Core JavaScript 1.5 Guide:Creating New Objects:Defining Getters and Setters

From MDC

[edit] Defining Getters and Setters

A getter is a method that gets the value of a specific property. A setter is a method that sets the value of a specific property. You can define getters and setters on any predefined core object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.

The following JS shell session illustrates how getters and setters could work for a user-defined object o. The JS shell is an application that allows developers to test JavaScript code in batch mode or interactively.

The o object's properties are:

• o.a - a number
• o.b - a getter that returns o.a plus 1
• o.c - a setter that sets the value of o.a to half of its value

js> o = new Object;
[object Object]
js> o = {a:7, get b() {return this.a+1; }, set c(x) {this.a = x/2}};
[object Object]
js> o.a
7
js> o.b
8
js> o.c = 50
js> o.a
25
js>

This JavaScript shell session illustrates how getters and setters can extend the Date prototype to add a year property to all instances of the predefined Date class. It uses the Date class's existing getFullYear and setFullYear methods to support the year property's getter and setter.

These statements define a getter and setter for the year property:

js> var d = Date.prototype;
js> d.__defineGetter__("year", function() { return this.getFullYear(); });
js> d.__defineSetter__("year", function(y) { this.setFullYear(y); });

These statements use the getter and setter in a Date object:

js> var now = new Date;
js> print(now.year);
2000
js> now.year=2001;
987617605170
js> print(now);
Wed Apr 18 11:13:25 GMT-0700 (Pacific Daylight Time) 2001

[edit] Summary

In principle, getters and setters can be either

• defined using object initializers, or
• added later to any object at any time using a getter or setter adding method.

When defining getters and setters using object initializers all you need to do is to prefix a getter method with get and a setter method with set. Of course, the getter method must not expect a parameter, while the setter method expects exactly one parameter (the new value to set). For instance:

o = {
  a:7,
  get b() { return this.a+1; },
  set c(x) { this.a = x/2; }
};

Getters and setters can also be added to an object at any time after creation using two special methods called __defineGetter__ and __defineSetter__. Both methods expect the name of the getter or setter as their first parameter, in form of a string. The second parameter is the function to call as the getter or setter. For instance (following the previous example):

o.__defineGetter__("b", function() { return this.a+1; });
o.__defineSetter__("c", function(x) { this.a = x/2; });

Which of the two forms to choose depends on your programming style and task at hand. If you already go for the object initializer when defining a prototype you will probably most of the time choose the first form. This form is more compact and natural. However, if you need to add getters and setters later – because you did not write the prototype or particular object – then the second form is the only possible form. The second form probably best represents the dynamic nature of JavaScript – but it can make the code hard to read and understand.