Fix exhaustiveness of error code switch in http status conversion (#520)

* Fix exhaustiveness of error code switch in http status conversion

* Add a dot at the end of an error message

* Add a newline before parameter documentation

Author: merlinnot

src/providers/https.ts

@@ -27,6 +27,7 @@ import * as _ from 'lodash';
 import { apps } from '../apps';
 import { HttpsFunction, optionsToTrigger, Runnable } from '../cloud-functions';
 import { DeploymentOptions } from '../function-configuration';
+import { assertNever } from '../utilities/assertions';
 
 export interface Request extends express.Request {
   rawBody: Buffer;
@@ -231,7 +232,7 @@ export class HttpsError extends Error {
         return 500;
       // This should never happen as long as the type system is doing its job.
       default:
-        throw new Error('Invalid error code: ' + this.code);
+        assertNever(this.code);
     }
   }
 

src/utilities/assertions.ts

@@ -0,0 +1,16 @@
+/**
+ * @file Provides common assertion helpers which can be used to improve
+ * strictness of both type checking and runtime.
+ */
+
+/**
+ * Checks that the given value is of type `never` — the type that’s left after
+ * all other cases have been removed.
+ *
+ * @param x A value of type `never`.
+ */
+export function assertNever(x: never): never {
+  throw new Error(
+    `Unhandled discriminated union member: ${JSON.stringify(x)}.`
+  );
+}