Difference between using var and not using var in JavaScript

I would say it's better to use "var" in most situations.
Local variables are always faster than the variables in global scope.
If you do not use "var" to declare a variable, the variable will be in global scope.
For more information, you can search "scope chain javascript" in Google.

Using var is always a good idea to prevent variables from cluttering the global scope and variables from conflicting with each other, causing unwanted overwriting.