Array method of J S, loop

One , function

1. Defined function

function  Function name ( Parameters 1, Parameters 2,… ) { 
     //JavaScript sentence ;
     [return  Return value ] 
  }

2. Call function       Event name ＝" Function name (  )" ;

3. Scope of variable （1） Global variables    （2） local variable

var i=20;
function first(){
    var i=5;
   document.write("<br/> function first Medium i The value is ："+i);
}
function second(){
   var t=document.getElementById("number").value;
   if(t>i){
      document.write(" function second Medium t The value is ："+t);}
   else{
     document.write(" In a global variable i The value is ："+i);
	 }
     first();
}

Two , Array （ A collection of one or more values with the same data type ）

Create array

var   Array name  = new Array(size);

new： Keywords representing arrays

size： Indicates that... Can be stored in the array Total number of elements put

***JavaScript The array in stores a series of values with a name , Use subscripts to distinguish each value in the array , Array subscript from 0 Start ***

Array assignment

On declaration, assign

var week= new Array (" Sunday "," Monday "……);

Assign values respectively

var week= new Array ( 7);
week[0] = " Sunday ";
week[1] = " Monday  ";
week[2] = " Tuesday ";
week[3] = " Wednesday ";
……

Accessing array elements

Directly access the elements of the array through the name and subscript of the array

  Array common methods and properties

Category	name	explain
attribute	length	Sets or returns the number of elements in an array
Method	join（）	Put all the elements of the array into a string , Separated by a separator
	sort()	Sort the array
	push()	Add one or more... To the end of the array Elements , And returns the new length

js Array syntax

push() Add an element to the end of the array  

pop() Delete array end element  

unshift() Add elements to the array header    

shift()  Delete the array header element

splice() Delete the elements in the array （ The first parameter is the starting index , The second is the number of deletions ）

3、 ... and , loop

What is cycle ？

A cycle is when conditions are met , To perform an operation repeatedly

The advantages of recycling

1. Resolve duplicate actions

2. Reduce the amount of code written , Make the code structure clear

3. Improve the readability of the code

The structure of the cycle

1. The loop condition

2. Cyclic operation

Classification of cycles

1.for Loop structure

2.while Loop structure

    // 1、for loop      Subscript  / key /  key    【 Known condition , Known range （ First judge , After execution ）】
    /* for(let i = 0; i < arr.length;i++){
        document.write(arr[i]+"<br/>");
    }
 */

    // 2、 while() loop    Subscript  / key /  key    【 Unknown condition , Unknown range （ First judge , After execution ）】
    /* let i = 0;
    while(i < arr.length){
        document.write(arr[i]+"<br/>");
        i++;
    } */

    //3、do while() loop    Subscript  / key /  key     【 At least once （ Execute first , Post judgment ）】
    /* let i = 0;
    do{
        // Loop code block 
        document.write(arr[i]+"<br/>");
        i++;
    }

    while(i < arr.length); */

    // 4、for of   loop   ES5     value  / value  /  Elements   【 Loop through groups 】
    /* for(let value of arr){
        document.write(value+"<br/>");
    } */

    // 5、 for in  loop   ES6     Subscript  / key /  key 
    /* for(let i in arr){
        document.write(arr[i]+"<br/>");
    } */

    // 6、 forEach()  Array methods     Anonymous callback function 

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