JavaScript: The Definitive Guide, 6th Edition

Property Attributes

In addition to a name and value, properties have attributes that specify whether they can be written, enumerated, and configured. In ECMAScript 3, there is no way to set these attributes: all properties created by ECMAScript 3 programs are writable, enumerable, and configurable, and there is no way to change this. This section explains the ECMAScript 5 API for querying and setting property attributes. This API is particularly important to library authors because:

• It allows them to add methods to prototype objects and make them nonenumerable, like built-in methods.

• It allows them to “lock down” their objects, defining properties that cannot be changed or deleted.

For the purposes of this section, we are going to consider getter and setter methods of an accessor property to be property attributes. Following this logic, we’ll even say that the value of a data property is an attribute as well. Thus, we can say that a property has a name and four attributes. The four attributes of a data property are value, writable, enumerable, and configurable. Accessor properties don’t have a value attribute or a writable attribute: their writability is determined by the presence or absence of a setter. So the four attributes of an accessor property are get, set, enumerable, and configurable.

The ECMAScript 5 methods for querying and setting the attributes of a property use an object called a property descriptor to represent the set of four attributes. A property descriptor object ...