JavaScript: The Definitive Guide, Fourth Edition

Name

Object.propertyIsEnumerable( ) — will property be seen by a for/in loop?

Availability

JavaScript 1.5; JScript 5.5; ECMAScript v3

Synopsis

                  object.propertyIsEnumerable(propname)

Arguments

propname

A string that contains the name of a property of object.

Returns

true if object has a noninherited property with the name specified by propname and if that property is “enumerable,” which means that it would be enumerated by a for/in loop on object.

Description

The for/in statement loops through the “enumerable” properties of an object. Not all properties of an object are enumerable, however: properties added to an object by JavaScript code are enumerable, but the predefined properties (such as methods) of built-in objects are not usually enumerable. The propertyIsEnumerable( ) method provides a way to distinguish between enumerable and nonenumerable properties. Note, however, that the ECMAScript specification states that propertyIsEnumerable( ) does not examine the prototype chain, which means that it only works for local properties of an object and does not provide any way to test the enumerability of inherited properties.

Example

var o = new Object( ); // Create an object o.x = 3.14; // Define a property o.propertyIsEnumerable("x"); // true: property x is local and enumerable o.propertyIsEnumerable("y"); // false: o doesn't have a property y o.propertyIsEnumerable("toString"); // false: toString property is inherited Object.prototype.propertyIsEnumerable("toString"); // false: nonenumerable ...