Update return type of RetryCallback (#3851)

* Update return type

* add type tests

test/types/retry-handler.test-d.ts

@@ -0,0 +1,49 @@
+import { expectType, expectAssignable, expectNotAssignable } from 'tsd'
+import { Dispatcher, RetryHandler } from '../..'
+
+// Test the basic structure of RetryCallback
+expectType<RetryHandler.RetryCallback>((err, context, callback) => {
+  expectType<Error>(err)
+  expectType<{
+    state: RetryHandler.RetryState;
+    opts: Dispatcher.DispatchOptions & {
+      retryOptions?: RetryHandler.RetryOptions;
+    };
+  }>(context)
+  expectType<RetryHandler.OnRetryCallback>(callback)
+})
+
+// Test that RetryCallback returns void
+const testCallback = (() => {}) as RetryHandler.RetryCallback
+const testContext = {
+  state: {} as RetryHandler.RetryState,
+  opts: {} as Dispatcher.DispatchOptions & {
+    retryOptions?: RetryHandler.RetryOptions;
+  }
+}
+
+expectType<void>(testCallback(new Error(), testContext, () => {}))
+
+// Test that the function is assignable to RetryCallback
+expectAssignable<RetryHandler.RetryCallback>(testCallback)
+
+// Test that an incorrectly typed function is not assignable to RetryCallback
+expectNotAssignable<RetryHandler.RetryCallback>((() => {}) as (
+  err: string,
+  context: number,
+  callback: boolean
+) => void)
+
+// Test the nested types
+const contextTest: Parameters<RetryHandler.RetryCallback>[1] = {
+  state: {} as RetryHandler.RetryState,
+  opts: {
+    method: 'GET',
+    path: 'some-path',
+    retryOptions: {} as RetryHandler.RetryOptions
+  }
+}
+expectType<RetryHandler.RetryState>(contextTest.state)
+expectType<
+  Dispatcher.DispatchOptions & { retryOptions?: RetryHandler.RetryOptions }
+>(contextTest.opts)

types/retry-handler.d.ts

@@ -32,7 +32,7 @@ declare namespace RetryHandler {
       };
     },
     callback: OnRetryCallback
-  ) => number | null
+  ) => void
 
   export interface RetryOptions {
     /**