Why can I put this code snippet only in the same page and not in an external js file?

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Javascript

PHP

Hi,

I'm finishing the development of my time tracker php application.

After working a little with it (It's wonderful having something that suits perfectly your needs) i've noticed it would be a good thing having the possibility to set the number of lines to be shown in each page.

I've put a small editbox that in javascript it sets a cookie when enter key is pressed and then reloads the page to show the changes.

It works perfectly if I put the javascript code in the head of the php document.

But it doesn't work when I put the code inside a js file that is linked in the same head section of the same php document.

The function is this one:

function changeLiniesPerPagina(e)
{
  if(e.keyCode == 13)
  {
    document.cookie = "cookieName="+document.getElementById("lines").value+"; expires=Fri, 31 Dec 2049 12:00:00 UTC; path=/;";
    location.reload();
  }
}

If I use the "Inspect" option in the browser, I can see the message it appears when the function is located in an external js file is:

VM715 phpfilename.php:27 Uncaught ReferenceError: changeLiniesPerPagina is not defined
    at HTMLInputElement.onkeypress (VM715 phpfilename.php:27)
onkeypress @ VM715 phpfilename.php:27

Any idea?

Thank you very much!

What I have tried:

Checked the syntax, put the code snippet in different js files...