[Identity] DefaultAzureCredential should properly handle the managedl…

…dentityClientId optional parameter (#13973)

While checking if the `DefaultAzureCredential` had issues while it's used on the Azure Functions, I realized we weren't handling the `managedIdentityClientId` optional parameter properly as well as we we're currently handling the `process.env.AZURE_CLIENT_ID`.

This pull request fixes this.

Related to: #13872

sdk/identity/identity/CHANGELOG.md

@@ -4,10 +4,11 @@
 
 ## 1.2.4-beta.2 (Unreleased)
 
-- Replaced the use of the 'express' module with a Node-native http server, shrinking the resulting identity module considerably
+- Bug fix: Now if the `managedIdentityClientId` optional parameter is provided to `DefaultAzureCredential`, it will be properly passed through to the underlying `ManagedIdentityCredential`. Related to customer issue: [13973](https://github.com/Azure/azure-sdk-for-js/pull/13973).
 - `DefaultAzureCredential`'s implementation for browsers was simplified to throw a simple error instead of trying credentials that were already not supported for the browser.
 - Breaking Change: `InteractiveBrowserCredential` for the browser now requires the client ID to be provided.
 - Documentation was added to elaborate on how to configure an AAD application to support `InteractiveBrowserCredential`.
+- Replaced the use of the 'express' module with a Node-native http server, shrinking the resulting identity module considerably
 - Bug fix: `ManagedIdentityCredential` now also properly handles `EHOSTUNREACH` errors. Fixes issue [13894](https://github.com/Azure/azure-sdk-for-js/issues/13894).
 
 ## 1.2.4-beta.1 (2021-02-12)

sdk/identity/identity/src/credentials/defaultAzureCredential.ts

@@ -42,15 +42,20 @@ export class DefaultAzureCredential extends ChainedTokenCredential {
   constructor(tokenCredentialOptions?: DefaultAzureCredentialOptions) {
     const credentials = [];
     credentials.push(new EnvironmentCredential(tokenCredentialOptions));
-    credentials.push(new ManagedIdentityCredential(tokenCredentialOptions));
-    if (process.env.AZURE_CLIENT_ID) {
+
+    // In case a user assigned ID has been provided.
+    const managedIdentityClientId =
+      tokenCredentialOptions?.managedIdentityClientId || process.env.AZURE_CLIENT_ID;
+
+    if (managedIdentityClientId) {
       credentials.push(
-        new ManagedIdentityCredential(
-          tokenCredentialOptions?.managedIdentityClientId || process.env.AZURE_CLIENT_ID,
-          tokenCredentialOptions
-        )
+        new ManagedIdentityCredential(managedIdentityClientId, tokenCredentialOptions)
       );
+    } else {
+      // If the user didn't provide an ID, we'll try with a system assigned ID.
+      credentials.push(new ManagedIdentityCredential(tokenCredentialOptions));
     }
+
     credentials.push(new AzureCliCredential());
     credentials.push(new VisualStudioCodeCredential(tokenCredentialOptions));