laipower/wp-content/themes/generatepress/assets/js/back-to-top.js

( function() {
	'use strict';

	// Feature Test
	if ( 'querySelector' in document && 'addEventListener' in window ) {
		var goTopBtn = document.querySelector( '.generate-back-to-top' );

		var trackScroll = function() {
			var scrolled = window.pageYOffset;
			var coords = goTopBtn.getAttribute( 'data-start-scroll' );

			if ( scrolled > coords ) {
				goTopBtn.style.opacity = '1';
				goTopBtn.style.visibility = 'visible';
			}

			if ( scrolled < coords ) {
				goTopBtn.style.opacity = '0';
				goTopBtn.style.visibility = 'hidden';
			}
		};

		// Function to animate the scroll
		var smoothScroll = function( anchor, duration ) {
			// Calculate how far and how fast to scroll
			var startLocation = window.pageYOffset;
			var endLocation = document.body.offsetTop;
			var distance = endLocation - startLocation;
			var increments = distance / ( duration / 16 );
			var stopAnimation;

			// Scroll the page by an increment, and check if it's time to stop
			var animateScroll = function() {
				window.scrollBy( 0, increments );
				stopAnimation();
			};

			// Stop animation when you reach the anchor OR the top of the page
			stopAnimation = function() {
				var travelled = window.pageYOffset;
				if ( travelled <= ( endLocation || 0 ) ) {
					clearInterval( runAnimation );
					document.activeElement.blur();
				}
			};

			// Loop the animation function
			var runAnimation = setInterval( animateScroll, 16 );
		};

		if ( goTopBtn ) {
			// Show the button when scrolling down.
			window.addEventListener( 'scroll', trackScroll );

			// Scroll back to top when clicked.
			goTopBtn.addEventListener( 'click', function( e ) {
				e.preventDefault();

				if ( generatepressBackToTop.smooth ) {
					smoothScroll( document.body, goTopBtn.getAttribute( 'data-scroll-speed' ) || 400 );
				} else {
					window.scrollTo( 0, 0 );
				}
			}, false );
		}
	}
}() );