anchorlink-offset.js

// From https://wpbeaches.com/anchor-link-offset-to-other-pages-with-fixed-header-size/

jQuery(document).ready(function($) {
  
  // set a variable for the anchor link which is the location.hash
  var anchorLink = $(window.location.hash);
    // test to see if the link is a anchor link, if not the length will have no value, this is done to avoid js errors on non anchor links
		if ( anchorLink.length ) {
      // set an element as the fixed entity, header in this case and get its height
			var offsetSize = $("header").innerHeight();
      // fire the animation from the top of the page to the anchor link offsetting by the fixed elements height, the number is the speed of the animation
			$("html, body").animate({scrollTop: anchorLink.offset().top - offsetSize }, 500);
    }

});