fix(amplify): cannot map branch to domain root

packages/@aws-cdk/aws-amplify/README.md

@@ -86,6 +86,7 @@ amplifyApp.addCustomRule({
 Add a domain and map sub domains to branches:
 ```ts
 const domain = amplifyApp.addDomain('example.com');
+domain.mapRoot(master); // map master branch to domain root
 domain.mapSubDomain(master, 'www');
 domain.mapSubDomain(dev); // sub domain prefix defaults to branch name
 ```

packages/@aws-cdk/aws-amplify/lib/domain.ts

@@ -95,12 +95,22 @@ export class Domain extends Resource {
 
   /**
    * Maps a branch to a sub domain
+   *
+   * @param branch The branch
+   * @param prefix The prefix. Use '' to map to the root of the domain. Defaults to branch name.
    */
   public mapSubDomain(branch: IBranch, prefix?: string) {
     this.subDomains.push({ branch, prefix });
     return this;
   }
 
+  /**
+   * Maps a branch to the domain root
+   */
+  public mapRoot(branch: IBranch) {
+    return this.mapSubDomain(branch, '');
+  }
+
   protected validate() {
     if (this.subDomains.length === 0) {
       return ['The domain doesn\'t contain any subdomains'];
@@ -112,7 +122,7 @@ export class Domain extends Resource {
   private renderSubDomainSettings() {
     return this.subDomains.map(s => ({
       branchName: s.branch.branchName,
-      prefix: s.prefix || s.branch.branchName,
+      prefix: s.prefix === undefined ? s.branch.branchName : s.prefix,
     }));
   }
 }
@@ -127,7 +137,7 @@ export interface SubDomain {
   readonly branch: IBranch;
 
   /**
-   * The prefix
+   * The prefix. Use '' to map to the root of the domain
    *
    * @default - the branch name
    */

packages/@aws-cdk/aws-amplify/test/domain.test.ts

@@ -63,6 +63,45 @@ test('create a domain', () => {
   });
 });
 
+test('map a branch to the domain root', () => {
+  // GIVEN
+  const stack = new Stack();
+  const app = new amplify.App(stack, 'App', {
+    sourceCodeProvider: new amplify.GitHubSourceCodeProvider({
+      owner: 'aws',
+      repository: 'aws-cdk',
+      oauthToken: SecretValue.plainText('secret'),
+    }),
+  });
+  const prodBranch = app.addBranch('master');
+
+  // WHEN
+  const domain = app.addDomain('amazon.com');
+  domain.mapRoot(prodBranch);
+
+  // THEN
+  expect(stack).toHaveResource('AWS::Amplify::Domain', {
+    AppId: {
+      'Fn::GetAtt': [
+        'AppF1B96344',
+        'AppId',
+      ],
+    },
+    DomainName: 'amazon.com',
+    SubDomainSettings: [
+      {
+        BranchName: {
+          'Fn::GetAtt': [
+            'Appmaster71597E87',
+            'BranchName',
+          ],
+        },
+        Prefix: '',
+      },
+    ],
+  });
+});
+
 test('throws at synthesis without subdomains', () => {
   // GIVEN
   const app = new App();