javascript的ES6学习总结(第二部分)

2019-04-21 07:05:45来源:博客园 阅读 ()

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1.数组循环

介绍数组循环之前,先回顾一下ES5数组的循环

(1)数组遍历(代替普通的for):arr.forEach(callback(val,index,arr){todo}) //val是数组的当前项,index是数组的键名(下标),arr是整个数组

let arr = ['today','tomorrow','the day after tommrrow','three days from now'];
arr.forEach(function(val,index,arr){
    console.log(val,index,arr);//today 0 (4) ["today", "tomorrow", "the day after tommrrow", "three days from now"]...
});

(2)数组映射:arr.map(callback(item,index,arr){todo... 正常情况下,需要配合return,返回的是一个新数组,若没有return,相当于forEach,一般用于在有return的情况下}) //item是数组的当前项,index是数组的键名(下标),arr是整个数组

let arr = [
    {title:'aaaa',read:100,hot:true},
    {title:'bbbb',read:100,hot:true},
    {title:'cccc',read:100,hot:true},
    {title:'dddd',read:100,hot:true}
];
let newArr = arr.map((item,index,arr)=>{
    // console.log(val,index,arr);
    let json = {};
    json.t = item.title;
    json.r = item.read;
    json.h = item.hot == true && '真棒!!!';
    return json;
});
console.log(newArr);//(4) [Object, Object, Object, Object]

(3)数组过滤:arr.filter(callback(item,index,arr){todo... 正常情况下,需要配合return,返回值为true的元素,返回值为false的元素则被过滤掉}) //item是数组的当前项,index是数组的键名(下标),arr是整个数组

let arr = [
    {title:'aaaa',read:100,hot:true},
    {title:'bbbb',read:100,hot:false},
    {title:'cccc',read:100,hot:true},
    {title:'dddd',read:100,hot:false}
];
let newArr = arr.filter((item,index,arr)=>{
    // return item.hot == true;
    return item.hot;
});
console.log(newArr);//(2) [Object, Object]

(4)数组单项检测:arr.some(callback(item,index,arr){todo... 正常情况下,需要配合return,如果有一个值为true,则返回true,否则返回false}) //item是数组的当前项,index是数组的键名(下标),arr是整个数组

let arr = ['one','two','three','four'];
let b = arr.some((val,index,arr)=>val=='one');
console.log(b);//true

注:此方法可封装成一个检测数组中是否存在某一项的函数

let arr = ['apple','banana','orange'];
function findInArray(arr,item){
    return arr.some((val,index,arr)=> val == item);
}
console.log(findInArray(arr,'orange'));//true

(5)数组多项检测:arr.every(callback(item,index,arr){todo... 正常情况下,需要配合return,如果所有的值都为true,则返回true,否则返回false}) //item是数组的当前项,index是数组的键名(下标),arr是整个数组

let arr = ['one','two','three','four',6,8,2];
let b = arr.every((val,index,arr)=>val);
console.log(b);//true

(6)数组简化:arr.reduce(callback(prev,cur,index,arr){todo... 正常情况下,需要配合return,返回计算后的结果},传递给函数的初始值(可选,默认为第一项的值)) //prev是数组的前一项(如果第一次循环,则prev默认为0),cur是数组的当前项,index是数组的键名(下标),arr是整个数组;

arr.reduceRight()用法和arr.reduce类似,只不过计算顺序是从右向左

//数组求和
let arr = [1,2,3,4,5];
let res = arr.reduce((prev,cur,index,arr)=>{
    return prev+cur;
},0);
console.log(res);//15//数组求积
let arr = [1,2,3,4,5];
let res = arr.reduce((prev,cur,index,arr)=>{
    return prev*cur;
},1);
console.log(res);//120
// 数组求最大值
let arr = [1,2,3,298,4,5,199];
let res = arr.reduce((prev,cur,index,arr)=>{
    return (cur>prev?cur:prev);
});
console.log(res);//298
// 求阶乘(ES2017新增幂运算符**,例如Math.pow(2,3)可写成2**3。)
let arr = [2,2,3];
let res = arr.reduce((prev,cur,index,arr)=>{
return Math.pow(prev,cur);//也可以写成return prev**cur
});
console.log(res);//64

