> o = { x: 1 } { x: 1 } > b = Object.create(o) {} > b.y = 2 2 > b.propertyIsEnumerable('x') false > b.propertyIsEnumerable('y') true > Object.prototype.propertyIsEnumerable('toString') false > a = {x:1, y:2 } { x: 1, y: 2 } > b = Object.create(a) {} > b.z = 3 3 > for(i in b) console.log(b[i]) 3 1 2 undefined > for(i in b) console.log(i) z x y undefined > b.propertyIsEnumerable("toString") false