What is a good way to make an event occur when the page has loaded OR on each resize?

I need a query to check the viewport width each time the document is ready or whenever the viewport gets resized. The reason is that I want the main navigation to look and behave differently for different viewport widths.

//EventListener to get viewport width on load
window.addEventListener("load", function () {
    window.loaded = true;
});

//get Window Size on Resize
var width = $(window).width();
$(window).on('resize', function reportWindowSize() {
  if ($(this).width() != width) {
    width = $(this).width();
    console.log(width);
  }
});

Then I think I need a query like this:

If (page loaded OR viewport width changes) {
   if (viewport width > x) {
     do things

} else if (viewport width < x) {
     do other things
  }
};

What would be a good way to form If (page loaded OR viewport width changes) into a JavaScript or jQuery expression?

As Terry pointed out in his comment, you can create a function and reuse it in each of the scenarios that you need it.

I.E.

function checkWidth() {
  let windowWidth = $(window).width();
  let maxWidth = 1000;
  if(windowWidth < maxWidth) {
    // do your stuff
  }
}

$(document).ready(function() {
  checkWidth();
});

$(window).on('resize', function() {
  checkWidth();
});

Another solution would be to use Media Queries inside JS.

You can watch for a media query as you would in CSS with the following code:

if (matchMedia) {
  let mq = window.matchMedia("(min-width: 500px)");
  mq.addListener(checkWidth);
  checkWidth(mq);
}

// media query change
function checkWidth(mq) {
  if (mq.matches) {
  // do your stuff
  }
}

You need to call checkWidth after adding the event listener in order to ensure that it executes when the page is loaded.

I got this example from: https://www.sitepoint.com/javascript-media-queries/