refactor!: use a union type for SignatureType (#331)

This commit refactors the SignatureType enum into an array of strings
declared [as const](microsoft/TypeScript#29510).
This allows the SignatureType type to be expressed as a union type,
which works a bit better when parsing a users provided string.

This is some simple refactoring in preparation for declarative function
signatures.

BREAKING CHANGE: exported SignatureType type is converted from an enum
to a union type

src/index.ts

@@ -36,7 +36,7 @@ import {resolve} from 'path';
 import {getUserFunction} from './loader';
 import {ErrorHandler} from './invoker';
 import {getServer} from './server';
-import {SignatureType} from './types';
+import {SignatureType, isValidSignatureType} from './types';
 
 // Supported command-line flags
 const FLAG = {
@@ -54,10 +54,6 @@ const ENV = {
   SOURCE: 'FUNCTION_SOURCE',
 };
 
-enum NodeEnv {
-  PRODUCTION = 'production',
-}
-
 const argv = minimist(process.argv, {
   string: [FLAG.PORT, FLAG.TARGET, FLAG.SIGNATURE_TYPE],
 });
@@ -68,17 +64,14 @@ const CODE_LOCATION = resolve(
 const PORT = argv[FLAG.PORT] || process.env[ENV.PORT] || '8080';
 const TARGET = argv[FLAG.TARGET] || process.env[ENV.TARGET] || 'function';
 
-const SIGNATURE_TYPE_STRING =
-  argv[FLAG.SIGNATURE_TYPE] || process.env[ENV.SIGNATURE_TYPE] || 'http';
-const SIGNATURE_TYPE =
-  SignatureType[
-    SIGNATURE_TYPE_STRING.toUpperCase() as keyof typeof SignatureType
-  ];
-if (SIGNATURE_TYPE === undefined) {
+const SIGNATURE_TYPE = (
+  argv[FLAG.SIGNATURE_TYPE] ||
+  process.env[ENV.SIGNATURE_TYPE] ||
+  'http'
+).toLowerCase();
+if (!isValidSignatureType(SIGNATURE_TYPE)) {
   console.error(
-    `Function signature type must be one of: ${Object.values(
-      SignatureType
-    ).join(', ')}.`
+    `Function signature type must be one of: ${SignatureType.join(', ')}.`
   );
   // eslint-disable-next-line no-process-exit
   process.exit(1);
@@ -108,7 +101,7 @@ getUserFunction(CODE_LOCATION, TARGET).then(userFunction => {
 
   SERVER.listen(PORT, () => {
     ERROR_HANDLER.register();
-    if (process.env.NODE_ENV !== NodeEnv.PRODUCTION) {
+    if (process.env.NODE_ENV !== 'production') {
       console.log('Serving function...');
       console.log(`Function: ${TARGET}`);
       console.log(`Signature type: ${SIGNATURE_TYPE}`);

src/router.ts

@@ -39,7 +39,7 @@ export function registerFunctionRoutes(
   userFunction: HandlerFunction,
   functionSignatureType: SignatureType
 ) {
-  if (functionSignatureType === SignatureType.HTTP) {
+  if (functionSignatureType === 'http') {
     app.use('/favicon.ico|/robots.txt', (req, res) => {
       // Neither crawlers nor browsers attempting to pull the icon find the body
       // contents particularly useful, so we send nothing in the response body.
@@ -57,7 +57,7 @@ export function registerFunctionRoutes(
       const handler = makeHttpHandler(userFunction as HttpFunction);
       handler(req, res, next);
     });
-  } else if (functionSignatureType === SignatureType.EVENT) {
+  } else if (functionSignatureType === 'event') {
     app.post('/*', (req, res, next) => {
       const wrappedUserFunction = wrapEventFunction(
         userFunction as EventFunction | EventFunctionWithCallback

src/server.ts

@@ -101,19 +101,19 @@ export function getServer(
   app.disable('x-powered-by');
 
   if (
-    functionSignatureType === SignatureType.EVENT ||
-    functionSignatureType === SignatureType.CLOUDEVENT
+    functionSignatureType === 'event' ||
+    functionSignatureType === 'cloudevent'
   ) {
     // If a Pub/Sub subscription is configured to invoke a user's function directly, the request body
     // needs to be marshalled into the structure that wrapEventFunction expects. This unblocks local
     // development with the Pub/Sub emulator
     app.use(legacyPubSubEventMiddleware);
   }
 
-  if (functionSignatureType === SignatureType.EVENT) {
+  if (functionSignatureType === 'event') {
     app.use(cloudeventToBackgroundEventMiddleware);
   }
-  if (functionSignatureType === SignatureType.CLOUDEVENT) {
+  if (functionSignatureType === 'cloudevent') {
     app.use(backgroundEventToCloudEventMiddleware);
   }
 

src/types.ts

@@ -16,8 +16,22 @@
 // executing the client function.
 export const FUNCTION_STATUS_HEADER_FIELD = 'X-Google-Status';
 
-export enum SignatureType {
-  HTTP = 'http',
-  EVENT = 'event',
-  CLOUDEVENT = 'cloudevent',
-}
+/**
+ * List of function signature types that are supported by the framework.
+ */
+export const SignatureType = ['http', 'event', 'cloudevent'] as const;
+
+/**
+ * Union type of all valid function SignatureType values.
+ */
+export type SignatureType = typeof SignatureType[number];
+
+/**
+ * Type guard to test if a provided value is valid SignatureType
+ * @param x the value to test
+ * @returns true if the provided value is a valid SignatureType
+ */
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+export const isValidSignatureType = (x: any): x is SignatureType => {
+  return SignatureType.includes(x);
+};

test/integration/cloudevent.ts

@@ -16,7 +16,6 @@ import * as assert from 'assert';
 import * as functions from '../../src/functions';
 import * as sinon from 'sinon';
 import {getServer} from '../../src/server';
-import {SignatureType} from '../../src/types';
 import * as supertest from 'supertest';
 
 const TEST_CLOUD_EVENT = {
@@ -224,7 +223,7 @@ describe('CloudEvent Function', () => {
       let receivedCloudEvent: functions.CloudEventsContext | null = null;
       const server = getServer((cloudevent: functions.CloudEventsContext) => {
         receivedCloudEvent = cloudevent as functions.CloudEventsContext;
-      }, SignatureType.CLOUDEVENT);
+      }, 'cloudevent');
       await supertest(server)
         .post('/')
         .set(test.headers)

test/integration/http.ts

@@ -15,7 +15,6 @@
 import * as assert from 'assert';
 import * as express from 'express';
 import {getServer} from '../../src/server';
-import {SignatureType} from '../../src/types';
 import * as supertest from 'supertest';
 
 describe('HTTP Function', () => {
@@ -73,7 +72,7 @@ describe('HTTP Function', () => {
             query: req.query.param,
           });
         },
-        SignatureType.HTTP
+        'http'
       );
       const st = supertest(server);
       await (test.httpVerb === 'GET'

test/integration/legacy_event.ts

@@ -15,7 +15,6 @@
 import * as assert from 'assert';
 import * as functions from '../../src/functions';
 import {getServer} from '../../src/server';
-import {SignatureType} from '../../src/types';
 import * as supertest from 'supertest';
 
 const TEST_CLOUD_EVENT = {
@@ -175,7 +174,7 @@ describe('Event Function', () => {
       const server = getServer((data: {}, context: functions.Context) => {
         receivedData = data;
         receivedContext = context as functions.CloudFunctionsContext;
-      }, SignatureType.EVENT);
+      }, 'event');
       const requestHeaders = {
         'Content-Type': 'application/json',
         ...test.headers,