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spheres, we could simply create more objects like the one we already have, as seen in
Listing 7.48.
Listing 7.48 Creating more sphere objects
"test should create more spheres based on existing":
function () {
var circle = new Circle(6);
var sphere = Object.create(circle);
sphere.area = function () {
return 4 *;
var sphere2 = Object.create(sphere);
sphere2.radius = 10;
assertEquals(1257, Math.round(sphere2.area()));
The Object.create function in Listing 7.49 is simpler than the previous
Function.prototype.inherit method because it only needs to create a
single object whose prototype is linked to the object argument.
Listing 7.49 Implementing Object.create
if (!Object.create) {
(function () {
function F() {}
Object.create = function (object) {
F.prototype = object;
return new F();
We create an intermediate constructor like before and assign the object argu-
ment to its prototype property. Finally we create a new object from the intermediate
constructor and return it. The new object will have an internal [[Prototype]] prop-
erty that references the original object, making it inherit directly from the object
argument. We could also update our Function.prototype.inherit func-
tion to use this method.
ES5 codifies the Object.create function, which “creates a new object with
a specified prototype”. Our implementation does not conform, because it does not
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