Leak detection and vacancy filling (VIII)

Leak filling （ 8、 ... and ）

css Selector weight

JS> inline style >ID Selectors > Class selectors > Pseudo class selector = Attribute selector > Pseudo element selector > Universal selector

The difference between pseudo elements and pseudo class elements

1. Pseudo class is in the existing DOM On the basis of labels , Generate corresponding styles according to different states , The state changes dynamically according to user behavior ; The pseudo element is in a nonexistent DOM Add a style to it on ;
2. Pseudo class elements use colons （：）, Pseudo elements use double colons （::）;

• animation-timing-function：linear、ease、jump、step
• cellIndex yes td Attributes of the tag
• join() Convert an array into a string , If there is no value in parentheses , Comma separated by default
• reduce() It's an accumulator ： Sum array values 、 Find the maximum value of array 、 Array weight removal

Reference article ：https://www.cnblogs.com/amujoe/p/11376940.html

['a','b','c'].join() //'a,b,c'
['a','b','c']+'' //'a,b,c'
['a','b','c'].reduce((a,b)=>a+b,'') //'abc'
['a','b','c'].join('') //'abc'

• Add an array to a string , Convert to their respective strings and add
• +'3’ yes number type
• new Number Is a built-in function constructor . Although it looks like a number , But it's not a real number ： It has a bunch of extra features , It's an object .

translator's note ：== Will cause implicit type conversion , The object type on the right will be unpacked automatically as Number type .

However , When we use === When the operator , Both types and values need to be equal ,new Number It's not a number , It's an object type . Both return to false.

TCP The protocol is implemented by using continuous ARQ Protocol and sliding window protocol , To ensure the correctness of data transmission , So as to provide reliable transmission

Arrow function does not have its own this,arguments,super or new.target

Script

In the middle ： The script will be loaded first , But during loading , Not finished loading , It will make the script unable to access the elements in .
Put the bottom ： The script loads after loading , Elements that can ensure that the script can access effectively .
Exceptions ： When there is no access element in the script , The two loading methods have the same effect .

http Request return code

Use Promise encapsulation Ajax operation

return new Promise(function(resolve, reject){
    
   var req = new XMLHttpRequest();
   req.open("POST", "www.baidu.com", true);
   req.onload = function(){
    
     if(req.readyState === 4 && req.status === 200){
    
       resolve(req.response);
     } else {
    
       reject(req.statusText);
     }
   }
   req.onerror = function(){
    
     reject(Error(" Network anomalies "));
   }
 });