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：

    // a person instance
    var person = {
        name: 'Andrew',
        job: 'sales manager'
    };

    // before freeze
    // existing properties maybe be changed or removed, new properties may be added
    person.name = 'Bruce';  
    person.age = 30;

    console.log(person.name);  // Bruce
    console.log(person.age);   // 30

    // freeze the person
    Object.freeze(person);

    // isFrozen
    console.log( Object.isFrozen(person) );  // true

    // after freeze
    // fail silently, or strict mode throw a TypeError
    person.name = 'James'; 
    person.age = '35';  
    person.gender = 'male'; 

    console.log(person.name);    // Bruce
    console.log(person.age);     // 30
    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：

    // a person again
    var person = {
        name: {
            firstname: 'Bruce',
            lastname: 'Lee'
        },
        age: 35,
        job: 'sales manager',
        children: ['lucy', 'linda']
    };

    console.log('name:' + person.name.firstname + ' ' + person.name.lastname);  // name:Bruce Lee
    console.log('children:' + person.children);       // lucy, linda

    // freeze the person 
    Object.freeze(person);

    // has been frozen!
    console.log(Object.isFrozen(person));

    // change name
    person.name.firstname = 'Andrew';
    person.name.lastname = 'Carnegie';

    // add one more child
    person.children.push('Tom');

    console.log('name:' + person.name.firstname + ' ' + person.name.lastname);  // name:Andrew Carnegie
    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：

    function deepFreeze(o) {
        var prop, propkey;

        Object.freeze(o);

        for(propkey in o) {
            prop = o[propkey];

            if( !o.hasOwnProperty(propkey) || !(typeof prop === 'object') || Object.isFrozen(prop) ) {
                continue;
            } 
            // Recursively call deepFreeze
            deepFreeze(prop);
        }
    };

    // if we use deepFreeze to replace with Object.freeze in the previous example, 
    // 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”。

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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：

    // a person again
    var person = {
        name: 'Andrew',
        age: 25,
        children: ['Tom'],
        edu: {
            major: 'computer science',
            university: 'Yale'
        }
    };

    // before seal
    // new properties may be added, exisiting properties may be changed or removed
    person.name = 'Bruce';
    person.gender = 'male';
    delete person.age;

    // looking up the structure 
    console.dir(person);

    // seal the person
    Object.seal(person);

    // has been sealed
    console.log( Object.isSealed(person) ); // true

    // after seal
    // existing (writable) properties can be changed
    person.name = 'Jackson';
    console.log(person.name);  // Jackson

    // or change value through Object.defineProperty
    Object.defineProperty(person, 'name', {value: 'Ann'});
    console.log(person.name);  // Anna
    
    // add one more child
    person.children.push('lucy');
    console.log(person.children);  // tom, lucy

    // internal object is still extensible, unless you seal it too (eg. Object.seal(person.edu) )
    person.edu.degree = 'master';
    console.log(person.edu.degree);  // master

    // silently fail or in strict mode throw TypeError
    // when add or delete property
    person.job = 'Front End Developer'; 
    delete person.name; 
    // TypeError when add property through Object.defineProperty
    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。