Scope of Script Tags
Here is the simple test for you, assume that the following
var a = 3;
alert(a + 1);
What will be the result in the pop-up alert?
If you said browser will display an error like "a is not defined", you are wrong :) Browsers interpret all the script tags inside the page (also the imported .js files) in the same scope. Therefore, number a is a global variable and alert will display 4 in the pop-up.