Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

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

Bugs

The specification is apparently in error when it restricts
propertyIsEnumerable( ) to check only noninherited
properties. Internet Explorer 5.5 implements this method as
specified. Netscape 6.0 implements it so that it does consider the
prototype chain. Although this is the way the method was probably
intended to work, it violates the specification, and Netscape 6.1 has
been modified to match the IE 5.5. Because of the error in the
specification, this method is less useful than it should be.

See Also

Function.prototype, Object.hasOwnProperty( ); Chapter 8