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In certain cases, a property may have the only purpose to provide some
information that is computed from other information items. For such a
derived property, there is no point to
allow the user entering a value for it. Examples of derived properties of a
class Person are the attributes age and
initials. The value of age is computed on demand
from the value of the attribute dateOfBirth and the current
year. The value of initials is computed from the values of the
attributes firstName and lastName.
Unlike in the case of standard properties, the value of a derived property cannot be set by the user. Rather, it is computed from the values of other properties. Consequently, it does not have a setter, but only a getter method.
When the value of a derived property is computed on demand, only, by its getter method
performing its computation, the derived property is called virtual. The value of such a virtual derived property is not stored. When using
JavaScript's get/set methods in an ES6 class definition, this means that we only define a get method for age, but no internal property like _age, and no
set
method:
var Person = class { constructor (slots) { // assign default values to mandatory properties this._name = ""; // string (non-empty) this._dateOfBirth = 0; // Date // is constructor invoked with a non-empty slots argument? if (typeof slots === "object" && Object.keys( slots).length > 0) { // assign properties by invoking implicit setters this.name = slots.name; this.dateOfBirth = slots.dateOfBirth; } } get name () {return this._name;} set name (n) {this._name = n;} get dateOfBirth () {return this._dateOfBirth;} set dateOfBirth (d) {this._dateOfBirth = d;} get age () { var today = new Date(); return today.getFullYear() - this.dateOfBirth.getFullYear(); } }