(7)ES6新增for...of循环:

let arr = ['a','b','c','d'];
// 遍历值
for(let val of arr){
    console.log(val);
}
// 遍历下标
for(let index of arr.keys()){
    console.log(index);
}
// 遍历某一项
for(let item of arr.entries()){
    console.log(item[0],item[1]);
}
// or
for(let [key,val] of arr.entries()){
    console.log(key,val);
}

(8)ES6新增Array.from()方法:将类数组对象(只要有length属性就可以)转换为数组,也可以复制数组

<ul>
    <li>11</li>
    <li>22</li>
    <li>33</li>
    <li>44</li>
</ul>

<script type="text/javascript">
window.onload= function(){
    let aLi = document.querySelectorAll('ul li');
    // let arrLi = Array.from(aLi);//ES6方法
  // let arrLi = [...aLi];//ES6方法
let arrLi = [].slice.call(aLi);//ES5方法 arrLi.pop(); console.log(arrLi); } </script>
function show(){
    let args = Array.from(arguments);
    // let args = [...arguments];//也可以用拓展运算符
    args.push(6);
    console.log(args);//[1, 2, 3, 4, 5, 6]
}
show(1,2,3,4,5);
let str = 'hello';
// let arr = str.split('');
// let arr = [...str];
let arr = Array.from(str);
console.log(arr);//["h", "e", "l", "l", "o"]
let json = {
    0:'one',
    1:'two',
    2:'three',
    length:3
}
let arr = Array.from(json);
console.log(arr);//["one", "two", "three"]

(9)ES6新增Array.of()方法:把一组值转成数组

let arr = Array.of('apple','banana','orange');
console.log(arr);//["apple", "banana", "orange"]

(10)ES6新增Array.find()查找元素方法:查找,找出第一个符合条件的数组成员,如果没有找到返回undefined

let arr = [1,2221,344,876,55,56,78];
let res = arr.find((val,index,arr)=>{
    return val > 333;
});
console.log(res);//2221

(11)ES6新增Array.findIndex()查找元素索引方法:查找,找出第一个符合条件的数组成员的位置,如果没有找到返回-1

let arr = [1,2221,344,876,55,56,78];
let res = arr.findIndex((val,index,arr)=>{
    return val > 333;
});
console.log(res);//1

(12)ES6新增Array.fill()填充数组方法:Array.fill(要填充的内容,开始填充的位置,结束的位置)

let arr = new Array(10);
arr.fill('默认',1,3);
console.log(arr);//[undefined × 1, "默认", "默认", undefined × 7]

(13)ES7(ES2016)新增Array.includes()查找数组中是否存在某一项方法,如果有返回true,否则返回false

let arr = [1,2,3,4];
let b = arr.includes(2);
console.log(b);//true

2.对象简洁语法及对象新增

(1)对象简洁语法

let name = 'pilot';
let age = 18;
let json = {
    name,//相当于name:name
    age//相当于age:age
}
console.log(json);//Object {name: "pilot", age: 18}
let name = 'pilot';
let age = 18;
let json = {
    name,//相当于name:name
    age,//相当于age:age
    showA(){//注意:不要替换为箭头函数
        return this.name;
    },
    showB(){
        return this.age;
    }
}
console.log(json.showA(),json.showB());//pilot 18

(2)ES6对象新增Object.is()方法:比较两个值是否相等

console.log(NaN == NaN);//false
console.log(NaN != NaN);//true
console.log(NaN === NaN);//false
console.log(NaN !== NaN);//true
let b = Object.is(NaN,NaN);
console.log(b);//true
console.log(+0 == -0);//true
let b = Object.is(+0,-0);
console.log(b);//false
console.log(Object.is('aaa','aab'));//false

