Methods of wrapping classes and strings

One . Packaging
stay js Three packaging classes are provided for us in , Through these three packaging classes , Data of basic data type can be converted into objects
String（）
You can convert the basic data type string to String object
Number（）
You can convert numbers of basic data types to Number object
Boolean（）
You can convert the Boolean value of the basic data type to Boolean object
But notice , We will not use objects of basic data types in actual development applications , If you use , When making some comparisons or judgments, it will bring unexpected results

This thing is not available to developers , Who is it for ？ The answer is to call it itself ！
We know that methods and attributes can be added to objects , But it cannot be added to the basic data type , When we call properties and methods on the values of some basic data types , The browser will temporarily use the wrapper class to convert it to an object , Then call the object's properties and methods , After calling , And then convert it to the basic data type

Two . Create a string
var str = “Hello Word”
At the bottom, strings are stored as character arrays
[“H”,“e”,“l”,“l”,“o”,“w”,“o”,“r”,“d”]
length attribute
Can be used to get the length of the string

Introduction to common methods of string –>
1.charAt（）
Can return a character at a specified position in a string , It gets the specified characters according to the index

2.charCodeAt（）
You can get the character encoding of the character at the specified position （Unicode code ）

3.String.formCharCode（）
You can get characters according to character encoding

4.concat（）
Can be used to connect two or more strings , Function and + equally

5.indexOf（）
This method can retrieve whether a string contains the specified content
If there is , The index that appears first will be returned
without , Then return to -1
You can also specify a second parameter , Specify where to start searching

6.lastIndexOf（）
The method and indexOf（） It's almost the same , The difference is
indexOf（） It's from the past to the future
and lastIndexOf（） It's back to front

7.slice（）
The specified content can be intercepted from the string , Does not affect the original string , Instead, the intercepted content is returned to
Parameters ：
first , Index of starting position （ Including start position ）
the second , Index of end position （ Does not include end position ）, If you omit the second parameter , Will intercept all the following
You can also pass a negative number as an argument , Negative numbers will be calculated from behind

8.substring（）
A string can be used to intercept , and slice（） similar
Parameters ：
first , Index of starting position （ Including start position ）
the second , Index of end position （ Does not include end position ）
The difference is , This method cannot accept negative values as parameters , If you pass a negative value , It is used by default 0, And it will automatically adjust the position of parameters , If the second parameter is less than the first , Then automatically exchange

9.split（）
You can split a string into an array
Parameters ：
You need a string as an argument , It will be split into arrays according to the string

10.toUpperCase（）
Replace a string with uppercase and return

11.toLowerCase（）
Replace a string with lowercase and return