diff --git a/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection.js b/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection.js
index fa1aeb2f4d..ecddba2ad2 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection.js
@@ -18,13 +18,13 @@
    * Possible CSS classes indicating the state of selection. Dynamically added/
    * removed based on where the user clicks within the diff.
    */
-  var SelectionClass = {
+  const SelectionClass = {
     COMMENT: 'selected-comment',
     LEFT: 'selected-left',
     RIGHT: 'selected-right',
   };
 
-  var getNewCache = function() { return {left: null, right: null}; };
+  const getNewCache = () => { return {left: null, right: null}; };
 
   Polymer({
     is: 'gr-diff-selection',
@@ -43,11 +43,11 @@
     ],
 
     listeners: {
-      'copy': '_handleCopy',
-      'down': '_handleDown',
+      copy: '_handleCopy',
+      down: '_handleDown',
     },
 
-    attached: function() {
+    attached() {
       this.classList.add(SelectionClass.RIGHT);
     },
 
@@ -59,19 +59,19 @@
       return this._cachedDiffBuilder;
     },
 
-    _diffChanged: function() {
+    _diffChanged() {
       this._linesCache = getNewCache();
     },
 
-    _handleDown: function(e) {
-      var lineEl = this.diffBuilder.getLineElByChild(e.target);
+    _handleDown(e) {
+      const lineEl = this.diffBuilder.getLineElByChild(e.target);
       if (!lineEl) {
         return;
       }
-      var commentSelected =
+      const commentSelected =
           this._elementDescendedFromClass(e.target, 'gr-diff-comment');
-      var side = this.diffBuilder.getSideByLineEl(lineEl);
-      var targetClasses = [];
+      const side = this.diffBuilder.getSideByLineEl(lineEl);
+      const targetClasses = [];
       targetClasses.push(side === 'left' ?
           SelectionClass.LEFT :
           SelectionClass.RIGHT);
@@ -80,23 +80,23 @@
         targetClasses.push(SelectionClass.COMMENT);
       }
       // Remove any selection classes that do not belong.
-      for (var key in SelectionClass) {
+      for (const key in SelectionClass) {
         if (SelectionClass.hasOwnProperty(key)) {
-          var className = SelectionClass[key];
-          if (targetClasses.indexOf(className) === -1) {
+          const className = SelectionClass[key];
+          if (!targetClasses.includes(className)) {
             this.classList.remove(SelectionClass[key]);
           }
         }
       }
       // Add new selection classes iff they are not already present.
-      for (var i = 0; i < targetClasses.length; i++) {
-        if (!this.classList.contains(targetClasses[i])) {
-          this.classList.add(targetClasses[i]);
+      for (const _class of targetClasses) {
+        if (!this.classList.contains(_class)) {
+          this.classList.add(_class);
         }
       }
     },
 
-    _getCopyEventTarget: function(e) {
+    _getCopyEventTarget(e) {
       return Polymer.dom(e).rootTarget;
     },
 
@@ -108,7 +108,7 @@
      * @param {!string} className
      * @return {boolean}
      */
-    _elementDescendedFromClass: function(element, className) {
+    _elementDescendedFromClass(element, className) {
       while (!element.classList.contains(className)) {
         if (!element.parentElement ||
             element === this.diffBuilder.diffElement) {
@@ -119,20 +119,20 @@
       return true;
     },
 
