Fix #377 ExpressReceiver's RespondFn implementation doesn't accept a string

src/ExpressReceiver.spec.ts

@@ -14,6 +14,8 @@ import ExpressReceiver, {
   verifySignatureAndParseBody,
 } from './ExpressReceiver';
 
+import { RespondArguments } from './types/utilities';
+
 describe('ExpressReceiver', () => {
 
   const noopLogger: Logger = {
@@ -379,4 +381,18 @@ describe('ExpressReceiver', () => {
 
   });
 
+  // Just copied the implementation as the method is private and it's a bit hard to write a unit test
+  describe('RespondFn implementation', () => {
+    it('should work with both a string and a RespondArguments', () => {
+      const respond = (response: string | RespondArguments): void => {
+        const validResponse: RespondArguments =
+          (typeof response === 'string') ? { text: response } : response;
+        assert.equal(validResponse.text, 'hello');
+      };
+      respond('hello');
+      respond({ text: 'hello' });
+      respond({ text: 'hello', blocks: [] });
+    });
+  });
+
 });

src/ExpressReceiver.ts

@@ -10,6 +10,7 @@ import crypto from 'crypto';
 import tsscmp from 'tsscmp';
 import { ErrorCode, errorWithCode } from './errors';
 import { Logger, ConsoleLogger } from '@slack/logger';
+import { RespondArguments } from './types/utilities';
 
 // TODO: we throw away the key names for endpoints, so maybe we should use this interface. is it better for migrations?
 // if that's the reason, let's document that with a comment.
@@ -101,8 +102,10 @@ export default class ExpressReceiver extends EventEmitter implements Receiver {
     };
 
     if (req.body && req.body.response_url) {
-      event.respond = (response): void => {
-        this.axios.post(req.body.response_url, response)
+      event.respond = (response: string | RespondArguments): void => {
+        const validResponse: RespondArguments =
+          (typeof response === 'string') ? { text: response } : response;
+        this.axios.post(req.body.response_url, validResponse)
           .catch((e) => {
             this.emit('error', e);
           });