1. Array
A collection of one or more values of the same data type.
arr.length array length
2. Create an array
1. Use literals to create an array
2. Use the method of constructor Array to create an array
three,
1. push can add a string at the last position
After the push method is executed, it will return the length of an array
2. reverse reverse order
3、 join
Use the join method to separate by commas by default
Parameters can be added in the join( ) method, and what parameters are used as the separator
Split with ' 'null character, which means there is no separator in the middle
4. sort array sorting
Sorting the array will change the original array (this method is suitable for single-digit sorting) 
(2) Multi-digit sorting (using functions)
Sort ab from smallest to largest, sort ba from largest to smallest
5、concat
Merge two arrays and generate a new array
6. pop deletes the last item in the array
7. unshift adds an item to the front of the original array
unshift will return the length of the new array
8. Shift deletes the first item in the array
9、indexOf
indexOf returns the index of the character in the array
★ If the character does not exist in the array, it will return -1
10. splice adds or deletes the specified position
splice has three parameters
The first parameter indicates the position of the array
The second parameter indicates how many to delete
The third parameter indicates adding several
11. Add splice at the specified position