-    _handleCopy: function(e) {
-      var commentSelected = false;
-      var target = this._getCopyEventTarget(e);
+    _handleCopy(e) {
+      let commentSelected = false;
+      const target = this._getCopyEventTarget(e);
       if (target.type === 'textarea') { return; }
       if (!this._elementDescendedFromClass(target, 'diff-row')) { return; }
       if (this.classList.contains(SelectionClass.COMMENT)) {
         commentSelected = true;
       }
-      var lineEl = this.diffBuilder.getLineElByChild(target);
+      const lineEl = this.diffBuilder.getLineElByChild(target);
       if (!lineEl) {
         return;
       }
-      var side = this.diffBuilder.getSideByLineEl(lineEl);
-      var text = this._getSelectedText(side, commentSelected);
+      const side = this.diffBuilder.getSideByLineEl(lineEl);
+      const text = this._getSelectedText(side, commentSelected);
       if (text) {
         e.clipboardData.setData('Text', text);
         e.preventDefault();
@@ -148,19 +148,20 @@
      * @param {boolean} Whether or not a comment is selected.
      * @return {string} The selected text.
      */
-    _getSelectedText: function(side, commentSelected) {
-      var sel = window.getSelection();
+    _getSelectedText(side, commentSelected) {
+      const sel = window.getSelection();
       if (sel.rangeCount != 1) {
         return; // No multi-select support yet.
       }
       if (commentSelected) {
         return this._getCommentLines(sel, side);
       }
-      var range = GrRangeNormalizer.normalize(sel.getRangeAt(0));
-      var startLineEl = this.diffBuilder.getLineElByChild(range.startContainer);
-      var endLineEl = this.diffBuilder.getLineElByChild(range.endContainer);
-      var startLineNum = parseInt(startLineEl.getAttribute('data-value'), 10);
-      var endLineNum = parseInt(endLineEl.getAttribute('data-value'), 10);
+      const range = GrRangeNormalizer.normalize(sel.getRangeAt(0));
+      const startLineEl =
+          this.diffBuilder.getLineElByChild(range.startContainer);
+      const endLineEl = this.diffBuilder.getLineElByChild(range.endContainer);
+      const startLineNum = parseInt(startLineEl.getAttribute('data-value'), 10);
+      const endLineNum = parseInt(endLineEl.getAttribute('data-value'), 10);
 
       return this._getRangeFromDiff(startLineNum, range.startOffset, endLineNum,
           range.endOffset, side);
@@ -176,9 +177,9 @@
      * @param {!string} side The side that is currently selected.
      * @return {string} The selected diff text.
      */
-    _getRangeFromDiff: function(startLineNum, startOffset, endLineNum,
-        endOffset, side) {
-      var lines = this._getDiffLines(side).slice(startLineNum - 1, endLineNum);
+    _getRangeFromDiff(startLineNum, startOffset, endLineNum, endOffset, side) {
+      const lines =
+          this._getDiffLines(side).slice(startLineNum - 1, endLineNum);
       if (lines.length) {
         lines[lines.length - 1] = lines[lines.length - 1]
             .substring(0, endOffset);
@@ -193,17 +194,13 @@
      * @param {!string} side The side that is currently selected.
      * @return {Array.string} An array of strings indexed by line number.
      */
-    _getDiffLines: function(side) {
+    _getDiffLines(side) {
       if (this._linesCache[side]) {
         return this._linesCache[side];
       }
-      var lines = [];
-      var chunk;
-      var key = side === 'left' ? 'a' : 'b';
-      for (var chunkIndex = 0;
-          chunkIndex < this.diff.content.length;
-          chunkIndex++) {
-        chunk = this.diff.content[chunkIndex];
+      let lines = [];
+      const key = side === 'left' ? 'a' : 'b';
+      for (const chunk of this.diff.content) {
         if (chunk.ab) {
           lines = lines.concat(chunk.ab);
         } else if (chunk[key]) {
@@ -222,16 +219,16 @@
      * @param {!string} side The side that is currently selected.
      * @return {string} The selected comment text.
      */
-    _getCommentLines: function(sel, side) {
-      var range = GrRangeNormalizer.normalize(sel.getRangeAt(0));
-      var content = [];
+    _getCommentLines(sel, side) {
+      const range = GrRangeNormalizer.normalize(sel.getRangeAt(0));
+      const content = [];
       // Query the diffElement for comments.
-      var messages = this.diffBuilder.diffElement.querySelectorAll(
-          '.side-by-side [data-side="' + side +
-          '"] .message *, .unified .message *');
+      const messages = this.diffBuilder.diffElement.querySelectorAll(
+          `.side-by-side [data-side="${side
+          }"] .message *, .unified .message *`);
 
-      for (var i = 0; i < messages.length; i++) {
-        var el = messages[i];
+      for (let i = 0; i < messages.length; i++) {
+        const el = messages[i];
         // Check if the comment element exists inside the selection.
         if (sel.containsNode(el, true)) {
           // Padded elements require newlines for accurate spacing.
@@ -262,10 +259,10 @@
      * @param {Range} range The normalized selection range.
      * @return {string} The text within the selection.
      */
-    _getTextContentForRange: function(domNode, sel, range) {
+    _getTextContentForRange(domNode, sel, range) {
       if (!sel.containsNode(domNode, true)) { return ''; }
 
