Summary of front-end technology

precompiled

function fn(a,c){
            console.log(a) //f a(){}
            var a = 123
            console.log(a) / / 123
            console.log(c) //f c(){}
            function a(){}
            if(false) {var d=678
            }
            console.log(d)//undefined
            console.log(b)//undefined
            var b =function() {}
            console.log(b)//f(){}
            function c(){}
            console.log(c)//f c(){}
        }
        fn(1.2)
        / / the precompiled
        // Scope creation phase precompile phase
        // What was done during precompilation
        The AO object is accessed by the JS engine itself
        // the function declaration overwrites the variable declaration. // The function declaration overwrites the variable declarationAO: {a:undefined 1 function a(){}
        c:undefined 2 function c(){}
        d:undefined
        b:undefined
        }


Copy the code

This points to the problem

Used directly in functions

   function get(content){
         console.log(content)
     }
     get('hello')// can be thought of as syntactic sugar for the following statements
     get.call(window.'hello')
Copy the code

A function is called as a method of an object (to whom it is called).

  var person = {
        name:'Joe'.run:function(time){
            console.log(`The ${this.name}In running the most${time}Min is not ')
        }
    }
    person.run(30)// can be thought of as syntactic sugar for the following statements
    person.run.call(person,30)
Copy the code

List the topic

    var name=222
       var a ={
           name:111.say:function(){
               console.log(this.name)
           }
       }
       var fun = a.say
       fun() //fun,call(window) 222
       a.say()//a.say.call(a) 111

       var b={
           name:333.say:function(fun){
               fun()
           }
       }
       b.say(a.say)// When passed in, this is equivalent to fun()= a.sage, which is equivalent to the first type 222
       b.say = a.say
       b.say()//b calls equal to console.log(this.name) copied to B's say method 333

Copy the code

This in the arrow function

• The this in the arrow function is bound when the function is defined, not when the function is executed

• In the arrow function, the this point is fixed, not because the arrow function has a mechanism to bind this. The actual reason is that the arrow function does not have its own this, so the internal this is the this of the outer code block. Because it does not have this, it cannot be used as a constructor

       var x =11
          var obj={
              x:22.say:() = >{
                  console.log(this.x)
              }
          }
          obj.say()   // The output is 11
  Copy the code

       var obj={
                 birth:1990.getAge:function(){
                     var b=this.birth
                     var fn=() = >new Date().getFullYear()-this.birth
                     return fn()
                 }
             }
             console.log(obj.getAge())// Output is 2021-1990=31
  Copy the code

  Since the arrow function is defined in getAge() and its parent is the obj inner scope, the this in its arrow function points to the parent OBj object, this.birth is 1990

Depth and light copy in js

Assignment: when we put an object is assigned to a new variable, assign a is actually the addresses of objects in the stack, rather than a pile of data, namely two object point to the same storage space, no matter which object changed, actually is to change the content of the storage space, so the two objects are linked

Shallow copy: The basic data types of objects before and after the copy do not affect each other. However, the reference types of objects before and after the copy do affect each other because they share the same memory block

Deep copy: Creates a new area of the heap to hold objects, the original object and the new object are not affected by each other

• Assignment code

  var person = {
             name:"Zhang".hobby: ['learning'.'Knock code'.'eat the melon']}var person1 = person
         person1.name = "Bill"
         person1.hobby[0] ='play'
         console.log(person)// Both name:" Lisi "hobby:[' play ',' code ',' eat ']
         console.log(person1)
  Copy the code

• Shallow copy code (base data types are not affected, reference data types are changed at the same time)

  var person = {
             name:"Zhang".hobby: ['learning'.'Knock code'.'eat the melon']}function shallowCopy(obj){
             var target = {}
             for(var i in obj){
                 if(obj.hasOwnProperty(i)){
                     target[i]=obj[i]
                 }
             }
             return target
         }
  
  var person1 = shallowCopy(person)
  person1.name='the king 2'
  person1.hobby[0] ="Play"
  console.log(person)// Joe plays code and eats melon
  console.log(person1)// Wang er play code eat melon
  Copy the code

