Here is the simple test for you, assume that the following
<script type="text/javascript">
var a = 3;
</script>
<script type="text/javascript">
alert(a + 1);
</script>
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.
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