[App Config] Better error message for AppConfigClient with invalid connection strings (#18356)

* better error message for AppConfigClient

* changelog

* (connectionStringOrEndpoint || "").match

* connectionStringOrEndpoint?.match

Author: HarshaNalluru

sdk/appconfiguration/app-configuration/CHANGELOG.md

@@ -13,6 +13,8 @@
 
 ### Other Changes
 
+- Throws a better error message if provided invalid connection strings such as `undefined` to the `AppConfigurationClient` constructor. [#18356](https://github.com/Azure/azure-sdk-for-js/pull/18356)
+
 ## 1.3.0 (2021-07-26)
 
 ### Features Added

sdk/appconfiguration/app-configuration/recordings/node/authentication/recording_invalid_connection_string_gives_a_decent_error_message.js

@@ -162,9 +162,7 @@ export class AppConfigurationClient {
       appConfigEndpoint = connectionStringOrEndpoint;
     } else {
       appConfigOptions = (tokenCredentialOrOptions as InternalAppConfigurationClientOptions) || {};
-
-      const regexMatch = connectionStringOrEndpoint.match(ConnectionStringRegex);
-
+      const regexMatch = connectionStringOrEndpoint?.match(ConnectionStringRegex);
       if (regexMatch) {
         appConfigCredential = new AppConfigCredential(regexMatch[2], regexMatch[3]);
         appConfigEndpoint = regexMatch[1];

sdk/appconfiguration/app-configuration/src/appConfigurationClient.ts

@@ -23,14 +23,6 @@ describe("Authentication", () => {
   afterEach(async function() {
     await recorder.stop();
   });
-
-  it("invalid connection string gives a decent error message", () => {
-    assert.throws(
-      () => new AppConfigurationClient("an invalid connection string"),
-      /Invalid connection string\. Valid connection strings should match the regex 'Endpoint=\(\.\*\);Id=\(\.\*\);Secret=\(\.\*\)'/
-    );
-  });
-
   it("token authentication works", async function() {
     const client = new AppConfigurationClient(
       credsAndEndpoint.endpoint,
@@ -45,3 +37,19 @@ describe("Authentication", () => {
     });
   });
 });
+
+describe("AppConfigurationClient constructor error cases", () => {
+  it("invalid connection string gives a decent error message", () => {
+    assert.throws(
+      () => new AppConfigurationClient("an invalid connection string"),
+      /Invalid connection string\. Valid connection strings should match the regex 'Endpoint=\(\.\*\);Id=\(\.\*\);Secret=\(\.\*\)'/
+    );
+  });
+
+  it("undefined connection string gives a decent error message", () => {
+    assert.throws(
+      () => new AppConfigurationClient(undefined as any),
+      /Invalid connection string\. Valid connection strings should match the regex 'Endpoint=\(\.\*\);Id=\(\.\*\);Secret=\(\.\*\)'/
+    );
+  });
+});