• Deep-copy code (data is not affected before and after copying)

  var person = {
             name:"Zhang".hobby: ['learning'.'Knock code'.'eat the melon']}function deepClone(obj){
          var cloneObj = new obj.constructor()
          if(obj===null) return obj
          if(obj instanceof Date) return new Datr(obj)
          if(obj instanceof RegExp) return new RegExp(obj)
          if(typeofobj ! = ='object') return obj
          for(var i in obj){
              if(obj.hasOwnProperty(i)){
                  cloneObj[i]=deepClone(obj[i])
              }
          }
          return cloneObj
      }
      Parse (json.stringify (person))// Var person1 = json.parse (json.stringify (person)
      var person1 = deepClone(person)
      person1.name="Hututu"
      person1.hobby[0] ="Playing mahjong"
      console.log(person1)// Both have no effect at all
      console.log(person)
  Copy the code

  Shallow copy implementation

  • CloneObj in Lodash
  • . Expansion operator
  • Array.prototype.concat()
  • Array.prototype.slice()

  Deep copy implementation

  • Json.parse (json.stringify ()) will raise an exception if there are regular expressions, Date objects, regular objects, and Promises in the object
  • Recursive implementation (as in the code above)
  • cloneDeep()
  • jquery.extend()

Image stabilization

You can use closures

 var input = document.querySelector('input')
      function debounce(delay){
          let timer
          return function(value){
              clearTimeout(timer)
              timer=setTimeout(function(){
                  console.log(value)
                  
              },delay)
          }
      }
      var debounceFunc = debounce(1000)
      input.addEventListener('keyup'.function(e){
          debounceFunc(e.target.value)
      })
Copy the code

Using closures, the return function can also use a timer, which is generated only once when debounce is called

The throttle

Do one thing at a time, no matter how many clicks, and wait until the second thing is done. Closures solve throttling

document.querySelector('button').addEventListener('click',thro(handle,2000))
        function thro(func,wait){
            let timeout
            return function(){
                if(! timeout){ timeout =setTimeout(function(){
                        func()
                        timeout=null
                    },wait)
                }
            }
        }
        function handle(){
            console.log(Math.random())
        }
Copy the code

The underlying principles of closures

 function a(){
         var aa=123
         function b(){
             var bb = 234
             console.log(aa)
         }
         return b
     }
     var res = a()
     res()// can output 123
Copy the code

After a() is executed, the corresponding scope chain of A will be broken, but B can access the scope chain of A when defining. This scope chain will not be broken, so res() can still access aa=123, which can output 123. The closure can keep the variable in memory all the time.

Closures implement the singleton pattern

var createLogin = function () {
        var div = document.createElement('div')
        div.innerHTML = "I'm the div that pops up."
        div.style.display = 'none'
        document.body.appendChild(div)
        return div
    }

    var getSingle = function(fn){
        var result
        return function(){
            return result || (result=fn.apply(this.arguments))}}var create = getSingle(createLogin)
    document.querySelector('button').onclick=function(){
        var loginLay = create()
        loginLay.style.display = "block"
    }
Copy the code

Create only one div, no matter how many times you click

Js runtime mechanism

Js is a single thread, because JS is inseparable from the operation of the user. Assuming that JS is multi-threaded, it is impossible to create and delete the same DOM at the same time when operating dom

arguments

function get(){
    console.log(arguments)
}
get(1.2.3)
// Arguments is an array object of class
/ / by Array. Prototype. Slice. The call (the arguments) is transformed into an Array
// It can also be converted to an array using the expansion operator in ES6
Copy the code

Why does b in the code become a global variable when this function is called

 function func(){
            let a=b=3
        }
        func()
        console.log(b)/ / 3
        console.log(a)//error
Copy the code

Because let a=b=3 is equivalent to let a=(b=3), b is not declared, so it will become a global variable when it is created. After func() is executed, b is a global variable, so it can be accessed, but a is in func() scope and cannot be accessed externally

Which operations cause memory leaks

• closure
• Unexpected global variables
• The forgotten timer
• An out-of-dom reference (for example, var div=document.querySelector(‘div’) is then removed, but the reference to div is not removed, and the reference to div remains in memory)

Higher-order functions

A function that takes a function as an argument or return value

function highOrder(params,callback){
    return callback(params)
}
Copy the code

Write a map

 var arr=[1.2.3]
        function map(arr,mapCallback){
            // Check if the parameters are correct
            if(!Array.isArray(arr)||! arr.length||typeofmapCallback! = ="function") {return []}
            else{
                let result = []
                for(let i=0; i<arr.length; i++){ result.push(mapCallback(arr[i],i,arr))//1 for each item 2, index 3 is passed into the array
                }
                return result
            }
        }

        var res = map(arr,(item) = >{
            return item*2
        })

        console.log(res)/ / 2 minus 2
Copy the code

Js event loop mechanism

• Asynchronous operations in JS, such as fetch setTimeout setInterval, will be pushed into the call stack and the message will be put into the message queue. The message queue will be executed after the call stack is empty

