1
检索一个对象的值，可以用[] 括住一个字符串表达式（是一个合法的标识符而且不是保留字），也可以用.(点)
推荐用点，因为这样更紧凑而且可读性好。
如果检索一个不存在的属性，将返回undefined。
2
Attempting to retrieve values from undefined will throw a TypeError exception. This
can be guarded against with the && operator:
flight.equipment // undefined
flight.equipment.model // throw "TypeError"
flight.equipment && flight.equipment.model // undefined
3
对象通过引用来传递，他们永远不会被复制。
Objects are passed around by reference. They are never copied:
4
原型（prototype） 不是很懂，把原版的英文copy下来。
Every object is linked to a prototype object from which it can inherit properties. All
objects created from object literals are linked to Object.prototype, an object that
comes standard with JavaScript.
When youmake a new object, youcan select the object that should be its prototype.
The mechanism that JavaScript provides to do this is messy and complex, but it can
be significantly simplified. We will add a create method to the Object function. The
create method creates a new object that uses an old object as its prototype. There
will be much more about functions in the next chapter.
if (typeof Object.create !== 'function') {
Object.create = function(o) {
var F = function() {};
F.prototype = o;
return new F();
};
}
var another_stooge = Object.create(stooge);
The prototype link has no effect on updating. When we make changes to an object,
the object’s prototype is not touched:
another_stooge['first-name'] = 'Harry';
another_stooge['middle-name'] = 'Moses';
another_stooge.nickname = 'Moe';
The prototype link is used only in retrieval. If we try to retrieve a property value from
an object, and if the object lacks the property name, then JavaScript attempts to
retrieve the property value from the prototype object. And if that object is lacking the
property, then it goes to its prototype, and so on until the process finally bottoms out
with Object.prototype. If the desired property exists nowhere in the prototype chain,
then the result is the undefined value. This is called delegation.
The prototype relationship is a dynamic relationship. If we add a new property to a
prototype, that property will immediately be visible in all of the objects that are
based on that prototype:
stooge.profession = 'actor';
another_stooge.profession // 'actor'
We will see more about the prototype chain in Chapter 6.
5
检查对象的属性，（不包括函数和原型链上的属性）
可以使用hasOwnProperty函数
The other approach is to use the hasOwnProperty method, which returns true if the
object has a particular property. The hasOwnProperty method does not look at the
prototype chain:
flight.hasOwnProperty('number') // true
flight.hasOwnProperty('constructor') // false
6
delete可以删除对象的属性
another_stooge.nickname // 'Moe'
// Remove nickname from another_stooge, revealing
// the nickname of the prototype.
delete another_stooge.nickname;
another_stooge.nickname // 'Curly'