I have two different functions for events on click, one works, the other does not, but if you swap the positions, on the contrary, the second one which became the first works, and the first one does not.
Both of these functions are written for different HTML pages. Apparently this is the problem. The interpreter stumbles and cannot read property addeventlistener of null?
but how else to do it? not to write a separate script for each page?

Answer 1, authority 100%

The answer is very simple, check before adding a listener to an element.

For example, before the lines where you add the listener, do something like this:

if (humburger) {
  humburger.addEventListener ....
}

And also for the second case

Answer 2

In the html file where you connect, write the defer attribute.

& lt; script src = "script.js" defer & gt; & lt; / script & gt;