JS input / output statements, variables

#1. Output

1.prompt(’ Fill in the contents to be output ‘)： Browser pop-up input box , User can input

2.alert(’ Fill in the contents to be output ‘)： Pop up warning box , Output 、 Show the , Browser pop-up warning

3.console.log(’ Fill in the contents to be output ‘)： Console output , For programmers to test , Browser console printouts

#2. Variable

1. Variable Overview ： Variables are containers for storing data , We can get the data through the variable name , Modify and use data , Variables are the memory space that the programmer applies for in memory to store data .

A complete variable consists of a variable name and a storage space .

 

 

Every time a variable is generated , A space will be generated in our memory , When you store data in variables , The data we write will be stored in memory .

Each variable created is , We all need to generate a name for the variable , When we need to use this variable , You can directly use the variable name to replace the variable

2. Use of variables

　　1. Declare variables

　　 Because the present tense ES6 So we use let Method to declare variables , Variables need to be declared with let For the back Variable name .

　　age It is declared by our programmers Variable name .

　　2. assignment

　　 We can use = To assign values to our variable names , Our value is the value to be stored in the variable space  

　　

 

3. Variable syntax extension

　　1. Update variables

　　 After a variable has been reassigned , Its original value will be overwritten , The variable value will be subject to the next assignment .

　　2. Declare multiple variables at the same time

　　 When multiple variables are declared at the same time , Just write one let , Multiple variable names are in English ‘ comma ’ separate .

　　

 

 　　3. A special case

　　

 

4. Variable naming conventions

　　1. By letter (A-Za-z)、 Numbers (O-9)、 Underline (_)、 Dollar symbol ($ ) form , Such as : usrAge, num01,_name

　　2. Case sensitive .var app; and var App; It's two variables

　　3. Cannot start with a number .18age It's wrong.

　　4. It can't be a keyword 、 Reserved words . for example : var、for、while

　　5. Variable names must have meaning .MMD BBD

　　6. Follow the hump nomenclature . Initial lowercase , The first letter of the following words needs to be capitalized .myFirstName

　　7. Recommended translation website : youdao   ICIBA