jquery source's position in html file is important or not

Question

This is an example of click button to pop up an alert.

Answer 1

Scripts are executed in order they are encountered in the DOM¹

So yes, order between scripts matters.

In this case, your script runs before jQuery does, so attempting to access $ results in a ReferenceError. To "fix" it, put your script under the jQuery script element. It doesn't matter if the scripts are in or at the bottom of the : but the relative order of the scripts is important.

Here is a trivial mockup of the invalid ordering:

If these the order of two script elements were reversed then "foo" would be alerted, as expected.

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¹ (Except, perhaps, for those marked as async=true, which I will not talk about.)