Day 21 (Js)

1.外挂:

Auto Rename Tag
Bracket Pair Colorizer
Chinese (Traditional)
Live Server

2.

HTML:网页内容 h1
CSS:美化 hover
JavaScript: 互动 => 存取 =>指令
计画 回应使用者

3.物件Object:万物皆物件

属性 + 事件() = 物件Object
标签 透过 浏览器 ==> 变成物件Object

4.小背包()

Method|Function =>函式 副程序 方法
给他小背包才会执行

没有小背包() 属性
https://www.w3schools.com/jsref/dom_obj_document.asp

5.BOM、DOM

https://youtu.be/tMmpaR0EnnA?t=32

(1)Browser Object Model (BOM)
https://www.w3schools.com/js/js_window.asp

(2)The HTML DOM (Document Object Model)
DOM树
https://www.w3schools.com/js/js_htmldom.asp

6.变数的命名：[A-Z][a-z]_[0-9]

https://www.w3schools.com/js/js_syntax.asp

不可以
Hyphens: first-name, last-name, master-card, inter-city.

可以
(1)Underscore:

first_name, last_name, master_card, inter_city.

(2)Upper Camel Case (Pascal Case):
FirstName, LastName, MasterCard, InterCity.

(3)Lower Camel Case:
JavaScript programmers tend to use camel case that starts with a lowercase letter:
firstName, lastName, masterCard, interCity.

7.变数：

(1)仓库，等一下拿出来用

(2)先宣告变数再使用，不要用了才宣告
虽然宣告会往上放to the top
(Hoisting is JavaScript's default behavior of moving declarations to the top.)
https://www.w3schools.com/js/js_hoisting.asp
但是还是会出错误:

      console.log(x4);
      var x4 = 25;

8.字串

(1) (Template Literals)：2015後，可以塞${变数}，好用

      `字串`
      var temp = "插入文字";
      var msg3 = `在我们的${temp}中间`;
      console.log(msg3); 

(2)''
(3)""

9.var、let、const的差异

先宣告 var、let、const

    var a = 1;
    let b = 2;
    const c = 3;

(1)更改

var、let 可以被更改
const 不能被更改

      a = 11;
      b = 22;
      c = 99; //会错误

(2)if判断句中，作用域范围

var {}外也不受影响
let {}失效，但这样才是合理的

      if (1 == 1) {
        var x = "rabbit";
        let y = "bunny";
        console.log(x); //rabbit
        console.log(y); //bunny
      }

      console.log(x); //rabbit
      console.log(y); //失效

(3)宣告相同的变数名称

var 更新宣告值
let 不能重复宣告

       var cat = 99;
       var cat = 55;
       console.log(cat); //55

       let cat = 99;
       let cat = 55;
       console.log(cat); //Identifier 'cat' has already been declared

(4)宣告往上拉

behavior of moving declarations to the top
var 被往上拉而有反应
let 反正就是错误。(ReferenceError: Cannot access 变数名称 before initialization(???))

var

        x3 = 20;
        console.log(x3);  //20
        var x3; 

        console.log(x4);  //undefined
        var x4 = 25;

let

         y3 = 20;
         console.log(x3);
         let x3;

         console.log(x4);
         let y4 = 25;

(5)if内的var在外面还能用，function则否

         finction aaa(){
         var bbb = 10 //生命周期在finction内
         }

10. match

regexp只是个说明(可以放甚麽? 可以放字串 |或 RegExp 正规表示法)
:RegExpMatchArray回传甚麽样的值

11.F12 开发者工具

Console 检查用
Network 载入时间
Application 暂时性存放资料

12.VS Code 常用快捷键:

Ctrl + D => 整体改变
Alt +上下 => 移动
Alt + Shift +上下 => 复制
Alt + Shift 多行选取

我需要的分隔线XD
console.log('\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n');