ECMAScript 5th Edition - Test-Driven JavaScript Development - page 165

assert(circle.isPrototypeOf(sphere));
assertEquals(circle, Object.getPrototypeOf(sphere));
}
You might recognize Object.create from Section 7.5.1, The Object.
create Method, in Chapter 7, Objects and Prototypal Inheritance, in which we did
in fact implement exactly such a method. The ES5 Object.create does us one
better—it can also add properties to the newly created object, as seen in Listing 8.8.
Listing 8.8 Create an object with properties
"test Object.create with properties": function () {
var circle = { /* ... */ };
var sphere = Object.create(circle, {
radius: {
value: 3,
writable: false,
configurable: false,
enumerable: true
}
});
assertEquals(3, sphere.radius);
}
As you might have guessed, Object.create sets the properties using Ob-
ject.defineProperties (which in turn uses Object.defineProperty ).
Its implementation could possibly look like Listing 8.9.
Listing 8.9 Possible Object.create implementation
if (!Object.create && Object.defineProperties) {
Object.create = function (object, properties) {
function F () {}
F.prototype = object;
var obj = new F();
if (typeof properties != "undefined") {
Object.defineProperties(obj, properties);
}
return obj;
};
}