Javascript Global Variable and Local Variable

 Javascript variables can be divided into Global Variable (global variable) and Local Variable (regional variable). The difference between these two variables lies in the position, method and calling area of ​​the declaration. The advantage of using global variables is the convenience of calling, but some Sometimes in a specific function, there may be an independent script that uses local variables to ensure that the operation of the entire program is not affected. The following are the main differences and sample applications of the two variables.

• Global Variable : It can be declared within or outside the function, and can be called within the entire web page, so although there can be countless Global Variables with different names in the entire web page, there is only one independent Global Variable name. If the name is repeated, the variable value will be overwritten. When the web page is closed, Global Variable will also become invalid.
• Local Variable : It can only be declared through the keyword var in the function. Each different function can have the same Local Variable variable name. In other words, the Local Variable between each function does not interfere with each other and cannot Called in other places outside the function, when the function finishes its work, the Local Variable also becomes invalid.

It is not difficult to see the difference between the above two variables. Only in the function, the variable declared through var can be regarded as Local Variable. If the var keyword is not used to declare, no matter where the variable is declared , Will all belong to the category of Global Variable. Please pay more attention to this detail to avoid errors.

Examples of Javascript Global Variable and Local Variable

<script type="text/javascript">
function Test(){
　var var1='A';
　var2='B';
　document.write('var1 in function='+var1+'<br>');　// output A
　document.write('var2 in function='+var2+'<hr>');　// output B
}
var var3='C';

Test();　// call Test function

document.write('var2 not in function ='+var2+'<br>');　// Output B
document.write('var3 not in function='+var3+'<hr>');　// Output C
</script>

The output of the above example

var1 in function=A
var2 in function=B

---

var2 not in function=B
var3 not in function=C

First prepare a Test function, in which we declare two variables, var1 and var2. According to the explanation in the first paragraph, we can clearly know that var1 is a local variable and var2 is a global variable. Declare a global variable of var3, and then we will call to see the result. In the first wave of calling two variables in the Test function, both variables can be output smoothly, and then var2 and var3 can also be output smoothly when calling var2 and var3 outside the function, but why didn't we call var1 outside the function? Because var1 is only declared in the function and declared through var, the call outside the function will be invalid, and even JavaScript will not recognize the variable var1, causing the script behind to lose its effect.