-      var text = '';
+      let text = '';
       if (domNode instanceof Text) {
         text = domNode.textContent;
         if (domNode === range.endContainer) {
@@ -275,9 +272,8 @@
           text = text.substring(range.startOffset);
         }
       } else {
-        for (var i = 0; i < domNode.childNodes.length; i++) {
-          text += this._getTextContentForRange(domNode.childNodes[i],
-              sel, range);
+        for (const childNode of domNode.childNodes) {
+          text += this._getTextContentForRange(childNode, sel, range);
         }
       }
       return text;
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection_test.html b/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection_test.html
index 3eeba90f48..39555b4b3a 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-selection/gr-diff-selection_test.html
@@ -101,13 +101,13 @@ limitations under the License.
 </test-fixture>
 
 <script>
-  suite('gr-diff-selection', function() {
-    var element;
-    var sandbox;
+  suite('gr-diff-selection', () => {
+    let element;
+    let sandbox;
 
-    var emulateCopyOn = function(target) {
-      var fakeEvent = {
-        target: target,
+    const emulateCopyOn = function(target) {
+      const fakeEvent = {
+        target,
         preventDefault: sandbox.stub(),
         clipboardData: {
           setData: sandbox.stub(),
@@ -118,7 +118,7 @@ limitations under the License.
       return fakeEvent;
     };
 
-    setup(function() {
+    setup(() => {
       element = fixture('basic');
       sandbox = sinon.sandbox.create();
       sandbox.stub(element, '_getCopyEventTarget');
@@ -145,11 +145,11 @@ limitations under the License.
       };
     });
 
-    teardown(function() {
+    teardown(() => {
       sandbox.restore();
     });
 
-    test('applies selected-left on left side click', function() {
+    test('applies selected-left on left side click', () => {
       element.classList.add('selected-right');
       element._cachedDiffBuilder.getSideByLineEl.returns('left');
       MockInteractions.down(element);
@@ -160,7 +160,7 @@ limitations under the License.
           'removes selected-right');
     });
 
-    test('applies selected-right on right side click', function() {
+    test('applies selected-right on right side click', () => {
       element.classList.add('selected-left');
       element._cachedDiffBuilder.getSideByLineEl.returns('right');
       MockInteractions.down(element);
@@ -170,41 +170,41 @@ limitations under the License.
           element.classList.contains('selected-left'), 'removes selected-left');
     });
 
-    test('ignores copy for non-content Element', function() {
+    test('ignores copy for non-content Element', () => {
       sandbox.stub(element, '_getSelectedText');
       emulateCopyOn(element.querySelector('.not-diff-row'));
       assert.isFalse(element._getSelectedText.called);
     });
 
-    test('asks for text for left side Elements', function() {
+    test('asks for text for left side Elements', () => {
       element._cachedDiffBuilder.getSideByLineEl.returns('left');
       sandbox.stub(element, '_getSelectedText');
       emulateCopyOn(element.querySelector('div.contentText'));
       assert.deepEqual(['left', false], element._getSelectedText.lastCall.args);
     });
 
-    test('reacts to copy for content Elements', function() {
+    test('reacts to copy for content Elements', () => {
       sandbox.stub(element, '_getSelectedText');
       emulateCopyOn(element.querySelector('div.contentText'));
       assert.isTrue(element._getSelectedText.called);
     });
 
-    test('copy event is prevented for content Elements', function() {
+    test('copy event is prevented for content Elements', () => {
       sandbox.stub(element, '_getSelectedText');
       element._cachedDiffBuilder.getSideByLineEl.returns('left');
       element._getSelectedText.returns('test');
-      var event = emulateCopyOn(element.querySelector('div.contentText'));
+      const event = emulateCopyOn(element.querySelector('div.contentText'));
       assert.isTrue(event.preventDefault.called);
     });
 
-    test('inserts text into clipboard on copy', function() {
+    test('inserts text into clipboard on copy', () => {
       sandbox.stub(element, '_getSelectedText').returns('the text');
-      var event = emulateCopyOn(element.querySelector('div.contentText'));
+      const event = emulateCopyOn(element.querySelector('div.contentText'));
       assert.deepEqual(
           ['Text', 'the text'], event.clipboardData.setData.lastCall.args);
     });
 
-    test('copies content correctly', function() {
+    test('copies content correctly', () => {
       // Fetch the line number.
       element._cachedDiffBuilder.getLineElByChild = function(child) {
         while (!child.classList.contains('content') && child.parentElement) {
@@ -216,9 +216,9 @@ limitations under the License.
       element.classList.add('selected-left');
       element.classList.remove('selected-right');
 
