JS notes (III)

Catalog

1. Array

1.1 Create array

1.2 Index of array

1.3 Traversal array

1.4 Length of array

1.5 Add array elements

1.6 Flip array

1.7 Bubble sort

2. function

2.1 Use of functions

2.2 The parameters of the function

2.3 The return value of the function

2.4 arguments Use

2.5  Two ways to declare functions

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1. Array

1.1 Create array

JavaScript There are two ways to create arrays in ：

• utilize  new  Create array
• Create an array using array literals

utilize  new  Create array  

var  Array name   = new Array(); // Create a new empty array 

Create an array using array literals

​
// 1. Create an empty array using array literal  
var  Array name  =[];
// 2. Create an array with an initial value using array literal 
var  Array name  =['a','b','b'];
// 3. Array can store any type of data , Like strings , Numbers , Boolean value, etc 
var arrStus =['haha',6,true,6.1,'a'];

​

• The literal of the array is square brackets  []
• Declaring an array and assigning values is called array initialization

1.2 Index of array

Indexes ： The ordinal number used to access an array element （ Array index from 0 Start ）

// Define an array 
var arr = [1,2,3];
// Gets the number... In the array 3 Elements 
alert(arr[2]);

1.3 Traversal array

adopt for The circular index traverses each item in the array

// for Loop through groups 
var arr = ['a','b', 'c'];
for (var i = 0; i < arr.length; i++){
    console.log(arr[i]);
}

1.4 Length of array

Use “ Array name .length” You can return the number of array elements

var arr = [1,2,3];
alert(arr.length);  //  Output 3

length Attributes will dynamically change with the number of elements in the array

1.5 Add array elements

1    It can be modified by length Length to achieve the purpose of array expansion

var arr = ['a','b','c'];
arr.length =5;
console.log(arr);

The index number is 3   4  Array element of did not give a value , Is to declare that the variable is not given a value , The default value is  undefined

2   Add new array elements by modifying the array index

var arr = ['a','b','c'];
arr[3]='d';
console.log(arr);// Output a b c d

1.6 Flip array

 var arr=['a','b','c'];
        var new_Arr=[];
        for(var i=arr.length-1;i>=0;i--){
            new_Arr[new_Arr.length]=arr[i];

        }
        console.log(new_Arr);

1.7 Bubble sort

  var arr=[1,2,6,5,19,4];
        for(var i=0;i<arr.length;i++){
            for(var j=0;j<=arr.length-i-1;j++){
                if(arr[j]>arr[j+1]){
                    var mid=arr[j];
                    arr[j]=arr[j+1];
                    arr[j+1]=mid;
                }
            }
        }
console.log(arr);

2. function

function ： It encapsulates a section of A block of code that can be repeatedly called and executed . A lot of code can be reused through this code block .

2.1 Use of functions

The function is used in two steps ： Declare functions and Call function

1 Declare functions

function  Function name (){
     // Function body code 
}

2 Call function

function  Function name (){
     // Function body code 
}
 Function name (); Execute the function body code by calling the function name 

Be careful ： Declaring the function itself does not execute code , Function body code is executed only when the function is called .

2.2 The parameters of the function

When you declare a function , You can add some parameters in parentheses after the function name , These parameters are called Shape parameter , And in the Call this function when , You also need to pass the corresponding parameters , These parameters are called Actual parameters .

Function of parameters  : stay Internal function Some values cannot be fixed , We can use parameters in Pass different values when calling a function go in

function  Function name ( Shape parameter 1,  Shape parameter 2 ,  Shape parameter 3...) { //  You can define as many parameters as you want , Separate with commas 
  //  The body of the function 
}

 Function name ( Actual parameters 1,  Actual parameters 2,  Actual parameters 3...); 

example Addition of two numbers

  function sum(a,b){
           alert(a+b);
       }
       sum(1,2);

Be careful ： stay JavaScript in , The default value of a parameter is undefined

2.3 The return value of the function

sometimes , We would want the function to return the value to the caller , At this time, by using return Statement can be implemented .

return The syntax format of the statement is as follows ：

//  Declare functions 
function  Function name （）{
    ...
    return   The value to be returned ;
}
//  Call function 
 Function name ();    //  At this point, you can call the function to get the function body return  Value after 

• In the use of return When the sentence is , Function will stop executing , And returns the specified value

• If the function does not return , The value returned is  undefined

• return The code after the statement is not executed
• return  Only one value can be returned . If you separate multiple values with commas , Subject to the last one

Function calls another function

Because each function is a separate block of code , Used to complete special tasks , Therefore, it is often used when functions call each other .

2.4 arguments Use

When we are not sure how many parameters are passed , It can be used  arguments  To get . stay JavaScript in ,arguments In fact, it is a built-in object of the current function . All functions have a built-in arguments object ,arguments Object stores all the arguments passed .

• arguments It stores the passed arguments

• arguments The display form is a pseudo array , So you can traverse . Pseudo arrays have the following characteristics

1. have length attribute
2. ​ Store data by index
3. ​ Without array push , pop Other methods

for example ： Use the function to find the maximum value of any number

 function maxNum() {
    var max = arguments[0];
    for (var i = 0; i < arguments.length; i++) {
        if (max < arguments[i]) {
            max = arguments[i];
        }
    }
    return max;
}
console.log(maxNum(2, 4, 5, 9)); // 9
console.log(maxNum(12, 4, 9)); // 12

2.5  Two ways to declare functions

1  Name the function

//  Declaration definition method 
function  Function name () {...}

//  call   
 Function name ();  

2  Anonymous functions

//  This is a function expression , Anonymous functions end with a semicolon 
var  Variable name  = function(){...};

//  The way it's called , The function call must be written below the function body 
 Variable name ();

• Because the function has no name , So it's also called Anonymous functions

• This Variable name   Stored inside is a function

• The code of the function call must be written after the function body