• The asynchronous operation of the Promise async await will be added to the microtask and will be executed as soon as the call stack empties and the microtask added to the call stack is executed as soon as possible

Js singleton mode

• Definition 1. only one instance 2. globally accessible
• Main problem solved: a globally used class is frequently created and destroyed
• When to use: When you want to control the number of instances and save systematic resources
• How to implement: determine whether the system has this singleton, if so return, not create
• Advantages of the singleton pattern: Having only one instance in memory reduces memory overhead especially for frequent instance creation and destruction (such as the cache of the home page)

Es6 Creates a singleton

class Person{
         constructor(name,sex,hobby){
             this.name = name
             this.sex = sex
             this.hobby = hobby
         }
         static getInstance(name,sex,hobby){
             if(!this.instance){
                 this.instance = new Person(name,sex,hobby)
             }
             return this.instance
         }
     }

     let person1 = Person.getInstance('hututu'.'male'.'eat the melon')
     let person2 = Person.getInstance('Handsome Hu'.'woman'.'Beat the beans')
     console.log(person1==person2)//true
     console.log(person1)//' hututu ',' male ',' melon '
     console.log(person2)
Copy the code

The strategy pattern

• Definition of a policy pattern: Define a series of algorithms, encapsulate them one by one, and make them interchangeable
• The strategy pattern refers to defining a set of algorithms and encapsulating them one by one. Separating the immutable from the changing is the theme of every design pattern, and policy patterns are no exception. The purpose of policy patterns is to separate the use of algorithms from the implementation of algorithms

	var registerForm = document.querySelector('.registerForm')

     var strategies = {
         isNonEmpty: function(value,errorMsg){
             if(value == ' ') {return errorMsg}
         },
         minLength:function(value,length,errorMsg){
             if(value.length<6) {return errorMsg}   
         },
         isMobile:function(value,errorMsg){
             if(!/ ^ 1 [3 5 | | 5] [0-9] {9} $/.test(value)){return errorMsg}
         }
     }

        // New Validator()
       var validataFun = function(){
           var validator = new Validator()
           // Add validation rules
           validator.add(registerForm.username,'isNonEmpty'.'User name cannot be empty')
           validator.add(registerForm.password,'minLength:6'.'Password length must not be less than 6 characters')
           validator.add(registerForm.username,'isMobile'.'Mobile phone number format is not correct')
           // Enable authentication
           var errorMsg = validator.start()
           return errorMsg
       }

       registerForm.onsubmit = function(){
           var errorMsg = validataFun()
           if(errorMsg){
               alert(errorMsg)
               return false}}// Encapsulate the policy class constructor class
       var Validator = function(){
           // Saves an array of validation rules
           this.cache=[]
       }

       Validator.prototype.add = function(dom,rule,errorMsg){
           var arr = rule.split(':')
           this.cache.push(function(){
               var strategy = arr.shift()
               arr.unshift(dom.value)
               arr.push(errorMsg)
               returnstrategies[strategy](... arr) }) } Validator.prototype.start =function(){
           for(var i=0,vaFunc; vaFunc =this.cache[i++];) {var msg = vaFunc()
               if(msg){
                   return msg
               }
           }
       }
Copy the code

BFC

• Definition: block-level formatting context, which refers to a separate block-level rendering area with only block-level BOX participation. This area has a set of rendering rules that constrain the layout of the block-level BOX, independent of the area outside

  Let’s start with a phenomenon

  A box father does not set height, and cannot lift itself when the son element is floating, i.e., the height of father is always 0, and the box does not form a BFC

• How do I create a BFC

  • The value of float is not None
  • The value of position is not static or relative
  • The value of display is inline-block, flex, or inline-flex
  • Overflow: Hidden (a better way not to affect the external layout)

• Other functions of the BFC

  • Margin collapse can be eliminated
  • Prevents elements from being overwritten by floating elements

Array flattening

• Arrays come with flattening methods

  const arr = [1[2[3[4.5]]].6]
  console.log(arr.flat(Infinity))
  Copy the code

• Regular add JSON. Parse (JSON. Stringify (arr))

• recursive

​