Javascript causes undefined and varundefined by using var incorrectly.

Javascript causes undefined and varundefined by using var incorrectly.

In javascript, variables are divided into local variables and global variables based on different scopes. The variables directly defined are global variables, which can be accessed by all scripts; the variables defined in the function are local variables, which are only valid in the function.
If the global variable and local variable use the same name, the local variable overwrites the global variable.
Sample Code:

<! DOCTYPE html> 
The running result is as follows:
Click test1 to bring up the local variable.
Click test2, undefined is displayed, and local variables are displayed.
 
Click test3 to bring up global variables and then local variables.
This is the difference between using var and not using it:
If var is used,The program will force a new variable to be defined.
If var is not used,The system will first search for the existence of the variable in the current context. A new variable will be redefined only if it does not exist.
In test3, all variables a are global variables. The first time the global variables are directly output, and the second time the global variables are directly assigned values.
Both test1 and test2 use var in the function to define a new variable with the same name. This will overwrite the global variable in the function. Therefore, in test2: the first output of a is the local variable a that has been overwritten, but the initial value is not given.UndefinedThis result. Using variables in this way is actually incorrect and should be avoided.
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