app-router: add support for parallel routes in useParams (#49595)

This PR changes `useParams` to return all dynamic params present in the
tree as opposed to only those on the canonical/URL path


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link NEXT-1108

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
Co-authored-by: Tim Neutkens <tim@timneutkens.nl>

packages/next/src/client/components/navigation.ts

@@ -129,26 +129,29 @@ export function useRouter(): import('../../shared/lib/app-router-context').AppRo
 interface Params {
   [key: string]: string
 }
-// TODO-APP: handle parallel routes
+
+// this function performs a depth-first search of the tree to find the selected
+// params
 function getSelectedParams(
   tree: FlightRouterState,
   params: Params = {}
 ): Params {
-  // After first parallel route prefer children, if there's no children pick the first parallel route.
   const parallelRoutes = tree[1]
-  const node = parallelRoutes.children ?? Object.values(parallelRoutes)[0]
 
-  if (!node) return params
-  const segment = node[0]
-  const isDynamicParameter = Array.isArray(segment)
-  const segmentValue = isDynamicParameter ? segment[1] : segment
-  if (!segmentValue || segmentValue.startsWith('__PAGE__')) return params
+  for (const parallelRoute of Object.values(parallelRoutes)) {
+    const segment = parallelRoute[0]
+    const isDynamicParameter = Array.isArray(segment)
+    const segmentValue = isDynamicParameter ? segment[1] : segment
+    if (!segmentValue || segmentValue.startsWith('__PAGE__')) continue
+
+    if (isDynamicParameter) {
+      params[segment[0]] = segment[1]
+    }
 
-  if (isDynamicParameter) {
-    params[segment[0]] = segment[1]
+    params = getSelectedParams(parallelRoute, params)
   }
 
-  return getSelectedParams(node, params)
+  return params
 }
 
 /**

test/e2e/app-dir/parallel-routes-and-interception/app/parallel-dynamic/[slug]/@bar/[id]/page.js

@@ -0,0 +1,6 @@
+'use client'
+import { useParams } from 'next/navigation'
+
+export default function Page() {
+  return <div id="bar">{JSON.stringify(useParams())}</div>
+}

test/e2e/app-dir/parallel-routes-and-interception/app/parallel-dynamic/[slug]/@foo/[id]/page.js

@@ -0,0 +1,6 @@
+'use client'
+import { useParams } from 'next/navigation'
+
+export default function Page() {
+  return <div id="foo">{JSON.stringify(useParams())}</div>
+}

test/e2e/app-dir/parallel-routes-and-interception/app/parallel-dynamic/[slug]/layout.js

@@ -0,0 +1,10 @@
+export default function Layout({ bar, foo }) {
+  return (
+    <div>
+      <h1>foo</h1>
+      {foo}
+      <h1>bar</h1>
+      {bar}
+    </div>
+  )
+}

test/e2e/app-dir/parallel-routes-and-interception/parallel-routes-and-interception.test.ts

@@ -305,6 +305,20 @@ createNextDescribe(
           'parallel for foo'
         )
       })
+
+      it('should display all parallel route params with useParams', async () => {
+        const browser = await next.browser('/parallel-dynamic/foo/bar')
+
+        await check(
+          () => browser.waitForElementByCss('#foo').text(),
+          `{"slug":"foo","id":"bar"}`
+        )
+
+        await check(
+          () => browser.waitForElementByCss('#bar').text(),
+          `{"slug":"foo","id":"bar"}`
+        )
+      })
     })
 
     describe('route intercepting', () => {