1. var is function-scoped, let is block-scoped; //for (const x of ['a', 'b']) {} is ok

2. let/const 有emporal dead zone (TDZ), 其实这和Java生命使用变量一直, 未声明前不可用. 但var声明但变量一旦进入这个变量声明但function, 即使还没到声明这行代码, 但是这个变量已经有了, 并且赋值为 undefined;

   let tmp = true;
   if (true) { // enter new scope, TDZ starts

    // Uninitialized binding for `tmp` is created
    console.log(tmp); // ReferenceError
   
    let tmp; // TDZ ends, `tmp` is initialized with `undefined`
    console.log(tmp); // undefined
   
    tmp = 123;
    console.log(tmp); // 123

   }
   console.log(tmp); // true
   例子2

   if (true) { // enter new scope, TDZ starts

    const func = function () {
        console.log(myVar); // OK!
    };
   
    // Here we are within the TDZ and
    // accessing `myVar` would cause a `ReferenceError`
   
    let myVar = 3; // TDZ ends
    func(); // called outside TDZ

   }

3. always use const, let, 避免 var

1. New integer literals, 直接可以二进制, 八进制, 十六进制了

   0xFF // ES5: hexadecimal
   255
   0b11 // ES6: binary
   3
   0o10 // ES6: octal
   8

2. New Number properties

   Number.isInteger(num)
   Number.isSafeInteger(number)
   Number.MIN_SAFE_INTEGER
   Number.MAX_SAFE_INTEGER
   Number.isNaN(num)
   Number.isFinite
   Number.parseFloat
   Number.parseInt

3. New Math methods

   Math.sign(-8) // -1
   Math.trunc(3.1)
   Math.log10(100)

1. From var to const/let;
2. From IIFEs(Immediately-Invoked Function Expression) to blocks;
3. From concatenating strings to template literals -> ``;
4. From function expressions to arrow functions, array function没有this;
5. Handling multiple return values
   --Multiple return values via arrays //const [, year, month, day] = [0, 2012, 2, 2]
   --Multiple return values via objects //const {foo, bar} = {'foo':'full', 'bar':3, 'name':'eric'}
6. From for and forEach() to for-of;
7. Handling parameter default values -> function foo(x=0, y=0) {};
8. From arguments to rest parameters (变长参数) foo(...args);
9. 通过spread operator (...) 去把Array 变成非数组序列;

10. From function expressions in object literals to method definitions;

    var obj = {

     foo: function () {
         ···
     },
     bar: function () {
         this.foo();
     }, // trailing comma is legal in ES5

    }

to

const obj = {
    foo() {
        ···
    },
    bar() {
        this.foo();
    },
}

11. 使用 class 和 extend 定义类和继承关系

    class Employee extends Person {}

12. From custom error constructors to subclasses of Error

    class MyError extends Error {}

13. From objects to Maps;
14. String 和 Array 新方法;

About ECMAScript 6

1. ECMA = European Computer Manufacturers Association, 是一个致力于信息和通讯标准化的组织. 更多信息
2. ES6 = ECMAScript 6 是一个脚本语言规范. 最早是用来规范JavaScript的, 现如今JavaScript也是这个规范最好的实现.其它实现比如 JScript 和 ActionScript. 更多信息,比如ECMAScript 2015, ECMAScript 2016, ECMAScript 2017, ECMAScript 2018, 参见 wiki
3. ES6 compatibility table
4. ECMAScript 由TC39 (Ecma Technical Committee 39)委员会制定,它主要由大的浏览器厂商组成;
5. ES6 亦 ECMAScript 2015 是多年后憋的一个大的release, 之后的ECMAScript 2016,等每年都会发布一些新的小更新;
6. JavaScript是Oracle(从Sun继承而来)的一个商标,所以 JavaScript的官方名称是ECMAScript.
7. 围绕 JavaScript 有: 1) JavaScript规范,即 ECMAScript 2015,2016,...; 2)JavaScript的Engine, 即规范实现者, 如V8; 3)JavaScript的使用者, 即应用开发人员;
8. ES6是ES5的超集, 没有移除任何feature. 为什么? 难以控制的升级, 老旧代码,世界各地的各种不同的浏览器
9. ES6的主要的features分类
   a. 更好的语法 如: Class, Module;
   b. 标准库的新功能, 如: Strings, Arrays, Promise, Map, Set;
   c. 新feature, 如: Generator, Proxy, WeakMap;

10. Strict mode was introduced in ECMAScript 5 to clean up the language.

    'use strict';

The bodies of modules and classes are implicitly in strict mode in ECMAScript 6 – there is no need for the 'use strict' marker. Given that virtually all of our code will live in modules in the future, ECMAScript 6 effectively upgrades the whole language to strict mode.

要根据 Node.js 的event loop 来理解这些 setImmediate, setTimeout, process.nextTick

Node.js actually provides its own implementation of these methods. Timers integrate very closely with the system, and despite the fact that the API mirrors the browser API, there are some differences in implementation.

console.log("start of hello");

setImmediate(()=>{
    console.log("in setImmediate");
    setImmediate(()=>{
        console.log("in setImmediate's setImmediate"); //the 3rd loop
    });
});

setTimeout(()=>{
    console.log("in setTimeout");
    setImmediate(()=>{
        console.log("in setTimeout's setImmediate"); //2nd loop 
    });
}, 0);

process.nextTick(()=>{
    console.log("in nextTick"); //next event action
    process.nextTick(()=>{
        console.log("in nextTick's nextTick"); // next event action
    });
});

console.log("end of hello");

process.nextTick 是当前event action 处理完之后, 下一个会处理的action.