【let var const】

Block level scope
let and const There are block-level scopes ,var No,
let and const You cannot repeat the description in the same scope ,var Sure
Recommended let

var i=99;
for(let i=0; i<5; i++){
     
// You cannot use var Declare variables , Use let Statement , because let It has the function of block level scope 
// If you are using var Statement , Then the printed results   Overall i Will be affected , The output is no longer 99
	console.log(i);    // Output results ： 0 1 2 3 4
}
console.log(i);        // Output results ： 99

JS File import method ： newly build aa.js file , stay html Introduce in the file

<script src="aa.js"></script>

var Meeting Direct change window object , It's not reasonable , When the declared variable is related to window The variables are exactly the same , It will cover window Variable , Make it unable to operate normally and display

var  web = "hdcms";
console.log(window.web);   //  At this time, the printout is hdcms,var Declared variables change global variables , It's not reasonable 

let Can't Direct change window object , When the declared variable is related to window The variables are exactly the same , Will not cover window Variable , More recommended

let web = "hdcms";
console.log(window.web);   //  At this time, the printout is undefine,let Declared variables do not change global variables 

Pass value ：

let a = 1;
let b = a;
console.log(a,b);    //1,1

Byref

let e = { name="a" };
let f = e;
console.log(e,f);     // { name="a"} { name="a" }
f.name = "123”;
console.log(e,f);     // { name="123"} { name="123" }

null and undefined
Function has no return value , The parameter has no value , Declared no assignment , namely No value is undefined

Background management system ： Is based on Vue and elementui Realized , Using the latest front-end technology stack , built-in i18n International solutions , Dynamic routing , Authority verification , Refine a typical business model , Provides a wealth of functional components

Dynamic routing ： Left navigation bar , Bread crumbs 、
index.jsion file ：children Child routes of （id、 level 、 Sub route name 、 route ）, Layer by layer
Vue file ： router
Bread crumbs ：

<el-breadcrumb-item 
	:to="{path:'/'}"
	v-for="item in breadcrumbList"
	:key="item.path"
>
	{
    {
    item.meta.title}}
</el-breadcrumb-item >

<router-view></router-view>
......
computed{
    
	breadcrumbList(){
    
	return this.$route.matched
	}
}

v-for loop , And click to trigger

<div
	v-for="item in list"
	:key="item.title"
>
	{
    {
     item.title}}
</div>

return{
    
	list:[
		{
    title:'A'},
		{
    title:'B'},
		{
    title:'C'},
		{
    title:'D'}
	]
}
methods:{
    
	handClick({
    title}){
    
		switch(title){
    
			case 'A':
				console.log(' I am a A');
				break
			case 'B':
				console.log(' I am a B');
				break
			case 'C':
				console.log(' I am a C');
				break
			case 'D':
				console.log(' I am a D');
				break
		}
	}
}

Interpolation expression ： { { flag ? ‘yes’ : ‘no’ }} data It defines flag The default is false
watch Listener attribute

What is a callback function ？
recursive ？
keyword this？
Arrow function ？ Simplify writing
Closure ？ The child function accesses the parent function
Shake proof . throttle