refactor: server/api-utils.ts (#21148)

- Add return type to function
- Add [isNotValidData](https://github.com/vercel/next.js/pull/21148/files#diff-64466461287cc1a70ce5597e3e1d18ac22755f9aeb7ff1865160a8c057540983R374) function for readability

packages/next/next-server/server/api-utils.ts

@@ -28,7 +28,7 @@ export async function apiResolver(
   apiContext: __ApiPreviewProps,
   propagateError: boolean,
   onError?: ({ err }: { err: any }) => Promise<void>
-) {
+): Promise<void> {
   const apiReq = req as NextApiRequest
   const apiRes = res as NextApiResponse
 
@@ -166,7 +166,9 @@ function parseJson(str: string): object {
  * Parse cookies from `req` header
  * @param req request object
  */
-export function getCookieParser(req: IncomingMessage) {
+export function getCookieParser(
+  req: IncomingMessage
+): () => NextApiRequestCookies {
   return function parseCookie(): NextApiRequestCookies {
     const header: undefined | string | string[] = req.headers.cookie
 
@@ -362,29 +364,24 @@ export function tryGetPreviewData(
   }
 }
 
+function isNotValidData(str: string): boolean {
+  return typeof str !== 'string' || str.length < 16
+}
+
 function setPreviewData<T>(
   res: NextApiResponse<T>,
   data: object | string, // TODO: strict runtime type checking
   options: {
     maxAge?: number
   } & __ApiPreviewProps
 ): NextApiResponse<T> {
-  if (
-    typeof options.previewModeId !== 'string' ||
-    options.previewModeId.length < 16
-  ) {
+  if (isNotValidData(options.previewModeId)) {
     throw new Error('invariant: invalid previewModeId')
   }
-  if (
-    typeof options.previewModeEncryptionKey !== 'string' ||
-    options.previewModeEncryptionKey.length < 16
-  ) {
+  if (isNotValidData(options.previewModeEncryptionKey)) {
     throw new Error('invariant: invalid previewModeEncryptionKey')
   }
-  if (
-    typeof options.previewModeSigningKey !== 'string' ||
-    options.previewModeSigningKey.length < 16
-  ) {
+  if (isNotValidData(options.previewModeSigningKey)) {
     throw new Error('invariant: invalid previewModeSigningKey')
   }