(3)ES6对象新增Object.assign()方法:用来合并对象,Object.assign(目标对象,source1,source2...),后面的值会覆盖前面的值

let json = {a:1};
let json2 = {b:2,a:2};
let json3 = {c:3};
let obj = Object.assign({},json,json2,json3);
console.log(obj);//{a: 2, b: 2, c: 3}
// 复制数组
let a = [1,2,3,4];
let b = Object.assign([],a);
console.log(b);//[1, 2, 3, 4]

(4)ES8(ES2017)新增Object.keys()、Object.entries()、Object.values()方法

let json = {
    a:1,
    b:2,
    c:3
}
for(let key of Object.keys(json)){
    console.log(key);//a,b,c
}
let {keys,values,entries} = Object;//解构
let json = {
    a:1,
    b:2,
    c:3
}
for(let key of keys(json)){
    console.log(key);//a,b,c
}
for(let value of values(json)){
    console.log(value);//1,2,3
}
for(let item of entries(json)){
    console.log(item);//["a", 1],["b", 2],["c", 3]
}
for(let [key,val] of entries(json)){
console.log(key,val);//a, 1,b, 2,c, 3
}

(5)ES9(ES2018)新增对象的”…“运算符

let {x,y,...z} = {x:1,y:2,a:3,b:4};//在Chrome最新版本测试
console.log(x,y,z);//1 2 {a: 3, b: 4}
let json = {a:3,b:4};
let json2 = {...json};//复制json对象(在Chrome最新版本测试)
delete json2.b;
console.log(json);//{a: 3, b: 4}
console.log(json2);//{a: 3}

3.Promise:解决异步回调问题

(1)Promise.resolve('aa'):将现有的东西,转成一个promis对象,resolve状态,成功状态;

Promise.reject('aa'):将现有的东西,转成一个promis对象,reject状态,失败状态;

Promise.all([p1,p2,p3]):把promise打包,扔到一个数组里面,打包完还是一个promise对象,必须确保所有的promise对象,都是resolve状态,都是成功状态;

Promise.race([p1,p2,p3]):把promise打包,扔到一个数组里面,打包完还是一个promise对象,只要有一个成功,就返回成功状态。

let p1 = Promise.resolve('aaa');
//等价于 let p1 = new Promise(resolve=>{resolve('aaa')});
p1.then(res=>{
    console.log(res);//aaa
});
let p1 = Promise.reject('aaa');
//等价于 let p1 = new Promise((resolve,reject)=>{reject('aaa')});
p1.then(res=>{
    console.log(res);
}).catch(err=>{
    console.log(err);//aaa
});
let p1 = Promise.resolve('aaa');
let p2 = Promise.resolve('bbb');
let p3 = Promise.resolve('ccc');
Promise.all([p1,p2,p3]).then(res=>{
    // console.log(res);//["aaa", "bbb", "ccc"]
    let [res1,res2,res3] = res;
    console.log(res1,res2,res3);//aaa bbb ccc
});
let p1 = Promise.resolve('aaa');
let p2 = Promise.reject('bbb');
let p3 = Promise.reject('ccc');
Promise.race([p1,p2,p3]).then(res=>{
    console.log(res);//aaa
});
new Promise(function(resolve,reject){
    //resolve,成功调用
    //reject,失败调用
})
let a = 103;
let promise = new Promise(function(resolve,reject){
    if(a == 10){
        resolve('成功');
    }else{
        reject('失败');
    }
});
// promise.then(success,fail);
promise.then(res=>{
    console.log(res);//成功
},err=>{
    console.log(err);//失败
})
let a = 103;
let promise = new Promise(function(resolve,reject){
    if(a == 10){
        resolve('成功');
    }else{
        reject('失败');
    }
});
// promise.then(success,fail);
// promise.then(res=>{
//     console.log(res);//成功
// },err=>{
//     console.log(err);//失败
// });
// promise.catch(err=>{//reject,发生错误,别名
//     console.log(err);//失败
// });

// promise.then(success,fail);
promise.then(res=>{
    console.log(res);//成功
}).catch(err=>{//reject,发生错误,别名
    console.log(err);//失败
});

