Javascript : How to add getter to an existing object

try defineProperty

Object.defineProperty(member, 'isGuest', {
  get: function() { return this.firstName=='Guest' }
});

You can use the new Object.defineProperty this way:

Object.defineProperty(
    member, 
    'prop', 
    {
        get: function() { 
            return this.lastName.toUpperCase()
        }
    }
);

In the past you had to use __defineGetter__ but now it has been deprecated.

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