Binding Properties of an Object to Its Own Properties

You want to base the value of one property on the value of another property.

Use one of the computed property macros like Ember.computed.alias or Ember.computed.gte

export default Ember.Object.extend({
    firstName : null,
    lastName : null,
    surname : Ember.computed.alias("lastName"),
    eligibleForRetirement: Ember.computed.gte("age", 65)
});

Ember.js includes a number of macros that will help create properties whose values are based on the values of other properties, correctly connecting them with bindings so they remain updated when values change. These all are stored on the Ember.computed object and documented in the API documentation