Introduction to arrays

Array （Array）
An array is also an object , It functions like our normal objects , It is also used to store some values , The difference is that ordinary objects use strings as property names , Arrays use numbers as index elements
–> Indexes
from 0 The starting integer is the index
Array storage is better than ordinary objects , In development, we often use arrays to store some data

One ：① Create an array object

var arr = new Array（）;

② Use literals to create arrays

var arr = [  ];

When creating arrays with literals , You can specify the elements in the array at the time of creation

var arr = [1,2,3,4,5];

–> The elements in the array can be any data type , It can also be an object , It can also be a function

arr = ["hello",1,true,null,undefined];
arr = [{
    name：" The Monkey King "},{
    name：" The sand monk "},{
    name：" Pig eight quit "}];
arr = [function（）{
    alert（1）},function（）{
    alert（2）}];
arr[0]（）// Call functions in the array 

Two ： Adding elements to an array
grammar ：

 Array [  Indexes  ] =  value  ;
arr[0] = 10 ;
arr[1] = 20 ;
arr[2] = 30 ;

3、 ... and ： Read the elements in the array
grammar ：

 Array [ Indexes ]
// If you read a nonexistent array , It does not report an error, but returns undefined
console.log（arr[0]）;

Four ： Gets the length in the array
have access to length Property to get the length of the array （ Number of elements ）
grammar ：

 Array .length
console.log（arr.length）;

For continuous arrays , Use length You can get the length of the array （ Number of elements ）
For discontinuous arrays , Use length You can get the maximum index of the array +1, Try not to create discontinuous arrays

5、 ... and ： modify length
If modified length Larger than the original length , Then the extra part will be empty
If modified length Less than the original length , Then the redundant part will be deleted

6、 ... and ： Adds an element to the last position of the array

arr[arr.length] = 40;

7、 ... and ： Array of 4 A way （ Commonly used ）
①push（）
This method can add one or more elements to the end of the array , And return the new length of the array
You can pass the element to be added as an argument to the method , In this way, these elements will be automatically added to the end of the array
This method returns the new length of the array as the return value

②pop（）
This method can delete the last element of the array , And return the deleted element as the return value

③unshift（）
Add one or more elements... To the beginning of the array , And returns the new array length
After inserting the element forward , The indexes of other elements will be adjusted in turn

④shift（）
You can delete the first element of the array , And return the deleted element as the return value

⑤slice（）
It can be used to extract the specified element from the array
This method does not change the element array , Instead, the intercepted elements are encapsulated into a new array and returned
Parameters
1. Index at the start of the interception , Include start index
2. Intercept the index of the end position , Does not include end index , The second parameter can be omitted , All elements from the beginning of the index will be intercepted
The index can also pass a negative value , From back to front
-1 The last one
-2 second to last

⑥splice（）
Use splice（） Will affect the original array , The specified element will be deleted from the original array , And return the deleted element as the return value
Parameters
first , Search index indicating the starting position
the second , Indicates the number of deletions
The third and beyond , You can pass some new elements , These elements are automatically inserted in front of the start position index

8、 ... and . Traversal of array
So called array traversal , Is to get all the elements in the array