Preserve the state of conditional reveals when navigating 'back' in the browser

CHANGELOG.md

@@ -7,6 +7,7 @@
 We’ve made fixes to GOV.UK Frontend in the following pull requests:
 
 - [#1838: Correctly camel case SVG attributes in the header and footer](https://github.com/alphagov/govuk-frontend/pull/1838)
+- [#1842: Preserve the state of conditional reveals when navigating 'back' in the browser](https://github.com/alphagov/govuk-frontend/pull/1842)
 
 ## 3.7.0 (Feature release)
 

src/govuk/components/checkboxes/checkboxes.js

@@ -1,5 +1,6 @@
 import '../../vendor/polyfills/Function/prototype/bind'
-import '../../vendor/polyfills/Event' // addEventListener and event.target normaliziation
+// addEventListener, event.target normalization and DOMContentLoaded
+import '../../vendor/polyfills/Event'
 import '../../vendor/polyfills/Element/prototype/classList'
 import { nodeListForEach } from '../../common'
 
@@ -29,14 +30,32 @@ Checkboxes.prototype.init = function () {
     // If we have content that is controlled, set attributes.
     $input.setAttribute('aria-controls', controls)
     $input.removeAttribute('data-aria-controls')
-    this.setAttributes($input)
-  }.bind(this))
+  })
+
+  // When the page is restored after navigating 'back' in some browsers the
+  // state of form controls is not restored until *after* the DOMContentLoaded
+  // event is fired, so we need to sync after the pageshow event in browsers
+  // that support it.
+  if ('onpageshow' in window) {
+    window.addEventListener('pageshow', this.syncAll.bind(this))
+  } else {
+    window.addEventListener('DOMContentLoaded', this.syncAll.bind(this))
+  }
+
+  // Although we've set up handlers to sync state on the pageshow or
+  // DOMContentLoaded event, init could be called after those events have fired,
+  // for example if they are added to the page dynamically, so sync now too.
+  this.syncAll()
 
   // Handle events
   $module.addEventListener('click', this.handleClick.bind(this))
 }
 
-Checkboxes.prototype.setAttributes = function ($input) {
+Checkboxes.prototype.syncAll = function () {
+  nodeListForEach(this.$inputs, this.syncWithInputState.bind(this))
+}
+
+Checkboxes.prototype.syncWithInputState = function ($input) {
   var inputIsChecked = $input.checked
   $input.setAttribute('aria-expanded', inputIsChecked)
 
@@ -53,7 +72,7 @@ Checkboxes.prototype.handleClick = function (event) {
   var isCheckbox = $target.getAttribute('type') === 'checkbox'
   var hasAriaControls = $target.getAttribute('aria-controls')
   if (isCheckbox && hasAriaControls) {
-    this.setAttributes($target)
+    this.syncWithInputState($target)
   }
 }
 

src/govuk/components/radios/radios.js

@@ -1,15 +1,17 @@
 import '../../vendor/polyfills/Function/prototype/bind'
-import '../../vendor/polyfills/Event' // addEventListener and event.target normaliziation
+// addEventListener, event.target normalization and DOMContentLoaded
+import '../../vendor/polyfills/Event'
 import '../../vendor/polyfills/Element/prototype/classList'
 import { nodeListForEach } from '../../common'
 
 function Radios ($module) {
   this.$module = $module
+  this.$inputs = $module.querySelectorAll('input[type="radio"]')
 }
 
 Radios.prototype.init = function () {
   var $module = this.$module
-  var $inputs = $module.querySelectorAll('input[type="radio"]')
+  var $inputs = this.$inputs
 
   /**
   * Loop over all items with [data-controls]
@@ -28,14 +30,32 @@ Radios.prototype.init = function () {
     // If we have content that is controlled, set attributes.
     $input.setAttribute('aria-controls', controls)
     $input.removeAttribute('data-aria-controls')
-    this.setAttributes($input)
-  }.bind(this))
+  })
+
+  // When the page is restored after navigating 'back' in some browsers the
+  // state of form controls is not restored until *after* the DOMContentLoaded
+  // event is fired, so we need to sync after the pageshow event in browsers
+  // that support it.
+  if ('onpageshow' in window) {
+    window.addEventListener('pageshow', this.syncAll.bind(this))
+  } else {
+    window.addEventListener('DOMContentLoaded', this.syncAll.bind(this))
+  }
+
+  // Although we've set up handlers to sync state on the pageshow or
+  // DOMContentLoaded event, init could be called after those events have fired,
+  // for example if they are added to the page dynamically, so sync now too.
+  this.syncAll()
 
   // Handle events
   $module.addEventListener('click', this.handleClick.bind(this))
 }
 
-Radios.prototype.setAttributes = function ($input) {
+Radios.prototype.syncAll = function () {
+  nodeListForEach(this.$inputs, this.syncWithInputState.bind(this))
+}
+
+Radios.prototype.syncWithInputState = function ($input) {
   var $content = document.querySelector('#' + $input.getAttribute('aria-controls'))
 
   if ($content && $content.classList.contains('govuk-radios__conditional')) {
@@ -65,7 +85,7 @@ Radios.prototype.handleClick = function (event) {
     // In radios, only radios with the same name will affect each other.
     var hasSameName = ($input.name === $clickedInput.name)
     if (hasSameName && hasSameFormOwner) {
-      this.setAttributes($input)
+      this.syncWithInputState($input)
     }
   }.bind(this))
 }