Convirted to be mostly ES6 compliant.

src/scripts/background.js

@@ -1,4 +1,4 @@
-function handleUpdated(tabId, changeInfo, tabInfo) {
+const handleUpdated = (tabId, changeInfo, tabInfo) => {
     if (changeInfo.url) {
         var http = "http://www.youtube.com/watch?v=";
         var https = "https://www.youtube.com/watch?v=";

src/scripts/betterYoutube.js

@@ -1,17 +1,17 @@
-function betterYoutubePlus() {
+const betterYoutubePlus = () => {
 
     // Declare variables
-    var slugInputTag, ytThumbImgMenu, ytEnhancerMenu;        // Menu systems
+    let slugInputTag, ytThumbImgMenu, ytEnhancerMenu;        // Menu systems
-    var ytThumbnailBttn, ytMaxDefaultImg, ytHqDefaultImg,    // Buttons & Images
+    let ytThumbnailBttn, ytMaxDefaultImg, ytHqDefaultImg,    // Buttons & Images
         ytLoopBttn, ytFloatBttn, ytAMaxDefaultImg, ytAHqDefaultImg;
 
-    var mastHead, mainContentArea, playerWindow;             // Youtube Player container
+    let mastHead, mainContentArea, playerWindow;             // Youtube Player container
-    var	video, containerOfPlyrWndow, poppedContainer;        // Video accessor
+    let	video, containerOfPlyrWndow, poppedContainer;        // Video accessor
-    var vdoPlyrAtts;                                         // Player attributes
+    let vdoPlyrAtts;                                         // Player attributes
-    var part, videoSlug, temp;                               // Image part
+    let part, videoSlug, temp;                               // Image part
-    var count = 0;
+    let count = 0;
 
-    function preSetupProc() {
+    const preSetupProc = () => {
         video               = document.getElementsByTagName("video")[0];        // Video Controler
         slugInputTag        = document.createElement("INPUT");
         slugInputTag.id     = "slugCopyZone";
@@ -27,7 +27,7 @@ function betterYoutubePlus() {
         slugInputTag.value  = vdoPlyrAtts.slice(32, 32+11);
     }
 
-    function setupProc() {
+    const setupProc = () => {
         poppedContainer   = document.createElement("DIV");
         ytThumbImgMenu    = document.createElement("DIV");
         ytEnhancerMenu    = document.createElement("DIV");
@@ -104,7 +104,7 @@ function betterYoutubePlus() {
     }
 
     // Functions
-    function showThumbImageVew(e) {
+    const showThumbImageVew = (e) => {
         videoSlug = video.baseURI.slice(32, 32+11);  // Used for setting up thumbnails
 
         if (ytThumbImgMenu.style.display == "block") {
@@ -135,7 +135,7 @@ function betterYoutubePlus() {
         return;
     }
 
-    function setLoop(e) {
+    const setLoop = (e) => {
         if (video.loop == false) {
             video.loop        = true;
             ytLoopBttn.title  = "Stop Loop...";
@@ -148,8 +148,8 @@ function betterYoutubePlus() {
         return;
     }
 
-    function toggleFloat() {
+    const toggleFloat = () => {
-        var playerWindow = document.getElementById("movie_player");  // Actual player
+        let playerWindow = document.getElementById("movie_player");  // Actual player
 
         if(poppedContainer.style.display == "none"){
             setVideoStyle("auto", "auto", "50%");
@@ -164,8 +164,8 @@ function betterYoutubePlus() {
         }
     }
 
-    function setVideoStyle(w, h, pr) {
+    const setVideoStyle = (w, h, pr) => {
-        var elm = document.getElementsByClassName("ytp-right-controls")[0];
+        let elm = document.getElementsByClassName("ytp-right-controls")[0];
         video.style.width  = w;
         video.style.height = h;
         elm.style.paddingRight = pr;
@@ -173,8 +173,8 @@ function betterYoutubePlus() {
         console.log(video.style);
     }
 
-    function manageVolume(e) {
+    const manageVolume = (e) => {
-        var delta;
+        let delta;
         e.preventDefault(); // Keep page from scrolling while in video area
 
         // Detect scroll direction
@@ -183,8 +183,8 @@ function betterYoutubePlus() {
         if (delta > 0) video.volume += 0.05; else if (delta < 0) video.volume -= 0.05;
     }
 
-    function dragVideo(elmnt) {
+    const dragVideo = (elmnt) => {
-        var pos1 = 0, pos2 = 0, pos3 = 0, pos4 = 0;
+        let pos1 = 0, pos2 = 0, pos3 = 0, pos4 = 0;
         elmnt.onmousedown = dragMouseDown;
 
         function dragMouseDown(e) {
@@ -198,7 +198,7 @@ function betterYoutubePlus() {
             document.onmousemove = elementDrag;
         }
 
-        function elementDrag(e) {
+        const elementDrag = (e) => {
             e = e || window.event;
             pauseEvent(e);
             // calculate the new cursor position:
@@ -211,13 +211,13 @@ function betterYoutubePlus() {
             elmnt.style.left = (elmnt.offsetLeft - pos1) + "px";
         }
 
-        function closeDragElement(e) {
+        const closeDragElement = (e) => {
             // stop moving when mouse button is released:
             document.onmouseup   = null;
             document.onmousemove = null;
         }
 
-        function pauseEvent(e) {
+        const pauseEvent = (e) => {
             if(e.stopPropagation) e.stopPropagation();
             if(e.preventDefault)  e.preventDefault();
             e.cancelBubble = true;
@@ -226,9 +226,9 @@ function betterYoutubePlus() {
         }
     }
 
-    function checkPg() {
+    const checkPg = () => {
         if (!document.getElementById("enhancerMenuIDRef")) {
-            var existCondition = setInterval(function() {
+            let existCondition = setInterval(function() {
                 if (document.getElementById("masthead-container")) {
                     clearInterval(existCondition);
                     preSetupProc();