From 1f60cd8cd74b530932aa84e4e6b9ad1b10891c0a Mon Sep 17 00:00:00 2001
From: Phil Hughes <me@iamphill.com>
Date: Tue, 31 Oct 2017 17:18:09 +0000
Subject: [PATCH] Increase lazy loader performance

This stops the checkElementsInView method from running when there aren't any lazy load images on the page. By calling this method we are causing an unnecessary reflow which on large pages is a massive performance issue
---
 app/assets/javascripts/lazy_loader.js | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/app/assets/javascripts/lazy_loader.js b/app/assets/javascripts/lazy_loader.js
index 3d64b121fa7..dbbf1637a47 100644
--- a/app/assets/javascripts/lazy_loader.js
+++ b/app/assets/javascripts/lazy_loader.js
@@ -1,5 +1,3 @@
-/* eslint-disable one-export, one-var, one-var-declaration-per-line */
-
 import _ from 'underscore';
 
 export const placeholderImage = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
@@ -21,7 +19,10 @@ export default class LazyLoader {
   }
   searchLazyImages() {
     this.lazyImages = [].slice.call(document.querySelectorAll('.lazy'));
-    this.checkElementsInView();
+
+    if (this.lazyImages.length) {
+      this.checkElementsInView();
+    }
   }
   startContentObserver() {
     const contentNode = document.querySelector(this.observerNode) || document.querySelector('body');
@@ -45,15 +46,13 @@ export default class LazyLoader {
   checkElementsInView() {
     const scrollTop = pageYOffset;
     const visHeight = scrollTop + innerHeight + SCROLL_THRESHOLD;
-    let imgBoundRect, imgTop, imgBound;
 
     // Loading Images which are in the current viewport or close to them
     this.lazyImages = this.lazyImages.filter((selectedImage) => {
       if (selectedImage.getAttribute('data-src')) {
-        imgBoundRect = selectedImage.getBoundingClientRect();
-
-        imgTop = scrollTop + imgBoundRect.top;
-        imgBound = imgTop + imgBoundRect.height;
+        const imgBoundRect = selectedImage.getBoundingClientRect();
+        const imgTop = scrollTop + imgBoundRect.top;
+        const imgBound = imgTop + imgBoundRect.height;
 
         if (scrollTop < imgBound && visHeight > imgTop) {
           LazyLoader.loadImage(selectedImage);