If you instantiate a person without an argument and try to reference this.gender later, you'll throw a script error. This is just avoiding that.

A way I'd prefer might look like this:

  function Person = function(gender) {
      this.gender = gender || 'neuter' //the sauce that defines a fallback
  }

[giberson]

This is not the case, because in the constructor person is being set to the argument. If the argument is not supplied (undefined), calling the instantiated object's gender property will result in undefined. [tested and confirmed]