fetchBaseQuery: return nullon empty body for JSON. Add DevWarnings.

fetchBaseQuery: return `null` instead of `undefined` on empty body for JSON. 
Also add dev errors if `baseQuery`/`queryFn` returns a wrong shape (#1699)

packages/toolkit/src/query/core/buildThunks.ts

@@ -302,6 +302,39 @@ export function buildThunks<
             baseQuery(arg, baseQueryApi, endpointDefinition.extraOptions as any)
         )
       }
+      if (
+        typeof process !== 'undefined' &&
+        process.env.NODE_ENV === 'development'
+      ) {
+        const what = endpointDefinition.query ? '`baseQuery`' : '`queryFn`'
+        let err: undefined | string
+        if (!result) {
+          err = `${what} did not return anything.`
+        } else if (typeof result !== 'object') {
+          err = `${what} did not return an object.`
+        } else if (result.error && result.data) {
+          err = `${what} returned an object containing both \`error\` and \`result\`.`
+        } else if (result.error === undefined && result.data === undefined) {
+          err = `${what} returned an object containing neither a valid \`error\` and \`result\`. At least one of them should not be \`undefined\``
+        } else {
+          for (const key of Object.keys(result)) {
+            if (key !== 'error' && key !== 'data' && key !== 'meta') {
+              err = `The object returned by ${what} has the unknown property ${key}.`
+              break
+            }
+          }
+        }
+        if (err) {
+          console.error(
+            `Error encountered handling the endpoint ${arg.endpointName}.
+              ${err}
+              It needs to return an object with either the shape \`{ data: <value> }\` or \`{ error: <value> }\` that may contain an optional \`meta\` property.
+              Object returned was:`,
+            result
+          )
+        }
+      }
+
       if (result.error) throw new HandledError(result.error, result.meta)
 
       return fulfillWithValue(

packages/toolkit/src/query/fetchBaseQuery.ts

@@ -54,7 +54,7 @@ const handleResponse = async (
 
   if (responseHandler === 'json') {
     const text = await response.text()
-    return text.length ? JSON.parse(text) : undefined
+    return text.length ? JSON.parse(text) : null
   }
 }
 

packages/toolkit/src/query/tests/apiProvider.test.tsx

@@ -6,7 +6,7 @@ import { waitMs } from './helpers'
 const api = createApi({
   baseQuery: async (arg: any) => {
     await waitMs()
-    return { data: arg?.body ? arg.body : undefined }
+    return { data: arg?.body ? arg.body : null }
   },
   endpoints: (build) => ({
     getUser: build.query<any, number>({

packages/toolkit/src/query/tests/buildThunks.test.tsx

@@ -74,7 +74,7 @@ test('passes the extraArgument property to the baseQueryApi', async () => {
 
 describe('re-triggering behavior on arg change', () => {
   const api = createApi({
-    baseQuery: () => ({ data: undefined }),
+    baseQuery: () => ({ data: null }),
     endpoints: (build) => ({
       getUser: build.query<any, any>({
         query: (obj) => obj,

packages/toolkit/src/query/tests/cleanup.test.tsx

@@ -7,7 +7,7 @@ import { render, waitFor } from '@testing-library/react'
 import { setupApiStore } from './helpers'
 
 const api = createApi({
-  baseQuery: () => ({ data: undefined }),
+  baseQuery: () => ({ data: null }),
   endpoints: (build) => ({
     a: build.query<unknown, void>({ query: () => '' }),
     b: build.query<unknown, void>({ query: () => '' }),

packages/toolkit/src/query/tests/createApi.test.ts

@@ -136,7 +136,7 @@ describe('wrong tagTypes log errors', () => {
   })
 
   beforeEach(() => {
-    baseQuery.mockResolvedValue({})
+    baseQuery.mockResolvedValue({ data: 'foo' })
   })
 
   test.each<[keyof typeof api.endpoints, boolean?]>([

packages/toolkit/src/query/tests/fetchBaseQuery.test.tsx

@@ -101,7 +101,7 @@ describe('fetchBaseQuery', () => {
       expect(res).toBeInstanceOf(Object)
       expect(res.meta?.request).toBeInstanceOf(Request)
       expect(res.meta?.response).toBeInstanceOf(Object)
-      expect(res.data).toBeUndefined()
+      expect(res.data).toBeNull()
     })
 
     it('should return an error and status for error responses', async () => {