ES6新特性以及一些规范

1.let:使变量成为块级变量，类似于C++,java类的变量

b = 2
if (b == 2) {
    let c = 2;
}
console.log(c) // 报错，因为c是一个块级变量，只存在if块里,我觉得这点特别棒

2.Object

2.1 使用字面量语法来创建对象
    a = new Array() // bad
    a = {} // good
2.2 简写对象方法

    //新特性
    a = {
        sayHello() {
            console.log("hello")
        }
    }
    //旧写法
    a = {
        sayHello: funtion() {
            console.log("Hello")
        }
    }
    a.sayHello();//都有效
    es6增加这个功能真的是！感天动地，省去了很多步骤
2.3 简写对象属性值方法
    name = "CJG"
    a = {
        name
    }
    console.log(a.name) // CJG
    只要你那个变量的名字和你想定义的属性的名字一样，就可以用这种方法简写！很方便吧
    不过呢，如果想用这个的话，就把所有的简写对象属性都放在前面，方便阅读,如
    a = {
        name,
        age,
        school: "SYSU"
    }

3.Array

3.1数组复制
    var s1 = [1,2,3,4]
    //bad
    var s2 = []
    for (let i = 0; i < s1.length; i++)
        s2[i] = s1[i]
    //good
    s2 = [...s1]
    用这种方法复制的话，少写了很多额外的代码,感觉特别好用
3.2在插入数组的时候，用push
    //bad
    array[length] =  newitem
    //good
    array.push(newitem)
3.3使用Array.from将类数组对象转化为数组
    ![](http://images2015.cnblogs.com/blog/993343/201607/993343-20160726212943997-287286683.png)

4.String

4.1 用''符号而不要用""符号来创建字符串
4.2 当字符串过长的时候，分成果断，并用+号连接起来
    //bad
    const errorMessage = 'This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowherefast.';

    // bad
    const errorMessage = 'This is a super long error that was thrown because \
    of Batman. When you stop to think about how Batman had anything to do \
    with this, you would get nowhere \
    fast.';

    // good
    const errorMessage = 'This is a super long error that was thrown because ' +
      'of Batman. When you stop to think about how Batman had anything to do ' +
      'with this, you would get nowhere fast.';
4.3 模板字符串
    let str = "CJG"
    let name
    //bad
    name = "My name is " + str
    //good
    name = `My name is ${str}`
    这样看起来更直观一点，插入什么变量一目了然
4.4 不要用eval()!!!!
4.5 尽量不要用转义符
    //bad
    const foo = '\'this\' \i\s \"quoted\"';
    //good
    const foo = `'this' is "quoted"`

5.Function

5.1不要用arguments,而用我们前面那种将类数组转化为数组的方法
    //bad
    function Hello(name) {
        console.log(`Hello, ${arguments[0]});
    }
    //good
    function Hello(...args) {
        console.log(`Hello, ${args[0]});
    }
5.2不要用function的构造函数去构造函数，因为用它就等于用eval()，可能会带来安全问题

6.Class & Constructors

6.1 在创建类的时候，用class关键字来创建，使用constructor来定义构造函数 // 更加直接
    class Student {
        constructor(name, age, school) {
            this.name = name;
            this.age = age;
            this.school = school;
        }
        sayName() {
            console.log(this.name);
        }
        saySchool() {
            console.log(this.school)
        }
    }
    let stu = new Student("ZHT", 20, "SYSU");
    stu.sayName() // ZHT
    stu.saySchool() // SYSU
6.2 要继承的时候，使用extends

` class goodStudent extends Student {

sayAge() {

console.log(this.age)

}

}

let goodStu = new goodStudent("CJG", 20, "SYSU);

goodStu.sayAge() // 20

6.3方法可以通过返回this来实现方法链式调用

class Person {

setName(name) {

this.name = name;

return this;

}

sayName() {

console.log(this.name);

return this

}

}

这样，我们就可以直接链式调用它的方法了

let p = new Person()

b.setName("cjg").sayName().setName("zht").sayName()

6.4使用class的时候，如果你没有声明构造函数的话，它会自己提供默认的构造函数,如果你不需要在构造函数做额外的事情(例如给某个变量赋值等),就没必要主动声明构造函数

//bad,没有必要，这是系统默认的

class goodStudent extends Student {

constructor(...args) {

super(...args);

}

}

//good 如果需要在构造函数做额外的工作，则主动声明构造函数

class goodStudent extends Student {

constructor(...args) {

super(...args);

this.age = 22;

}

}

7.Iterators

7.1在遍历数组的时候，使用javascript一些内置的函数，不要用for-in,防止你在遍历的时候，可能不小心修改了某些值，或者导致一些副作用
    const numbers = [1, 2, 3, 4, 5];
    let sum = 0;
    //bad
    for (let i = 0; i < numbers.length; i++)
        sum += numbers[i]
    //good
    numbers.forEach(number => sum += number)

8.Properties

8.1用.方法来访问对象的属性
    const test = {
        name: "test"
    }
    console.log(test.name)
8.2当用变量来访问属性的时候，用[]
    let n = "name"
    console.log(test[n])