Use keys 1 through 3 to switch between and click on focusable elements.

chrome/browser/resources/chromeos/switch_access/switch_access.js

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-var AutomationNode = chrome.automation.AutomationNode;
-
+let AutomationNode = chrome.automation.AutomationNode;
+
+let debuggingEnabled = true;
 /**
  * @constructor
  */
-var SwitchAccess = function() {
-  console.log("Switch access is enabled");
+let SwitchAccess = function() {
+  console.log('Switch access is enabled');
 
   // Currently selected node.
   /** @private {AutomationNode} */
   this.node_ = null;
 
+  // Root node (i.e., the desktop).
+  /** @private {AutomationNode} */
+  this.root_ = null;
+
   // List of nodes to push to / pop from in case this.node_ is lost.
   /** @private {!Array<!AutomationNode>} */
   this.ancestorList_ = [];
 
   chrome.automation.getDesktop(function(desktop) {
     this.node_ = desktop;
-    console.log("AutomationNode for desktop is loaded");
+    this.root_ = desktop;
+    console.log('AutomationNode for desktop is loaded');
     this.printDetails_();
 
-    document.addEventListener("keyup", function(event) {
-      if (event.key === "1") {
-        console.log("1 = go to previous element");
-        this.moveToPrevious_();
-      } else if (event.key === "2") {
-        console.log("2 = go to next element");
-        this.moveToNext_();
-      } else if (event.key === "3") {
-        console.log("3 = go to child element");
-        this.moveToFirstChild_();
-      } else if (event.key === "4") {
-        console.log("4 = go to parent element");
-        this.moveToParent_();
-      } else if (event.key === "5") {
-        console.log("5 is not yet implemented");
-        console.log("\n");
-      }
-      chrome.accessibilityPrivate.setFocusRing([this.node_.location]);
+    document.addEventListener('keyup', function(event) {
+      switch (event.key) {
+        case '1':
+          console.log('1 = go to previous element');
+          this.moveToPrevious_();
+          break;
+        case '2':
+          console.log('2 = go to next element');
+          this.moveToNext_();
+          break;
+        case '3':
+          console.log('3 = do default on element');
+          this.doDefault_();
+          break;
+      }
+      if (debuggingEnabled) {
+        switch (event.key) {
+          case '6':
+            console.log('6 = go to previous element (debug mode)');
+            this.debugMoveToPrevious_();
+            break;
+          case '7':
+            console.log('7 = go to next element (debug mode)');
+            this.debugMoveToNext_();
+            break;
+          case '8':
+            console.log('8 = go to child element (debug mode)');
+            this.debugMoveToFirstChild_();
+            break;
+          case '9':
+            console.log('9 = go to parent element (debug mode)');
+            this.debugMoveToParent_();
+            break;
+        }
+      }
+      if (this.node_)
+        chrome.accessibilityPrivate.setFocusRing([this.node_.location]);
     }.bind(this));
   }.bind(this));
 };
 
