Three ways to declare global variables based on javascript

JS in the declaration of global variables are mainly divided into explicit declarations or implicit declarations are described below respectively.

Declaration Method One:

The use of the VAR (keyword) + variable name (identifier) is declared outside the function as a global variable, otherwise the function declares a local variable. The method is explicitly declared in detail as follows:

<script>   var test = 5;//global variable   function A () {     var a = 3;//local variable     alert (a);   }   Function B () {     alert (test);   }   A ();//Call a method, then the contents of the method will be executed   

Declaration Method Two:

No var is used to assign a value directly to the identifier test, which implicitly declares the global variable test. Even though the statement is within a function, when the function is executed, test becomes a global variable.

<script>     test = 5;//global variable     function A () {       AA = 3;//is also a global variable       alert (test);     }     A ();  Output 5     //alert (AA);//The variable inside the method A () method can be used here, but the a method needs to be called before the AA becomes the global variable   

Declaration Method Three:

Use the window global object to declare that the properties of the global object are also global variables, as detailed below:

<script>   window.test = 50;   

This approach is often used when an anonymous function is executed to expose some functions to the global. As the last sentence in JQuery1.5
The code is as follows:

Window.jquery = window.$ = JQuery;

Advantages of global variables:

You can reduce the number of variables and reduce the time consumption caused by data transfer of actual parameters and formal parameters.

Disadvantages of global variables:

(1) The global variable is stored in a static storage area, which allocates memory for the program to run, and the program ends releasing the memory. Compared with the dynamic allocation and dynamic release of local variables, the lifetime is relatively long, so too many global variables will occupy more memory units.

(2) The global variable destroys the encapsulation performance of the function. function like a black box, usually through the function parameters and return values for input and output, the function of the internal implementation is relatively independent. However, if a global variable is used in a function, the statement in the function body can be accessed by bypassing the function parameter and the return value, which destroys the function's independence and causes the function to rely on the global variable. At the same time, the portability of the function is reduced.

(3) Global variables make the code readability of the function less readable. Because multiple functions may use global variables, the values of global variables may change at any time when the function is executed, which is detrimental to the troubleshooting and debugging of the program.
Therefore, if it is not a last resort, it is best not to use global variables.

Three ways to declare global variables based on javascript