fix(gatsby-plugin-page-creator): support index routes when using the …

…File System Route API (#31339)

Co-authored-by: LekoArts <lekoarts@gmail.com>

packages/gatsby-plugin-page-creator/src/__tests__/derive-path.ts

@@ -265,6 +265,74 @@ describe(`derive-path`, () => {
     ).toEqual(`foo/dolores/[...name]`)
   })
 
+  it(`supports index paths`, () => {
+    expect(
+      derivePath(
+        `{Page.path}`,
+        {
+          path: `/`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`index`)
+    expect(
+      derivePath(
+        `{Page.path}.js`,
+        {
+          path: `/`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`index.js`)
+    expect(
+      derivePath(
+        `foo/{Page.path}`,
+        {
+          path: `/`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`foo`)
+    expect(
+      derivePath(
+        `foo/{Page.path}/bar`,
+        {
+          path: `/`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`foo/bar`)
+    expect(
+      derivePath(
+        `foo/{Page.pathOne}/{Page.pathTwo}`,
+        {
+          pathOne: `/`,
+          pathTwo: `bar`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`foo/bar`)
+    expect(
+      derivePath(
+        `foo/{Page.pathOne}/{Page.pathTwo}`,
+        {
+          pathOne: `/`,
+          pathTwo: `/bar`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`foo/bar`)
+    expect(
+      derivePath(
+        `foo/{Page.path}/[...name]`,
+        {
+          path: `/`,
+        },
+        reporter
+      ).derivedPath
+    ).toEqual(`foo/[...name]`)
+  })
+
   it(`handles special chars`, () => {
     expect(
       derivePath(

packages/gatsby-plugin-page-creator/src/derive-path.ts

@@ -6,9 +6,12 @@ import {
   extractAllCollectionSegments,
   switchToPeriodDelimiters,
   stripTrailingSlash,
+  removeFileExtension,
 } from "./path-utils"
 
 const doubleForwardSlashes = /\/\/+/g
+// Match 0 or 1 of "/"
+const indexRoute = /^\/?$/
 
 // Generates the path for the page from the file path
 // product/{Product.id} => /product/:id, pulls from nodes.id
@@ -64,6 +67,14 @@ export function derivePath(
   // 4.  Remove double forward slashes that could occur in the final URL
   modifiedPath = modifiedPath.replace(doubleForwardSlashes, `/`)
 
+  // 5.  Remove trailing slashes that could occur in the final URL
+  modifiedPath = stripTrailingSlash(modifiedPath)
+
+  // 6.  If the final URL appears to be an index path, use the "index" file naming convention
+  if (indexRoute.test(removeFileExtension(modifiedPath))) {
+    modifiedPath = `index${modifiedPath}`
+  }
+
   const derivedPath = modifiedPath
 
   return {