object.propertyIsEnumerable

obj.propertyIsEnumerable(prop)

var o = {};
var a = [];
o.prop = 'is enumerable';
a[0] = 'is enumerable';

o.propertyIsEnumerable('prop'   // returns true
a.propertyIsEnumerable(0        // returns true

var a = ['is enumerable'];

a.propertyIsEnumerable(0          // returns true
a.propertyIsEnumerable('length'   // returns false

Math.propertyIsEnumerable('random'   // returns false
this.propertyIsEnumerable('Math'     // returns false

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