一、循环与迭代 Iteration

倘若使用手动迭代的方式，比如`(for i=0, i<9, i++)`,那么编程就会陷入到dirty-details之中。

ES6标准化了specifi-protocol的iterator-pattern.

    // an array is an iterable
    var arr = [ 10, 20, 30 ];

    for (let val of arr) {
        console.log(`Array value: ${ val }`);
    }
   
   //map and destruction
    var buttonNames = new Map();
    buttonNames.set(btn1,"Button 1");
    buttonNames.set(btn2,"Button 2");
    
    for (let [btn,btnName] of buttonNames) {
        btn.addEventListener("click",function onClick(){
            console.log(`Clicked ${ btnName }`);
        });
    }

    // values, keys and entries 
    for (let btnName of buttonNames.values()) {
        console.log(btnName);
    }
    // Button 1
    // Button 2

二、闭包 Closure

Closure的定义：

  Closure is when a function remembers and continues to access variables
  from outside its scope, even when the function is executed in a
  different scope.

示例应用：

    function counter(step = 1) {
        var count = 0;
        return function increaseCount(){
            count = count + step;
            return count;
        };
    }

    var incBy1 = counter(1);
    var incBy3 = counter(3);
    
    incBy1();       // 1
    incBy1();       // 2
    
    incBy3();       // 3
    incBy3();       // 6
    incByf3();       // 9

在event中的应用：

    for (let [idx,btn] of buttons.entries()) {
        btn.addEventListener("click",function onClick(){
           console.log(`Clicked on button (${ idx })!`);
        });
    }

三、this关键词的function

Function内的this关键词要看其execution-context，首先定义 this-aware的function。

    function classroom(teacher) {
        return function study() {
            console.log(
                `${ teacher } says to study ${ this.topic }`
            );
        };
    }
    var assignment = classroom("gaowei");

然后将其分派到object中：

    var homework = {
        topic: "JS",
        assignment: assignment
    };

    homework.assignment();
    // gaowei says to study JS 

另外一种调取方法：

    var otherHomework = {
        topic: "Math"
    };

    assignment.call(otherHomework);
    // Kyle says to study Math

四、Prototype

定义homework，虽有只有topic这一个property，但却linkage-connect到Object.prototype的多个属性。

> let homework = {
... topic: "JS",
... }
undefined
> homework.
homework.__defineGetter__      homework.__defineSetter__
homework.__lookupGetter__      homework.__lookupSetter__
homework.__proto__             homework.constructor
homework.hasOwnProperty        homework.isPrototypeOf
homework.propertyIsEnumerable  homework.toLocaleString
homework.toString              homework.valueOf

homework.topic

五、Object Linkage

查看下面的案例：

    var homework = {
        topic: "JS"
    };

    var otherHomework = Object.create(homework);
    
    otherHomework.topic;   // "JS"

    homework.topic;
    // "JS"

    otherHomework.topic;
    // "JS"
    
    otherHomework.topic = "Math";
    otherHomewokrk.topic;
    // "Math"
    
    homework.topic;
    // "JS" -- not "Math"