JS related interview questions and answers (1)

js What are the basic data types ？

• Number
• String
• Boolean
• Undefined
• Null

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Ajax How to use ？

establish XMLHttpRequest object
         grammar ：let xhr = new XMLHttpRequest();
Send request to server
        get The way ： call open Method , Parameter is （ Request mode , Request address + Request parameters , Asynchronous or not ）
                        call send() Method
        post The way ： call open Method , Parameter is （ Request mode , Request address , Asynchronous or not ）
                          Add request header ：xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
                         call send( Request parameters ) Method

Receive server response
         Use onreadystatechange event ：
        How to write it ：xhr.onreadystatechange=function(){
        if(xhr.readyState==4&&xhr.status==200){
        fun(xhr.responseText);
        }

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How to judge whether a data is NaN？

        NaN Is not a number and the data type is Number, And it doesn't mean itself

         utilize NaN Not equal to unique, not equal to their own characteristics ：
        How to write it ：funcion isNaN(n){
        if(n!=n){
        return true;
        }else{
        return false
        }}

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Null And Undefined difference

The same thing ： stay if In the judgment statement , Will be converted to false

Difference ： Convert to Number type ,null by 0,undefined by NaN
              null Indicates that a value defines , But it is assigned a null value ;undefined The expression defines , No assignment

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Characteristics of closure function ？

         A closure can be understood as a function defined within a function , When one of the internal functions is called outside the external function containing them , It's a closure .

How to write it ：

let fun = (function(){
var count=0;
return function(){
return ++count;
}})()

The nature of closures ：

         The outer function is nested with an inner function , Operate on local variables in sub functions , Take the child function as the return value of the parent function , Bind the return value of the parent function through a global variable , Thus, the life cycle of sub functions and local variables is prolonged , It realizes that the local variables inside the function can be operated outside the function

characteristic ：

• Function nested function
• Functions can reference external parameters and variables internally
• Parameters and variables are not recycled by the garbage collection mechanism

use ：

• Read the variables inside the function ;
• The values of these variables are always kept in memory , It will not be automatically cleared after the outer function call

shortcoming ：

• Can cause memory leaks