Basic knowledge of loop traversal and function

for of Special for looping arrays , Or an array structure , Can't traverse ordinary objects ,for of Is to take each piece of data

let arr = [1, 2, 3, 4]
        // value  Namely  for of  Ergodic   Array of   data 

        // let obj = {
    
        // name:" Finch ",
        // age:18
        // } 

        let arr1 = []
        for (let val of arr) {
    
            //  loop length Time 
            // value  Namely  arr Every item in the array 
            //  from arr in   Take out every item   Every time   Plug into the  arr1 Go inside 
            // console.log(value);
            arr1.push(val)
        }
        console.log(arr1);

for in You can traverse objects or arrays ( And it is mostly used for arrays ) Get the attribute value yes key

// Traversing objects 
 let obj ={
    
           name:" Finch ",
           age:18
       }

       for(let key in obj)
       console.log(obj[key]); // Finch  18

// Traversal array 
  let arr = [1,2,3,4]
    for(let index in arr){
    
        console.log(index); // Take the subscript  0,1,2,3
    }
    console.log(arr); // [1,2,3,4]

while and do while The difference between

while It is first to judge whether to execute or not
do while It's a first run , I'm making judgments

	// while
    //  times count   On the outside 
    let y = 0 // Fold times 
    let num = 1 // Thickness of paper 
    let hei = 8848000 // The height of zhumurama peak 

    while(true){
     //true Words   Come in 
        y++ // Fold once 
        num *= 2 // Fold once   The height of the paper 
        if(num > hei){
    
            console.log(y);
            break;
        }
    }


//do while 
 let j = 1;
    do{
    
        console.log(j);
        j++
    }while(false){
    

    }

What is a function ?

A function is a series of code or logic packed together to form a function , At certain times , Execute function , Code reuse

The methods of function definition are divided into declaration function and function expression

// Function declaration 
function add(){
    
}
add()

// Function expression 
let fn = function(){
    
  //  Write code 
  console.log(1);
  console.log("js It's too hard ");
  console.log(" Kill me and stop learning ");
}
fn()

Function definitions can be used first and then declared ( Variable Promotion , And will not report a mistake ), But it's better to declare before using
var Declared variable keywords also promote variables , however let Won't
Functions generally do not exist in arrays , It exists in ordinary objects

  let obj = {
    
      fn3: function(){
    console.log("fn3");}
   }
      console.log(obj.fn3);

Function expression , Immediate execution function ( Definition time , Use immediately ) The scope is mainly to create a separate scope , It will not be attached to the global system , Similar to block scope

  (function(){
    }())
   (function(){
    })()
   +function(){
    }()
   function(){
    }()
   ~function(){
    }()
   !function(){
    }()

	(function zhuque() {
     // interfere js
            let num = 10096
            console.log(num); //10096
        }())

        !function(){
    
            let num = 10086
            console.log(num); //10086
        }()

The parameters of the function

The parameters of a function are divided into formal parameters and actual parameters . When naming formal parameters , See the name and know the meaning
Formal parameters are written in the function definition () Inside , Its function is to accept the parameters from the arguments
The arguments are written when the function is executed () Inside , Its function is to pass parameters to formal parameters

If a formal parameter does not receive an argument Its formal parameter is undefined
When there are more formal parameters than arguments , First, execute one by one in order , The formal parameter not received is undefined
When there are more arguments than formal parameters , Receive in sequence first , The extra arguments are ignored
The normal operation is how many parameters you need Just write as many formal and actual parameters

 function zhuque(value,value2){
    
            console.log(" Zhuquezhen " + value + value2);
        }

        zhuque(" beautiful "," lovely "," generous ")
        zhuque(" lovely ")
        zhuque(" generous ") //  Rosefinch is so beautiful and lovely 
					   //  Rosefinch is so cute undefined
					   //  Rosefinch is so generous undefined

…rest What is it ?

…rest Is to accept the remaining parameters , Must be written at the end of the formal parameter ,

 function count(...rest){
    
          //  Parameters   Deposit in  rest  In this array 

            console.log(rest); // [1, 2, 3]

            let num = 0
            for(let value of rest){
    
                //  Each iteration calculation 
                num += value 
            }
            console.log(num); //6
        }
        count(1,2,3)

If …rest There is formal parameter reception in front , As the first rest The preceding shape participating arguments correspond to each other one by one , All the remaining arguments are given to rest

  function count(a,...rest){
    
            console.log(a); //1
            console.log(rest); //[2, 100, 100, 100]
        }
        count(1,2,100,100,100)

…rest You don't have to write …rest, You can also name other types , All are acceptable

        function count(a,...arr){
    
            console.log(a); //1
            console.log(arr); //[2, 3]
        }
        count(1,2,3)