Replace domdiff library with morphdom

We added domdiff to replace the DiffDOM library
here:

87f54d1e88

DiffDOM had updated its code to be written to the
ECMAScript 6 (ES6) standard and so needed extra
work to work with the older browsers in our
support matrix. This was recorded as an issue
here:

https://www.pivotaltracker.com/n/projects/1443052/stories/165380360

Domdiff didn't work (see below for more details)
so this replaces it with the morphdom library.
Morphdom supports the same browsers as us and is
relied on by a range of large open source
projects:

https://github.com/patrick-steele-idem/morphdom#what-projects-are-using-morphdom

It was tricky to find alternatives to DiffDOM so
if we have to source alternatives in future, other
options could be:
- https://github.com/choojs/nanomorph
- https://diffhtml.org/index.html (using its
  outerHTML method)

Why domdiff didn't work

Turns out that domdiff was replacing the page HTML
with the HTML from the AJAX response every time,
not just when they differed. This isn't a bug.
Domdiff is bare bones enough that it compares old
DOM nodes to new DOM nodes with ===. With our
code, this always results to false because our new
nodes are made from HTML strings from AJAX
response so are never the same node as the old
one.

app/assets/javascripts/modules/all.mjs

@@ -6,11 +6,16 @@
 //
 // Exported items will be added to the window.GOVUK namespace.
 // For example, `export { Frontend }` will assign `Frontend` to `window.Frontend`
+
+// GOVUK Frontend modules
 import Header from 'govuk-frontend/components/header/header';
 import Details from 'govuk-frontend/components/details/details';
 import Button from 'govuk-frontend/components/button/button';
 import Radios from 'govuk-frontend/components/radios/radios';
 
+// Modules from 3rd party vendors
+import morphdom from 'morphdom';
+
 /**
  * TODO: Ideally this would be a NodeList.prototype.forEach polyfill
  * This seems to fail in IE8, requires more investigation.
@@ -62,6 +67,13 @@ var Frontend = {
   "initAll": initAll
 }
 
-export {
-  Frontend
+var vendor = {
+  "morphdom": morphdom
+}
+
+// The exported object will be assigned to window.GOVUK in our production code
+// (bundled into an IIFE by RollupJS)
+export {
+  Frontend,
+  vendor
 }

app/assets/javascripts/updateContent.js

@@ -2,6 +2,7 @@
   "use strict";
 
   var queues = {};
+  var morphdom = global.GOVUK.vendor.morphdom;
   var defaultInterval = 2000;
   var interval = 0;
 
@@ -10,18 +11,10 @@
       1000
   ));
 
-  var getRenderer = $component => {
-    var key = $component.data('key'); // use closure to retain key when component is replaced
-    return response => {
-      $component = $(
-        global.domdiff(
-          $component.parent().get(0),
-          [$component.get(0)],
-          [$(response[key]).get(0)]
-        )[0]
-      );
-    };
-  };
+  var getRenderer = $component => response => morphdom(
+    $component.get(0),
+    $(response[$component.data('key')]).get(0)
+  );
 
   var getQueue = resource => (
     queues[resource] = queues[resource] || []

gulpfile.js

@@ -146,7 +146,6 @@ const javascripts = () => {
       paths.npm + 'hogan.js/dist/hogan-3.0.2.js',
       paths.npm + 'jquery/dist/jquery.min.js',
       paths.npm + 'query-command-supported/dist/queryCommandSupported.min.js',
-      paths.npm + 'domdiff/min.js',
       paths.npm + 'timeago/jquery.timeago.js',
       paths.npm + 'textarea-caret/index.js',
       paths.npm + 'cbor-js/cbor.js'

package.json

@@ -23,7 +23,6 @@
     "@babel/preset-env": "7.4.2",
     "cbor-js": "0.1.0",
     "del": "5.1.0",
-    "domdiff": "2.2.2",
     "govuk_frontend_toolkit": "8.1.0",
     "govuk-elements-sass": "3.1.2",
     "govuk-frontend": "2.13.0",
@@ -39,6 +38,7 @@
     "hogan": "1.0.2",
     "jquery": "3.5.0",
     "leaflet": "1.6.0",
+    "morphdom": "2.6.1",
     "query-command-supported": "1.0.0",
     "rollup": "1.23.1",
     "sass": "1.32.7",

tests/javascripts/updateContent.test.js

@@ -31,13 +31,12 @@ beforeAll(() => {
 
   $.ajax.mockImplementation(() => jqueryAJAXReturnObj);
 
-  // using require to execute the version we use in our our frontend build here can't add
-  // the domdiff variable to this scope like it does when executed in browsers because
-  // that version doesn't export it
-  // we use CommonJS version instead because it does (as the default property)
-  // see https://nodejs.org/en/knowledge/getting-started/what-is-require/ for more info
-  // also, we're not a browser so we need to manually attach domdiff to window
-  window.domdiff = require('domdiff/cjs').default;
+  // RollupJS assigns our bundled module code, including morphdom, to window.GOVUK.
+  // morphdom is assigned to its vendor property so we need to copy that here for the updateContent
+  // code to pick it up.
+  window.GOVUK.vendor = {
+    morphdom: require('morphdom')
+  };
   require('../../app/assets/javascripts/updateContent.js');
 
 });