[JavaScript] ES6语法小结

1. var versus let/const

let snack='Meow Mix';
function getFood(food){
    if(food){
        let snack='Friskies';
        return snack;
    }

    return snack;
}
getFood(false);    //'Meow Mix'

2. Replacing IIFEs with Blocks

{
    let food='Meow Mix';
}

food;    //Reference Error

3. Arrow Functions

function Persion(name){
    this.name=name;
}
Person.prototype.prefixName=function(arr){
    return arr.map(character=>this.name+character);
};

4. Strings

// ---- 4.1 .includes()
const string='food';
const substring='foo';
string.includes(substring);    //true

// ---- 4.2 .repeat()
'meow'.repeat(3);    //'meowmeowmeow'

5. Template Literals

// ---- 5.1 Special characters
let text=`This string contains "double quotes" which don't need to be escaped anymore.`;

// ---- 5.2 interpolation
const name='Tiger';
const age=13;
`My cat is named ${name} and is ${age} years old.`;

// ---- 5.3 Preserve new line
let text=(`cat
dog
nickelodeon`);

// ---- 5.4 Accept expressions
let today=new Date();
let text=`The time and date is ${today.toLocalString()}`;

6. Destructuring

// ---- 6.1 Destructuring Arrays
let [a,b,c,d]=[1,2,3,4];
a;    //1
b;    //2

// ---- 6.2 Destructuring Objects
let luke={occupation:'jedi',father:'anakin'};
let {occupation,father}=luke;
occupation;    //'jedi'
father;    //'anakin'

7. Modules

// ---- 7.1 Named Exports
export let name='David';
export let age=25;

export function sumTwo(a,b){
    return a+b;
}

// ---- 7.2 Export a list
function sumTwo(a,b){
    return a+b;
}
function sumThree(a,b,c){
    return a+b+c;
}

export {sumTwo,sumThree};

// ---- 7.3 Export default bindings
function sumTwo(a,b){
    return a+b;
}
function sumThree(a,b,c){
    return a+b+c;
}

let api={
    sumTwo,
    sumThree
};

export default api;    //export {api as default};

// ---- 7.4 Import an entire file
import 'underscore';

// ---- 7.5 Named imports
import {sumTwo,sumThree} from 'math/addtion';

// ---- 7.6 Rename the named imports
import {
    sumTwo as addTwoNumbers,
    sumThree as addThreeNumbers
} from 'math/addtion';

// ---- 7.7 Import all the things
import * as until from 'math/addtion';
const {sumTwo,sumThree}=util;

// ---- 7.8 Import from the default binding
import api from 'math/addtion';    //import {default as api} from 'math/addtion';

// ---- 7.9 Mix default
export {foo as default,foo1,foo2};

import foo,{foo1,foo2} from 'foos';

8. Parameters

// ---- 8.1 Default parameters
function addTwoNumbers(x=0,y=0){
    return x+y;
}

addTwoNumbers(2,4);    //6
addTwoNumbers(2);    //2
addTwoNumbers();    //0

// ---- 8.2 Rest parameters
function logArguments(...args){
    for(let arg of args){
        console.log(arg);
    }
}

// ---- 8.3 Option paramters
function initializeCanvas({height=600,width=400,lineStroke='black'}){
    //height,width,lineStroke
}

function initializeCanvas({height=600,width=400,lineStroke='black'}={}){
    //height,width,lineStroke
}

// ---- 8.4 Spread operator
Math.max(...[-1,100,9001,-32]);

let cities=['San Francisco','Los Angeles'];
let places=['Miami',...cities,'Chicago'];

9. Classes

class Person{
    constructor(name,age,gender){
        this.name=name;
        this.age=age;
        this.gender=gender;
    }

    incrementAge(){
        this.age++;
    }
}

class Personal extends Person{
    constructor(name,age,gender,occupation,hobby){
        super(name,age,gender);

        this.occupation=occupation;
        this.hobby=hobby;
    }

    incrementAge(){
        super.incrementAge();

        this.age+=20;
    }
}

10. Symbols

// ---- 10.1 Non-global symbol
const refreshComponent=Symbol();
React.Component.prototype[refreshComponent]=()=>{
    //
};

