JS day 2 (variables, variable usage, naming rules, syntax extensions)

One 、 What is a variable

          It's a box of things , Variables are data containers for storing , Refers to a section of memory space in a computer , We get data by variable name , Even the data can be modified

        The essence ： A variable is a piece of space requested by the program in memory for storing data

Two 、 Use of variables

        Variables are used in two steps ：

        1、 Declare variables          Open up memory space

var age;// Declare variables ：

          2、 assignment         Store data in memory space

age = 18;

       3、 Output results

console.log(age);

        4、 Initialization of a variable ： Declare a variable and assign a value , We call it initialization of variables

     //1、 Make a statement age The variable of 
        var age;
        //2、 assignment , Store values in variables 
        age = 18;
        //3、 Output results 
        // console.log(age);
        //4、 Initialization of a variable 
        var myname = ' Pillow can't wake up ';
        console.log(myname);
         // The variable is declared with a value of 18

        // Declare a variable and assign a value , We call it initialization of variables 

3、 ... and 、 Naming rules for variables

1、 Variable names can be given by Numbers Letter Underline   Element composition

2、 Cannot start with a number

3、 You can't use keywords and reserved words var for if class 

4、 To see the meaning of name

5、 Hump named ： Hump The first letter of every word should be capitalized Student Name

                        Little hump First word lowercase , The rest of the words start with single shoes student Name

Four 、 Syntax extension of variables

1、 Update variables : After a variable has been reassigned , Its original value will be overwritten , The value of the variable will be based on the last assigned value

Code display ：

<script>
        //1、 Update variables 
        var myname = ' teacher ';
        console.log(myname);
        myname = ' Pillow can't wake up ';
        console.log(myname);
    </script>

Result display ：

 2、 When multiple variables are declared , Just write one var, Multiple variable names are separated by commas

// When multiple variables are declared , Just write one var, Multiple variable names are separated by commas 
        // var age = 18
        // var address = ' Naruto ';
        // var gz = 2000;
        var age = 18,
            address = ' Naruto '
            gz = '2000'

3、 Special cases of declaring variables

（1） Only declare , No assignment , The result is undefined Undefined

Code display ：

var sex;
        console.log(sex);

Pictures show ：

 

（2） Neither declaration nor assignment , Using a variable directly will definitely report an error

Code display ：

 console.log(tel);

Pictures show ：

 

（3） Do not declare direct assignment to use , Can get results , But not recommended

Code display ：

        qq = 110;
        console.log(qq)

Pictures show ：

The best way to write ： Declare variables and assign values