Common APIs (Methods) for scope -number and string

What is scope ?

Scope : A range in which variables work Because every variable Have their own corresponding living environment
Global scope ： By script Packages can be called global scopes
Function scope
Each function Will generate an independent function scope
Variables declared within the scope of a function Can only be used within the scope of the current function Cannot access in other scopes
Function scope within function scope It belongs to the inclusion relationship The inner scope can access the outer scope But the scope inside cannot be accessed from the outside
Empathy The function scope cannot be accessed globally Function scope can access global
Block scope recognize { } Is the domain

           There is no intersection between scopes   Can't visit each other 

  //  Global scope 
        let a = 1 
        function zhuque(){
    
            console.log(a);
        }
        zhuque()

        //  Function scope 
        function zhuque(){
    
            let a = 123
            console.log(a);
        }
        zhuque()

        //  Block scope 
        if(true){
    
        
        }

Variables declared within the scope of a function , Can only be used within the scope of the current function , Cannot access... In another scope

 function zhuque(){
    
            let a = 123 // The statement in   Function scope 
            // console.log(b); // Dad cares about his son , It's impossible 
            function zhu1(){
    
                console.log(a); //123

                // let b = 456
            }
            zhu1()
            console.log(a); //123
        }
        zhuque()

// There is no intersection between scopes Can't visit each other

There is no intersection between scopes Can't visit each other

  function zhuque(){
    
        let a = 0
        function zhu1(){
    
            let b = 456
        }
        zhu1()
    }

    zhuque();

    function fn(){
    
        console.log(a);
    }
    fn()

// Variables cannot be returned

  function zhuque(){
    
        let a = 0
        return a 
    }

    let num = zhuque();
    console.log(num);


    function fn(){
    
        console.log(a);
    }
    fn()
       

When we are in a certain scope , When you need to use variables , First, find the variable from its scope
without , Just query the outer scope If there is no outer scope , Keep looking for the outer layer , Until the global scope is found
If there are no global variables , You're going to report a mistake

   let a = 1
        function fn(){
    

            let a = 2
            console.log(a); //2
            {
    
                console.log(a); //2
            }
        }

        fn()

        console.log(a); //1

        let fn2 = function(){
    
            {
    
                console.log(a); //1
            }
        }

        fn2()

No matter what scope As long as there is no statement Directly assigned Will become window Properties of

 b = 1
 window.b = 1

Number Methods (api)

parseInt()： Keep the whole number
parseFloat()： Keep floating point numbers
Number.isNaN()： Judgment is not NaN ( Returns a Boolean value )
Number.isFinte()： Judge whether it is a finite number ( Returns a Boolean value )
Number.isInteger()： Judge whether it's an integer ,( Returns a Boolean value )
toFixed()： Keep a few decimal places

1. Return integer ,(Number It can be omitted )

 let num = 3.12344
    console.log(Number.parseInt(num));//3
    console.log(parseInt(num));

1. String to number , Cut off the non numeric part

 let num = "123px"
    console.log(parseInt(num)); //123
    console.log(parseInt("a23b")); //NaN

2. Return floating point number ,(Number It can be omitted )

let num = 3.1548
    let num1 = "123.12px"
    console.log(Number.parseFloat(num));// 3.1548
    console.log(parseFloat(num1));

3. Judge whether a value is NaN, Returns a Boolean value

    let num = NaN

    if(Number.isNaN(num)){
    
        console.log(1);
    }

    if(Number.isNaN("abc" - 1)){
    
        console.log(1);
    }

4. Judge if it's a finite number ,( It can be omitted Number)

   let num = 123
    console.log(Number.isFinite(num)); //true

5. Judge whether it's an integer , Returns a Boolean value ( Don't omit Number)

   let num = 3.1548
    let num2 = 123
    console.log(Number.isInteger(num));//false
    console.log(Number.isInteger(num2));//true

6.toFixed() Keep a few decimal places

  let num = 3.12344
    console.log(num.toFixed(2)); //3.12
    console.log(+Number(3.1525).toFixed(3));

Common use of strings API

1.charAt()： Get a string item Parameter serial number , Take the character corresponding to the sequence number of the string

 let str = "zhuque520"
       console.log(str.charAt(2));//u
       console.log(str.charAt(str.length-1));//0

2.charCodeAt()： Take the sequence number corresponding to the string unicode code

 let str = "zhuque520"
    console.log(str.charCodeAt(str.length-1));//48
    console.log(str.charCodeAt(0));//122  Return string unicode Encoding value 

3.String.formCharcode(), adopt uniclde Sort value , Returns the corresponding character

 console.log(String.fromCharCode(122)); //z

4.substring(strNum,endNum) Intercepting string from startNum Intercept to endNum ( Left closed right away )
Returns the new string successfully intercepted

let str = "zhuque520"
    console.log(str.substring(0,3));//zhu

5.slice and substring Use the same ( It's all closed on the left and open on the right )

 let str = "zhuque520"
    console.log(str.slice(0,3));//zhu

6.*substr： Intercepting string , Cut by length

    let str = "zhuque520"
    console.log(str.slice(0,3));//zhu

7.*trim() Clear the left and right spaces , No matter in the middle

 let str = "  User groups  "
    console.log(str.trim());

8.replace(str,req) Replace string , Only the first retrieved string item will be replaced

 let str = "zhuque520"
    console.log(str.replace("zhuque"," Brother Yao "));

9.split() Cut the string into an array , Accept parameters , Is the cutting point ( According to what to cut )

    let str = "12345"
    // let arr = str.split() //['12345']
    // let arr = str.split(" ") // According to the space  ['12345']
    // let arr = str.split("")// By empty character  ['1', '2', '3', '4', '5']
    let arr = str.split("3") //['12', '45']
    
    console.log(arr);