4.模块化

(1)在es6之前,是不支持模块化的,但是社区制定一套模块规范:

Commonjs   主要服务端 nodeJa require('http')

AMD            requireJs,curlJs

CMD            seaJs

ES6模块化出来后,统一服务端和客户端模块规范:

import {xx} from ddd;

(2)基本概念:

注:需要放服务器环境

>如何定义模块?  

export 东西

export const a = 12;
export{
  a as aaa,
  a as bbb
}

>如何使用?  

import

import './modules/1.js';
import {aaa as a,bbb as b} from 'xxx'

使用模块:

<script type="module">

</script>

import特点:

1.import可以是相对路径,也可以是绝对路径

import 'https://cdn.bootcss.com/jquery/2.2.2/jquery.js'

2.import模块只会导入一次,无论引入多少次

3.import './modules/1.js';如果这么用,相当于引入文件

4.有提升效果,import会自动提升到顶部,首先执行

5.导出去模块内容,如果里面有定时器更改,外面也会改动

* 6.import动态引入:

import(),类似于node里面的require,可以动态引入,默认import语法不能写到if之类的语法里

优点:

1.按需加载

2.可以写if中

3.路径也可以写动态

4.因为返回值是promise对象,所以可以用then方法

例子:

one.html

<script type="module">
import {a,b,c} from './modules/1.js';
console.log(a,b,c);//12 15 65
</script>

1.js

export const a = 12;
export const b = 15;
export let c = 65;

two.html

<script type="module">
import {aaa as a,bbb as b,ccc as c} from './modules/2.js';
console.log(a,b,c);//12 5 102
</script>

2.js

const a =12;
const b =5;
const c =102;
export {
    a as aaa,//起别名
    b as bbb,
    c as ccc
}

three.html

<script type="module">
import * as modTwo from './modules/3.js';
console.log(modTwo.aaa);//125
</script>

3.js

const a =125;
const b =25;
const c =1012;
export {
    a as aaa,//起别名
    b as bbb,
    c as ccc
}

four.html

<script type="module">
import a,{cc,dd} from './modules/4.js';
console.log(a,cc,dd);//12 21 23
</script>

4.js

export default 12;
export const cc = 21;
export const dd = 23;

five.html

<script type="module">
//import有提升效果,import会自动提升到顶部,首先执行
console.log(a,cc,dd);//12 21 23
import a,{cc,dd} from './modules/5.js';
</script>

5.js

export default 12;
export const cc = 21;
export const dd = 23;

six.html

<script type="module">
//导出去模块内容,如果里面有定时器更改,外面也会改动
import {a,b} from './modules/6.js';
console.log(a);
setTimeout(() => {
    console.log(a);
}, 3000);
</script>

6.js

let a = 6;
let b = 12;
setTimeout(()=>{
    a = 12345;
},2000);
export {
    a,
    b
}

seven.html

<script type="module">
//动态加载模块,按需加载,可以写if中,路径可以是动态
import('./modules/7.js').then(res=>{
    console.log(res.a+res.b);//27
});
</script>

7.js

console.log('7模块加载了');
export const a = 12;
export const b = 22;

eight.html

<script type="module">
//动态加载模块结合Promise使用
Promise.all([
    import('./modules/1.js'),
    import('./modules/2.js')
]).then(([mod1,mod2])=>{
    console.log(mod1);
    console.log(mod2);
})
</script>

nine.html

<script type="module">
//动态加载模块结合async、await函数使用
async function main(){
    /* 方法1 */
    const mod1 = await import('./modules/1.js');
    const mod2 = await import('./modules/2.js');
    console.log(mod1,mod2);
    /* 方法2 */
    const [m1,m2] = await Promise.all([
        import('./modules/1.js'),
        import('./modules/2.js')
    ]);
    console.log(m1,m2);
}
main();
</script>

到此先写到这,第三部分抽时间会补上。


原文链接:https://www.cnblogs.com/abc-x/p/10744897.html
如有疑问请与原作者联系

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