Symbol('foo')===Symbol('foo');    //false

// ---- 10.2 Global symbol
Symbol.for('foo')===Symbol('foo');    //false
Symbol.for('foo')===Symbol.for('foo');    //true

11. Maps

// ---- 11.1 .set(),.get(),.has()
let map=new Map();
map.set('name','david');
map.get('name');    //david
map.has('name');    //true

// ---- 11.2 Any types as key
let map=new Map([
    ['name','david'],
    [true,'false'],
    [1,'one'],
    [{},'object'],
    [function(){},'function']
]);

for(let key of map.keys()){
    typeof key;    //string,boolean,number,object,function
}

for(let [key,value] of map.entries()){
    key,value;
}

12. WeakMaps

// ---- 12.1 No property name
let _age=new WeakMap();
class Person{
    constructor(age){
        _age.set(this,age);
    }

    incrementAge(){
        let age=_age.get(this)+1;
        _age.set(this,age);
    }
}

const person=new Person();
Reflect.ownKeys(person);    //[]

// ---- 12.2 Auto destroy
let map=new WeakMap();
let el=document.getElementById('someElement');

map.set(el,'reference');
map.get(el);    //'reference'

el.parentNode.removeChild(el);
el=null;
map.get(el);    //undefined

13. Promise

// ---- 13.1 Error handler
new Promise((resolve,reject)=>{
    reject('Failed to fulfill Promise');
}).catch(reason=>console.log(reason));

new Promise((resolve,reject)=>{
    throw 'Exception happens';
}).catch(error=>console.log(error));

// ---- 13.2 Parallel promise
let urls=[
    '/api/commits',
    '/api/issues/completed'
];

let promises=urls.map(url=>{
    return new Promise((resolve,reject)=>{
        $.ajax({url:url}).done(data=>resolve(data));
    });
});

Promise.all(promises)
    .then(results=>{
        //
    });

14. Generators

// ---- 14.1 yield
function* sillyGenerator(){
    yield 1;
    yield 2;

    return 3;
}
let gen=sillyGenerator();
gen.next();    //{value:1,done:false}
gen.next();    //{value:2,done:false}
gen.next();    //{value:3,done,true}

// ---- 14.2 yield promise
function* getData(){
    let entry1=yield request('some_api/item1');
    let data1=JSON.parse(entry1);

    let entry2=yield request('some_api/item2');
    let data2=JSON.parse(entry2);
}

function request(url){
    return new Promise((resolve,reject)=>{
        getJSON(url,resolve);
    });
}

function iterateGen(generator){
    var gen=generator();
    
    (function iterate(val){
        let ret=gen.next();
        if(!ret.done){
            ret.value.then(iterate);
        }
    }());
}

15. Getter and Setter functions

var person={
    firstName:'James',
    lastName:'Bond',
    get fullName(){
        return this.firstName+' '+this.lastName;
    },
    set fullName(name){
        let words=name.split(' ');
        this.firstName=words[0];
        this.lastName=words[1];
    }
};

person.fullName;    //James Bond
person.fullName='Bond 007';
person.fullName;    //Bond 007

16. Object

// ---- 16.1 Variable as property
var foo='bar';
var baz={foo};
baz;    //{foo:'bar'}

// ---- 16.2 Method
var obj={
    method(){
        //
    }
};

// ---- 16.3 Generator
var obj={
    *gen(){
        //
    }
};

// ---- 16.4 Return
function foo(x,y){
    return {x,y};
}

// ---- 16.5 Export
export {sumTwo,sumThree};

// ---- 16.6 String as property name
var propKey='foo';
let obj={
    [propKey]:true,
    ['a'+'bc']:123,
    ['h'+'ello'](){
        //
    }
};

17. Iterable/Iterator

let fib={
    [Symbol.iterator](){
        let pre=0,cur=1;
        return {
            next(){
                [pre,cur]=[cur,pre+cur];
                return {done:false,value:cur}
            }
        };
    }
};

for(var n of fib){
    if(n>10){
        break;
    }

    console.log(n);
}

参考：
es6-cheatsheet
ECMAScript 6入门 - 阮一峰
es6features