 SwitchAccess.prototype = {
   /**
+   * Set this.node_ to the previous interesting node. If no interesting node
+   * comes before this.node_, set this.node_ to the last interesting node.
+   *
+   * @private
+   */
+  moveToPrevious_: function() {
+    if (this.node_) {
+      let prev = this.getPreviousNode_(this.node_);
+      while (prev && !this.isInteresting_(prev)) {
+        prev = this.getPreviousNode_(prev);
+      }
+      if (prev) {
+        this.node_ = prev;
+        this.printNode_(this.node_);
+        return;
+      }
+    }
+
+    if (this.root_) {
+      console.log('Reached the first interesting node. Restarting with last.')
+      let prev = this.getYoungestDescendant_(this.root_);
+      while (prev && !this.isInteresting_(prev)) {
+        prev = this.getPreviousNode_(prev);
+      }
+      if (prev) {
+        this.node_ = prev;
+        this.printNode_(this.node_);
+        return;
+      }
+    }
+
+    console.log('Found no interesting nodes to visit.')
+  },
+
+  /**
+   * Set this.node_ to the next interesting node. If no interesting node comes
+   * after this.node_, set this.node_ to the first interesting node.
+   *
+   * @private
+   */
+  moveToNext_: function() {
+    if (this.node_) {
+      let next = this.getNextNode_(this.node_);
+      while (next && !this.isInteresting_(next))
+        next = this.getNextNode_(next);
+      if (next) {
+        this.node_ = next;
+        this.printNode_(this.node_);
+        return;
+      }
+    }
+
+    if (this.root_) {
+      console.log('Reached the last interesting node. Restarting with first.');
+      let next = this.root_.firstChild;
+      while (next && !this.isInteresting_(next))
+        next = this.getNextNode_(next);
+      if (next) {
+        this.node_ = next;
+        this.printNode_(this.node_);
+        return;
+      }
+    }
+
+    console.log('Found no interesting nodes to visit.');
+  },
+
+  /**
+   * Given a flat list of nodes in pre-order, get the node that comes after
+   * |node|.
+   *
+   * @param {!AutomationNode} node
+   * @return {AutomationNode}
+   * @private
+   */
+  getNextNode_: function(node) {
+    // Check for child.
+    let child = node.firstChild;
+    if (child)
+      return child;
+
+    // No child. Check for right-sibling.
+    let sibling = node.nextSibling;
+    if (sibling)
+      return sibling;
+
+    // No right-sibling. Get right-sibling of closest ancestor.
+    let ancestor = node.parent;
+    while (ancestor) {
+      let aunt = ancestor.nextSibling;
+      if (aunt)
+        return aunt;
+      ancestor = ancestor.parent;
+    }
+
+    // No node found after |node|, so return null.
+    return null;
+  },
+
+  /**
+   * Given a flat list of nodes in pre-order, get the node that comes before
+   * |node|.
+   *
+   * @param {!AutomationNode} node
+   * @return {AutomationNode}
+   * @private
+   */
+  getPreviousNode_: function(node) {
+    // Check for left-sibling. Return its youngest descendant if it has one.
+    // Otherwise, return the sibling.
+    let sibling = node.previousSibling;
+    if (sibling) {
+      let descendant = this.getYoungestDescendant_(sibling);
+      if (descendant)
+        return descendant;
+      return sibling;
+    }
+
+    // No left-sibling. Check for parent.
+    let parent = node.parent;
+    if (parent)
+      return parent;
+
+    // No node found before |node|, so return null.
+    return null;
+  },
+
+  /**
+   * Get the youngest descendant of |node| if it has one.
+   *
+   * @param {!AutomationNode} node
+   * @return {AutomationNode}
+   * @private
+   */
+  getYoungestDescendant_: function(node) {
+    if (!node.lastChild)
+      return null;
+
+    while (node.lastChild)
+      node = node.lastChild;
+
+    return node;
+  },
+
+  /**
+   * Returns true if |node| is interesting.
+   *
+   * @param {!AutomationNode} node
+   * @return {boolean}
+   * @private
+   */
+  isInteresting_: function(node) {
+    return node.state && node.state.focusable;
+  },
+
+  /**
    * Move to the previous sibling of this.node_ if it has one.
-   */
-  moveToPrevious_: function() {
-    var previous = this.node_.previousSibling;
+   *
+   * @private
+   */
+  debugMoveToPrevious_: function() {
+    let previous = this.node_.previousSibling;
     if (previous) {
       this.node_ = previous;
       this.printDetails_();
     } else {
-      console.log("Node is first of siblings");
-      console.log("\n");
+      console.log('Node is first of siblings');
+      console.log('\n');
     }
   },
 
