fix(@angular/build): allow a default application browser option

The application build system's `browser` option is now considered optional.
If not present, the value will be an `main.ts` file within the configured
project source root (`sourceRoot`). This change allows the removal of the
`browser` option from any project that uses the default generated value.

Author: clydin

packages/angular/build/src/builders/application/options.ts

@@ -177,7 +177,12 @@ export async function normalizeOptions(
     i18nOptions.flatOutput = true;
   }
 
-  const entryPoints = normalizeEntryPoints(workspaceRoot, options.browser, options.entryPoints);
+  const entryPoints = normalizeEntryPoints(
+    workspaceRoot,
+    projectSourceRoot,
+    options.browser,
+    options.entryPoints,
+  );
   const tsconfig = path.join(workspaceRoot, options.tsConfig);
   const optimizationOptions = normalizeOptimization(options.optimization);
   const sourcemapOptions = normalizeSourceMaps(options.sourceMap ?? false);
@@ -545,26 +550,25 @@ async function getTailwindConfig(
  */
 function normalizeEntryPoints(
   workspaceRoot: string,
+  projectSourceRoot: string,
   browser: string | undefined,
-  entryPoints: Set<string> | Map<string, string> = new Set(),
+  entryPoints: Set<string> | Map<string, string> | undefined,
 ): Record<string, string> {
   if (browser === '') {
     throw new Error('`browser` option cannot be an empty string.');
   }
 
   // `browser` and `entryPoints` are mutually exclusive.
-  if (browser && entryPoints.size > 0) {
+  if (browser && entryPoints) {
     throw new Error('Only one of `browser` or `entryPoints` may be provided.');
   }
-  if (!browser && entryPoints.size === 0) {
-    // Schema should normally reject this case, but programmatic usages of the builder might make this mistake.
-    throw new Error('Either `browser` or at least one `entryPoints` value must be provided.');
-  }
 
-  // Schema types force `browser` to always be provided, but it may be omitted when the builder is invoked programmatically.
   if (browser) {
     // Use `browser` alone.
     return { 'main': path.join(workspaceRoot, browser) };
+  } else if (!entryPoints) {
+    // Default browser entry if no explicit entry points
+    return { 'main': path.join(projectSourceRoot, 'main.ts') };
   } else if (entryPoints instanceof Map) {
     return Object.fromEntries(
       Array.from(entryPoints.entries(), ([name, entryPoint]) => {

packages/angular/build/src/builders/application/schema.json

@@ -616,7 +616,7 @@
     }
   },
   "additionalProperties": false,
-  "required": ["browser", "tsConfig"],
+  "required": ["tsConfig"],
   "definitions": {
     "assetPattern": {
       "oneOf": [

packages/angular/build/src/builders/application/tests/options/browser_spec.ts

@@ -42,6 +42,49 @@ describeBuilder(buildApplication, APPLICATION_BUILDER_INFO, (harness) => {
       harness.expectFile('dist/browser/main.js').content.toContain('console.log("main")');
     });
 
+    it('defaults to use `src/main.ts` if not present', async () => {
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        browser: undefined,
+      });
+
+      const { result } = await harness.executeOnce();
+
+      expect(result?.success).toBe(true);
+
+      harness.expectFile('dist/browser/main.js').toExist();
+      harness.expectFile('dist/browser/index.html').toExist();
+    });
+
+    it('uses project source root in default if `browser` not present', async () => {
+      harness.useProject('test', {
+        root: '.',
+        sourceRoot: 'source',
+        cli: {
+          cache: {
+            enabled: false,
+          },
+        },
+      });
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        browser: undefined,
+        index: false,
+      });
+
+      // Update app for a `source/main.ts` file based on the above changed `sourceRoot`
+      await harness.writeFile('source/main.ts', `console.log('main');`);
+      await harness.modifyFile('src/tsconfig.app.json', (content) =>
+        content.replace('main.ts', '../source/main.ts'),
+      );
+
+      const { result } = await harness.executeOnce();
+
+      expect(result?.success).toBe(true);
+
+      harness.expectFile('dist/browser/main.js').content.toContain('console.log("main")');
+    });
+
     it('fails and shows an error when file does not exist', async () => {
       harness.useTarget('build', {
         ...BASE_OPTIONS,