-      var selection = window.getSelection();
+      const selection = window.getSelection();
       selection.removeAllRanges();
-      var range = document.createRange();
+      const range = document.createRange();
       range.setStart(element.querySelector('div.contentText').firstChild, 3);
       range.setEnd(
           element.querySelectorAll('div.contentText')[4].firstChild, 2);
@@ -226,13 +226,13 @@ limitations under the License.
       assert.equal(element._getSelectedText('left'), 'ba\nzin\nga');
     });
 
-    test('copies comments', function() {
+    test('copies comments', () => {
       element.classList.add('selected-left');
       element.classList.add('selected-comment');
       element.classList.remove('selected-right');
-      var selection = window.getSelection();
+      const selection = window.getSelection();
       selection.removeAllRanges();
-      var range = document.createRange();
+      const range = document.createRange();
       range.setStart(
           element.querySelector('.gr-formatted-text *').firstChild, 3);
       range.setEnd(
@@ -242,14 +242,14 @@ limitations under the License.
           element._getSelectedText('left', true));
     });
 
-    test('respects astral chars in comments', function() {
+    test('respects astral chars in comments', () => {
       element.classList.add('selected-left');
       element.classList.add('selected-comment');
       element.classList.remove('selected-right');
-      var selection = window.getSelection();
+      const selection = window.getSelection();
       selection.removeAllRanges();
-      var range = document.createRange();
-      var nodes = element.querySelectorAll('.gr-formatted-text *');
+      const range = document.createRange();
+      const nodes = element.querySelectorAll('.gr-formatted-text *');
       range.setStart(nodes[2].childNodes[2], 13);
       range.setEnd(nodes[2].childNodes[2], 23);
       selection.addRange(range);
@@ -257,15 +257,15 @@ limitations under the License.
           element._getSelectedText('left', true));
     });
 
-    test('defers to default behavior for textarea', function() {
+    test('defers to default behavior for textarea', () => {
       element.classList.add('selected-left');
       element.classList.remove('selected-right');
-      var selectedTextSpy = sandbox.spy(element, '_getSelectedText');
+      const selectedTextSpy = sandbox.spy(element, '_getSelectedText');
       emulateCopyOn(element.querySelector('textarea'));
       assert.isFalse(selectedTextSpy.called);
     });
 
-    test('regression test for 4794', function() {
+    test('regression test for 4794', () => {
       element._cachedDiffBuilder.getLineElByChild = function(child) {
         while (!child.classList.contains('content') && child.parentElement) {
           child = child.parentElement;
@@ -276,9 +276,9 @@ limitations under the License.
       element.classList.add('selected-right');
       element.classList.remove('selected-left');
 
-      var selection = window.getSelection();
+      const selection = window.getSelection();
       selection.removeAllRanges();
-      var range = document.createRange();
+      const range = document.createRange();
       range.setStart(
           element.querySelectorAll('div.contentText')[1].firstChild, 4);
       range.setEnd(
@@ -287,12 +287,12 @@ limitations under the License.
       assert.equal(element._getSelectedText('right'), ' other');
     });
 
-    suite('_getTextContentForRange', function() {
-      var selection;
-      var range;
-      var nodes;
+    suite('_getTextContentForRange', () => {
+      let selection;
+      let range;
+      let nodes;
 
-      setup(function() {
+      setup(() => {
         element.classList.add('selected-left');
         element.classList.add('selected-comment');
         element.classList.remove('selected-right');
@@ -302,7 +302,7 @@ limitations under the License.
         nodes = element.querySelectorAll('.gr-formatted-text *');
       });
 
-      test('multi level element contained in range', function() {
+      test('multi level element contained in range', () => {
         range.setStart(nodes[2].childNodes[0], 1);
         range.setEnd(nodes[2].childNodes[2], 7);
         selection.addRange(range);
@@ -311,7 +311,7 @@ limitations under the License.
       });
 
 
-      test('multi level element as startContainer of range', function() {
+      test('multi level element as startContainer of range', () => {
         range.setStart(nodes[2].childNodes[1], 0);
         range.setEnd(nodes[2].childNodes[2], 7);
         selection.addRange(range);
@@ -319,7 +319,7 @@ limitations under the License.
             'a differ');
       });
 
-      test('startContainer === endContainer', function() {
+      test('startContainer === endContainer', () => {
         range.setStart(nodes[0].firstChild, 2);
         range.setEnd(nodes[0].firstChild, 12);
         selection.addRange(range);
@@ -328,7 +328,7 @@ limitations under the License.
       });
     });
 
-    test('cache is reset when diff changes', function() {
+    test('cache is reset when diff changes', () => {
       element._linesCache = {left: 'test', right: 'test'};
       element.diff = {};
       flushAsynchronousOperations();