一些JavaScript Quiz

a.x = a = { }, 深入理解赋值表达式

 

var o = {x : 1};
var a = o;

a.x = a = {name:100};

console.log(a.x);    // undefined
console.log(o.x);    // {name:100}

// a.x = a = {name:100};
// 等价于 a.x = (a = {name:100});
// 首先计算a.x的引用，然后计算(a = {name:100})的返回值

 

if 语句的简写

 

var condition = true, numb = 0;
if(condition) {
    alert('rain-man')
}
if(condition) {
    numb = 1 + 2;
}

等同于

 

var condition = true, numb = 0;
condition && alert('rain-man');
condition && (numb = 1 + 2);

&& 和 || 的计算取值

 

(true && 222);        // 222
!!(true && 222);    // true
(false && 222 );    // false

 

(false || 222);     // 222
!!(false || 222);   // true

!!variable 会返回和原值相等的boolean值

Object的构造

 

function Object() { [native code] }
Object.prototype = {
    constructor: function Object() { [native code] },
    hasOwnProperty: function hasOwnProperty() { [native code] },
    isPrototypeOf: function isPrototypeOf() { [native code] },
    propertyIsEnumerable: function propertyIsEnumerable() { [native code] },
    toLocaleString: function toLocaleString() { [native code] },
    toString: function toString() { [native code] },
    valueOf: function valueOf() { [native code] }
};

Object.prototype.constructor === Object;    // true

prototype中的一些细节

 

var A = function(){
    this.name = 'rain-man';
};
A.prototype = {
    name : 'cnblogs'
};
var o = new A();
console.log(o.name);    // 'rain-man'

创建对象，并保持原型链

 

var O = function(obj) {
    function T() {}
    T.prototype = obj;
    return new T();
};

var obj = {name: 'obj', age: 0 }, 
    obj1 = O(obj), 
    obj2 = O(obj1);

// 更改原型链的一处，所有原型链都会更改
obj.name = 'superclass';    
console.log(obj1.name);    // 'superclass'
console.log(obj2.name);    // 'superclass'

// 每一层可单独处理
obj1.name = 100;
console.log(obj1.name);    //100
delete obj1.name;    //暴漏原型链
console.log(obj1.name);    // 'superclass'