feature/485768 page conditions

src/server/plugins/engine/helpers.ts

@@ -1,4 +1,4 @@
-import { ControllerPath } from '@defra/forms-model'
+import { ControllerPath, Engine } from '@defra/forms-model'
 import Boom from '@hapi/boom'
 import { type ResponseToolkit } from '@hapi/hapi'
 import { format, parseISO } from 'date-fns'
@@ -155,6 +155,11 @@
 }
 
 export function getStartPath(model?: FormModel) {
+  if (model?.engine === Engine.V2) {
+    const startPath = normalisePath(model.def.pages.at(0)?.path)
+    return startPath ? `/${startPath}` : ControllerPath.Start
+  }
+
   const startPath = normalisePath(model?.def.startPage)
   return startPath ? `/${startPath}` : ControllerPath.Start
 }

src/server/plugins/engine/models/FormModel.ts

@@ -2,6 +2,7 @@ import {
   ConditionsModel,
   ControllerPath,
   ControllerType,
+  Engine,
   formDefinitionSchema,
   hasRepeater,
   type ConditionWrapper,
@@ -34,10 +35,12 @@ import {
 import { FormAction } from '~/src/server/routes/types.js'
 import { merge } from '~/src/server/services/cacheService.js'
 
+/**
+ * Responsible for instantiating the {@link PageControllerClass} and condition context from a form JSON
+ */
 export class FormModel {
-  /**
-   * Responsible for instantiating the {@link PageControllerClass} and condition context from a form JSON
-   */
+  /** The runtime engine that should be used */
+  engine: Engine
 
   /** the entire form JSON as an object */
   def: FormDefinition
@@ -78,6 +81,7 @@ export class FormModel {
       ]
     })
 
+    this.engine = def.engine
     this.def = def
     this.lists = def.lists
     this.sections = def.sections
@@ -228,6 +232,17 @@ export class FormModel {
     // Find start page
     let nextPage = findPage(this, startPath)
 
+    // For the V2 engine, we need to initialise `evaluationState` to null
+    // for all keys. This is because the current condition evaluation
+    // library (eval-expr) will throw if an expression uses a key that is undefined.
+    if (this.engine === Engine.V2) {
+      for (const page of this.pages) {
+        for (const key of page.keys) {
+          context.evaluationState[key] = null
+        }
+      }
+    }
+
     // Walk form pages from start
     while (nextPage) {
       const { collection, pageDef } = nextPage

src/server/plugins/engine/pageControllers/PageController.ts

@@ -18,6 +18,7 @@ import {
   normalisePath
 } from '~/src/server/plugins/engine/helpers.js'
 import { type FormModel } from '~/src/server/plugins/engine/models/index.js'
+import { type ExecutableCondition } from '~/src/server/plugins/engine/models/types.js'
 import {
   type FormContext,
   type PageViewModelBase
@@ -39,6 +40,7 @@ export class PageController {
   pageDef: Page
   title: string
   section?: Section
+  condition?: ExecutableCondition
   collection?: ComponentCollection
   viewName = 'index'
 
@@ -55,6 +57,11 @@ export class PageController {
     this.section = model.sections.find(
       (section) => section.name === pageDef.section
     )
+
+    // Resolve condition
+    if (pageDef.condition) {
+      this.condition = model.conditions[pageDef.condition]
+    }
   }
 
   get path() {

src/server/plugins/engine/pageControllers/QuestionPageController.ts

@@ -1,5 +1,7 @@
 import {
   ComponentType,
+  ControllerType,
+  Engine,
   hasComponents,
   hasNext,
   hasRepeater,
@@ -181,6 +183,33 @@ export class QuestionPageController extends PageController {
     // Walk from summary page (no next links) to status page
     let defaultPath = path === summaryPath ? statusPath : undefined
 
+    if (model.engine === Engine.V2) {
+      if (this.pageDef.controller !== ControllerType.Terminal) {
+        const { pages } = this.model
+        const pageIndex = pages.indexOf(this)
+
+        // The "next" page is the first found after the current which is
+        // either unconditional or has a condition that evaluates to "true"
+        const nextPage = pages.slice(pageIndex + 1).find((page) => {
+          const { condition } = page
+
+          if (condition) {
+            const conditionResult = condition.fn(evaluationState)
+
+            if (!conditionResult) {
+              return false
+            }
+          }
+
+          return true
+        })
+
+        return nextPage?.path ?? defaultPath
+      } else {
+        return defaultPath
+      }
+    }
+
     const nextLink = next.find((link) => {
       const { condition } = link
 

src/server/plugins/engine/pageControllers/TerminalPageController.ts

@@ -0,0 +1,19 @@
+import { type PageTerminal } from '@defra/forms-model'
+import Boom from '@hapi/boom'
+import { type ResponseObject, type ResponseToolkit } from '@hapi/hapi'
+
+import { QuestionPageController } from '~/src/server/plugins/engine/pageControllers/QuestionPageController.js'
+import { type FormContext } from '~/src/server/plugins/engine/types.js'
+import { type FormRequestPayload } from '~/src/server/routes/types.js'
+
+export class TerminalPageController extends QuestionPageController {
+  declare pageDef: PageTerminal
+
+  makePostRouteHandler(): (
+    request: FormRequestPayload,
+    context: FormContext,
+    h: Pick<ResponseToolkit, 'redirect' | 'view'>
+  ) => Promise<ResponseObject> {
+    throw Boom.methodNotAllowed('POST method not allowed for terminal pages')
+  }
+}

src/server/plugins/engine/pageControllers/helpers.test.ts

@@ -18,7 +18,8 @@ import {
   RepeatPageController,
   StartPageController,
   StatusPageController,
-  SummaryPageController
+  SummaryPageController,
+  TerminalPageController
 } from '~/src/server/plugins/engine/pageControllers/index.js'
 import definition from '~/test/form/definitions/blank.js'
 
@@ -37,6 +38,10 @@ describe('Page controller helpers', () => {
         controller = QuestionPageController
         break
 
+      case ControllerType.Terminal:
+        controller = TerminalPageController
+        break
+
       case ControllerType.Repeat:
         controller = RepeatPageController
         break

src/server/plugins/engine/pageControllers/helpers.ts

@@ -44,6 +44,10 @@ export function createPage(model: FormModel, pageDef: Page) {
       controller = new PageControllers.QuestionPageController(model, pageDef)
       break
 
+    case ControllerType.Terminal:
+      controller = new PageControllers.TerminalPageController(model, pageDef)
+      break
+
     case ControllerType.Summary:
       controller = new PageControllers.SummaryPageController(model, pageDef)
       break

src/server/plugins/engine/pageControllers/index.ts

@@ -3,6 +3,7 @@ export {
   QuestionPageController, // Export alongside alias
   QuestionPageController as PageController
 } from '~/src/server/plugins/engine/pageControllers/QuestionPageController.js'
+export { TerminalPageController } from '~/src/server/plugins/engine/pageControllers/TerminalPageController.js'
 export { SummaryPageController } from '~/src/server/plugins/engine/pageControllers/SummaryPageController.js'
 export { StatusPageController } from '~/src/server/plugins/engine/pageControllers/StatusPageController.js'
 export { FileUploadPageController } from '~/src/server/plugins/engine/pageControllers/FileUploadPageController.js'