   /**
    * Move to the next sibling of this.node_ if it has one.
-   */
-  moveToNext_: function() {
-    var next = this.node_.nextSibling;
+   *
+   * @private
+   */
+  debugMoveToNext_: function() {
+    let next = this.node_.nextSibling;
     if (next) {
       this.node_ = next;
       this.printDetails_();
     } else {
-      console.log("Node is last of siblings");
-      console.log("\n");
+      console.log('Node is last of siblings');
+      console.log('\n');
     }
   },
 
   /**
    * Move to the first child of this.node_ if it has one.
-   */
-  moveToFirstChild_: function() {
-    var child = this.node_.firstChild;
+   *
+   * @private
+   */
+  debugMoveToFirstChild_: function() {
+    let child = this.node_.firstChild;
     if (child) {
       this.ancestorList_.push(this.node_);
       this.node_ = child;
       this.printDetails_();
     } else {
-      console.log("Node has no children");
-      console.log("\n");
-    }
-  },
-
+      console.log('Node has no children');
+      console.log('\n');
+    }
+  },
+
   /**
    * Move to the parent of this.node_ if it has one. If it does not have a
    * parent but it is not the top level root node, then this.node_ lost track of
    * its neighbors, and we move to an ancestor node.
+   *
+   * @private
    */
-  moveToParent_: function() {
-    var parent = this.node_.parent;
+  debugMoveToParent_: function() {
+    let parent = this.node_.parent;
     if (parent) {
       this.ancestorList_.pop();
       this.node_ = parent;
       this.printDetails_();
     } else if (this.ancestorList_.length === 0) {
-      console.log("Node has no parent");
-      console.log("\n");
+      console.log('Node has no parent');
+      console.log('\n');
     } else {
       console.log(
-          "Node could not find its parent, so moved to recent ancestor");
-      var ancestor = this.ancestorList_.pop();
+          'Node could not find its parent, so moved to recent ancestor');
+      let ancestor = this.ancestorList_.pop();
       this.node_ = ancestor;
       this.printDetails_();
     }
   },
 
   /**
+   * Perform the default action on the currently selected node.
+   *
+   * @private
+   */
+  doDefault_: function() {
+    let state = this.node_.state;
+    if (state && state.focusable)
+      console.log('Node was focusable, doing default on it')
+    else if (state)
+      console.log('Node was not focusable, but still doing default');
+    else
+      console.log('Node has no state, still doing default');
+    console.log('\n');
+    this.node_.doDefault();
+  },
+
+  // TODO(elichtenberg): Move print functions to a custom logger class. Only
+  // log when debuggingEnabled is true.
+  /**
    * Print out details about the currently selected node and the list of
    * ancestors.
    *
    * @private
    */
   printDetails_: function() {
     this.printNode_(this.node_);
     console.log(this.ancestorList_);
-    console.log("\n");
+    console.log('\n');
   },
 
   /**
    * Print out details about a node.
    *
    * @param {AutomationNode} node
    * @private
    */
   printNode_: function(node) {
     if (node) {
-      console.log("Name = " + node.name);
-      console.log("Role = " + node.role);
-      if (!node.parent) {
-        console.log("At index " + node.indexInParent + ", has no parent");
-      } else {
-        var numSiblings = node.parent.children.length;
+      console.log('Name = ' + node.name);
+      console.log('Role = ' + node.role);
+      if (!node.parent)
+        console.log('At index ' + node.indexInParent + ', has no parent');
+      else {
+        let numSiblings = node.parent.children.length;
         console.log(
-            "At index " + node.indexInParent + ", there are "
-            + numSiblings + " siblings");
+            'At index ' + node.indexInParent + ', there are '
+            + numSiblings + ' siblings');
       }
-      console.log("Has " + node.children.length + " children");
+      console.log('Has ' + node.children.length + ' children');
     } else {
-      console.log("Node is null");
+      console.log('Node is null');
     }
     console.log(node);
+    console.log('\n');
   }
 };
 
-new SwitchAccess();
+window.switchAccess = new SwitchAccess();