ECMAScript5之Object学习笔记（二）

继续第二部分

Object.freeze(obj)

看字面意思就是“把一个对象冻结”。

下面我们来看个简单的例子以作说明：

 1     // a person instance
 2     var person = {
 3         name: 'Andrew',
 4         job: 'sales manager'
 5     };
 6 
 7     // before freeze
 8     // existing properties maybe be changed or removed, new properties may be added
 9     person.name = 'Bruce';  
10     person.age = 30;
11 
12     console.log(person.name);  // Bruce
13     console.log(person.age);   // 30
14 
15     // freeze the person
16     Object.freeze(person);
17 
18     // isFrozen
19     console.log( Object.isFrozen(person) );  // true
20 
21     // after freeze
22     // fail silently, or strict mode throw a TypeError
23     person.name = 'James'; 
24     person.age = '35';  
25     person.gender = 'male'; 
26 
27     console.log(person.name);    // Bruce
28     console.log(person.age);     // 30
29     console.log(person.gender);  // undefined

上面例子的特点是，person对象的属性像name，job，age的值都是“字面值”，并非对象，如果存在一个属性，其值是一个对象呢？

我们再来一个例子：

 1     // a person again
 2     var person = {
 3         name: {
 4             firstname: 'Bruce',
 5             lastname: 'Lee'
 6         },
 7         age: 35,
 8         job: 'sales manager',
 9         children: ['lucy', 'linda']
10     };
11 
12     console.log('name:' + person.name.firstname + ' ' + person.name.lastname);  // name:Bruce Lee
13     console.log('children:' + person.children);       // lucy, linda
14 
15     // freeze the person 
16     Object.freeze(person);
17 
18     // has been frozen!
19     console.log(Object.isFrozen(person));
20 
21     // change name
22     person.name.firstname = 'Andrew';
23     person.name.lastname = 'Carnegie';
24 
25     // add one more child
26     person.children.push('Tom');
27 
28     console.log('name:' + person.name.firstname + ' ' + person.name.lastname);  // name:Andrew Carnegie
29     console.log('children:' + person.children);   // lucy, linda, Tom

通过上面的2个例子，我们很容易得出结论。那么我们如何做到“深层”freeze呢。

就要用到一个递归函数，来让里面的每个对象都被freeze：

 1     function deepFreeze(o) {
 2         var prop, propkey;
 3 
 4         Object.freeze(o);
 5 
 6         for(propkey in o) {
 7             prop = o[propkey];
 8 
 9             if( !o.hasOwnProperty(propkey) || !(typeof prop === 'object') || Object.isFrozen(prop) ) {
10                 continue;
11             } 
12             // Recursively call deepFreeze
13             deepFreeze(prop);
14         }
15     };
16 
17     // if we use deepFreeze to replace with Object.freeze in the previous example, 
18     // then nothing will happen, even person's properties(aka. name, age, children ) are changed

利用的deepFreeze方法，我们就可以把person对象的属性全部“冻结”。

搞清楚Object.freeze的用途后，我们继续来了解下Object.seal

Object.seal(obj)

seal从字面意思上理解有“密封”的意思，我依旧通过例子来说明：

 1     // a person again
 2     var person = {
 3         name: 'Andrew',
 4         age: 25,
 5         children: ['Tom'],
 6         edu: {
 7             major: 'computer science',
 8             university: 'Yale'
 9         }
10     };
11 
12     // before seal
13     // new properties may be added, exisiting properties may be changed or removed
14     person.name = 'Bruce';
15     person.gender = 'male';
16     delete person.age;
17 
18     // looking up the structure 
19     console.dir(person);
20 
21     // seal the person
22     Object.seal(person);
23 
24     // has been sealed
25     console.log( Object.isSealed(person) ); // true
26 
27     // after seal
28     // existing (writable) properties can be changed
29     person.name = 'Jackson';
30     console.log(person.name);  // Jackson
31 
32     // or change value through Object.defineProperty
33     Object.defineProperty(person, 'name', {value: 'Ann'});
34     console.log(person.name);  // Anna
35     
36     // add one more child
37     person.children.push('lucy');
38     console.log(person.children);  // tom, lucy
39 
40     // internal object is still extensible, unless you seal it too (eg. Object.seal(person.edu) )
41     person.edu.degree = 'master';
42     console.log(person.edu.degree);  // master
43 
44     // silently fail or in strict mode throw TypeError
45     // when add or delete property
46     person.job = 'Front End Developer'; 
47     delete person.name; 
48     // TypeError when add property through Object.defineProperty
49     Object.defineProperty(person, 'job', {value: 'teacher'}); // TypeError

通过上面的例子，我们可以看到，seal一个对象后，使得它不可扩展，但是其属性值（该属性writable为true）是 可以改变的。还有一点就是若其属性是一个对象，姑且叫内部对象吧，那么这个内部对象还是可以进行扩展的，除非写一个递归的deepSeal方法， 这个可以参考上面的deepFreeze

第二部分就到这里，感兴趣的同学可以前往MDN查看，Object.freeze，Object.seal。