JavaScript: The Definitive Guide, 6th Edition

Property Getters and Setters

We’ve said that an object property is a name, a value, and a set of attributes. In ECMAScript 5^[9] the value may be replaced by one or two methods, known as a getter and a setter. Properties defined by getters and setters are sometimes known as accessor properties to distinguish them from data properties that have a simple value.

When a program queries the value of an accessor property, JavaScript invokes the getter method (passing no arguments). The return value of this method becomes the value of the property access expression. When a program sets the value of an accessor property, JavaScript invokes the setter method, passing the value of the right-hand side of the assignment. This method is responsible for “setting,” in some sense, the property value. The return value of the setter method is ignored.

Accessor properties do not have a writable attribute as data properties do. If a property has both a getter and a setter method, it is a read/write property. If it has only a getter method, it is a read-only property. And if it has only a setter method, it is a write-only property (something that is not possible with data properties) and attempts to read it always evaluate to undefined.

The easiest way to define accessor properties is with an extension to the object literal syntax:

var o = {
    // An ordinary data property 
    data_prop: value,

    // An accessor property defined as a pair of functions
    get accessor_prop() { /* function body here */ },
    set accessor_prop ...