var obj = {

    a: undefined,

    b: null,

    c: false

};

​

// a, b, c all found

for ( var prop in obj ) {

    document.writeln( "Object1: " + prop + "<br>" );

}

​

function Class(){

    this.a = undefined;

    this.b = null;

    this.c = false;

}

​

Class.prototype = {

    a: undefined,

    b: true,

    c: true,

    d: true,

    e: true

};

​

var obj2 = new Class();

​

// a, b, c, d, e found

for ( var prop in obj2 ) {

    document.writeln( "Object2: " + prop + "<br>");

}

​

function hasOwnProperty(obj, prop) {

    var proto = obj.__proto__ || obj.constructor.prototype;

    return (prop in obj) &&

        (!(prop in proto) || proto[prop] !== obj[prop]);

}

​

if ( Object.prototype.hasOwnProperty ) {

    var hasOwnProperty = function(obj, prop) {

        return obj.hasOwnProperty(prop);

    };

}

​

document.writeln( "<br>" );

// a, b, c found in modern browsers

// b, c found in Safari 2.0.1 and older

for ( var prop in obj2 ) {

    if ( hasOwnProperty(obj2, prop) ) {

        document.writeln( "Object2 w/ hasOwn: " + prop + "<br>" );

    }

}

​

document.writeln( "<br>" );

var img = new Image();

document.writeln( 'naturalWidth: ' +  hasOwnProperty(img,'naturalWidth') + '<br>' ); // FAILS here in IE8

​