Object.prototype.propertyIsEnumerable()

The propertyIsEnumerable() method returns a Boolean indicating whether the specified property is enumerable.

Syntax

obj.propertyIsEnumerable(prop)

Parameters

prop

The name of the property to test.

Return value

A Boolean indicating whether the specified property is enumerable.

Description

Every object has a propertyIsEnumerable method. This method can determine whether the specified property in an object can be enumerated by a for...in loop, with the exception of properties inherited through the prototype chain. If the object does not have the specified property, this method returns false.

Examples

A basic use of propertyIsEnumerable

The following example shows the use of propertyIsEnumerable on objects and arrays:

var o = {};
var a = [];
o.prop = 'is enumerable';
a[0] = 'is enumerable';
o.propertyIsEnumerable('prop');   // returns true
a.propertyIsEnumerable(0);        // returns true

User-defined versus built-in objects

The following example demonstrates the enumerability of user-defined versus built-in properties:

var a = ['is enumerable'];
a.propertyIsEnumerable(0);          // returns true
a.propertyIsEnumerable('length');   // returns false
Math.propertyIsEnumerable('random');   // returns false
this.propertyIsEnumerable('Math');     // returns false

Direct versus inherited properties

var a = [];
a.propertyIsEnumerable('constructor');         // returns false
function firstConstructor() {
  this.property = 'is not enumerable';
}
firstConstructor.prototype.firstMethod = function() {};
function secondConstructor() {
  this.method = function method() { return 'is enumerable'; };
}
secondConstructor.prototype = new firstConstructor;
secondConstructor.prototype.constructor = secondConstructor;
var o = new secondConstructor();
o.arbitraryProperty = 'is enumerable';
o.propertyIsEnumerable('arbitraryProperty');   // returns true
o.propertyIsEnumerable('method');              // returns true
o.propertyIsEnumerable('property');            // returns false
o.property = 'is enumerable';
o.propertyIsEnumerable('property');            // returns true
// These return false as they are on the prototype which 
// propertyIsEnumerable does not consider (even though the last two
// are iteratable with for-in)
o.propertyIsEnumerable('prototype');   // returns false (as of JS 1.8.1/FF3.6)
o.propertyIsEnumerable('constructor'); // returns false
o.propertyIsEnumerable('firstMethod'); // returns false

Specifications

Browser compatibility

[1] Starting in JavaScript 1.8.1 (in Gecko 1.9.2 (Firefox 3.6 / Thunderbird 3.1 / Fennec 1.0)), propertyIsEnumerable('prototype') returns false instead of true; this makes the result compliant with ECMAScript 5.

See also

• Enumerability and ownership of properties
• for...in
• Object.keys()
• Object.defineProperty()