fix: trim undefined from splat routes with no param (#4558)

Fixes #4547

---------

Co-authored-by: Manuel Schiller <[email protected]>

Author: tannerlinsley

packages/router-core/src/path.ts

@@ -330,6 +330,21 @@ export function interpolatePath({
         usedParams._splat = params._splat
         const segmentPrefix = segment.prefixSegment || ''
         const segmentSuffix = segment.suffixSegment || ''
+
+        // Check if _splat parameter is missing
+        if (!('_splat' in params)) {
+          isMissingParams = true
+          // For missing splat parameters, just return the prefix and suffix without the wildcard
+          if (leaveWildcards) {
+            return `${segmentPrefix}${segment.value}${segmentSuffix}`
+          }
+          // If there is a prefix or suffix, return them joined, otherwise omit the segment
+          if (segmentPrefix || segmentSuffix) {
+            return `${segmentPrefix}${segmentSuffix}`
+          }
+          return undefined
+        }
+
         const value = encodeParam('_splat')
         if (leaveWildcards) {
           return `${segmentPrefix}${segment.value}${value ?? ''}${segmentSuffix}`

packages/router-core/tests/path.test.ts

@@ -482,7 +482,7 @@ describe('interpolatePath', () => {
     })
   })
 
-  describe('named params (prefix + suffix', () => {
+  describe('named params (prefix + suffix)', () => {
     it.each([
       {
         name: 'regular',
@@ -523,6 +523,48 @@ describe('interpolatePath', () => {
       ).toBe(result)
     })
   })
+
+  describe('should handle missing _splat parameter for', () => {
+    it.each([
+      {
+        name: 'basic splat route',
+        path: '/hello/$',
+        params: {},
+        expectedResult: '/hello',
+      },
+      {
+        name: 'splat route with prefix',
+        path: '/hello/prefix{$}',
+        params: {},
+        expectedResult: '/hello/prefix',
+      },
+      {
+        name: 'splat route with suffix',
+        path: '/hello/{$}suffix',
+        params: {},
+        expectedResult: '/hello/suffix',
+      },
+      {
+        name: 'splat route with prefix and suffix',
+        path: '/hello/prefix{$}suffix',
+        params: {},
+        expectedResult: '/hello/prefixsuffix',
+      },
+      {
+        name: 'nested splat route',
+        path: '/users/$id/$',
+        params: { id: '123' },
+        expectedResult: '/users/123',
+      },
+    ])('$name', ({ path, params, expectedResult }) => {
+      const result = interpolatePath({
+        path,
+        params,
+      })
+      expect(result.interpolatedPath).toBe(expectedResult)
+      expect(result.isMissingParams).toBe(true)
+    })
+  })
 })
 
 describe('matchPathname', () => {