refactor(@angular/build): add baseline_browserslist tool

This tool uses `baseline-browser-mapping` to generate a `browserslist` configuration at build time from a given widely-available Baseline date.

I opted _not_ to use "downstream browsers". This refers to browsers like Opera, which technically uses Blink and shares the same featureset as Chrome for a particular version. I decided against this to maintain a stricter interpretation of Baseline, given that those browsers are not included in Baseline today. Developers can manually widen their own `.browserslistrc` if they really want to and are comfortable accepting the risks that brings.

Using `baseline-browser-mapping` as part of the build process means there is a potential risk that a bugfix in the package generates a different browserslist file which leads to a different build configuration that causes a problem for existing users. However, it's also just as likely (if not moreso) to fix a problem than cause one, so I'm inclined to call that WAI. If it becomes an issue in the future, we can potentially check in the generated `.browserslistrc` file itself rather than the Baseline date, meaning the list of browsers would be frozen until we explicitly update it between majors.

Author: dgp1130

packages/angular/build/package.json

@@ -49,17 +49,18 @@
     "lmdb": "3.2.6"
   },
   "devDependencies": {
-    "@angular/ssr": "workspace:*",
     "@angular-devkit/core": "workspace:*",
+    "@angular/ssr": "workspace:*",
+    "baseline-browser-mapping": "^2.2.0",
     "less": "4.3.0",
     "ng-packagr": "20.0.0-next.6",
     "postcss": "8.5.3",
     "rxjs": "7.8.2"
   },
   "peerDependencies": {
-    "@angular/core": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/compiler": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/compiler-cli": "0.0.0-ANGULAR-FW-PEER-DEP",
+    "@angular/core": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/localize": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/platform-browser": "0.0.0-ANGULAR-FW-PEER-DEP",
     "@angular/platform-server": "0.0.0-ANGULAR-FW-PEER-DEP",

packages/angular/build/tools/baseline_browserslist/BUILD.bazel

@@ -0,0 +1,53 @@
+load("@aspect_rules_js//js:defs.bzl", "js_binary")
+load("@aspect_rules_ts//ts:defs.bzl", "ts_config")
+load("//tools:defaults2.bzl", "jasmine_test", "ts_project")
+
+js_binary(
+    name = "baseline_browserslist",
+    data = [":baseline_browserslist_lib"],
+    entry_point = "index.mjs",
+    visibility = ["//packages/angular/build:__subpackages__"],
+)
+
+ts_project(
+    name = "baseline_browserslist_lib",
+    srcs = ["index.mts"],
+    source_map = True,
+    tsconfig = ":tsconfig",
+    deps = [":generate_browserslist"],
+)
+
+ts_project(
+    name = "generate_browserslist",
+    srcs = ["generate_browserslist.mts"],
+    source_map = True,
+    tsconfig = ":tsconfig",
+    deps = [
+        "//packages/angular/build:node_modules/baseline-browser-mapping",
+    ],
+)
+
+ts_project(
+    name = "generate_browserslist_test_lib",
+    testonly = True,
+    srcs = ["generate_browserslist_spec.mts"],
+    tsconfig = "//:test-tsconfig-esm",
+    deps = [
+        ":generate_browserslist",
+        "//:node_modules/@types/jasmine",
+    ],
+)
+
+jasmine_test(
+    name = "generate_browserslist_test",
+    data = [":generate_browserslist_test_lib"],
+)
+
+ts_config(
+    name = "tsconfig",
+    src = "tsconfig-build.json",
+    deps = [
+        "//:build-tsconfig-esm",
+        "//:node_modules/@types/node",
+    ],
+)

packages/angular/build/tools/baseline_browserslist/baseline_browserslist.bzl

@@ -0,0 +1,26 @@
+"""Generates a `browserslist` configuration from a Baseline date."""
+
+load("@aspect_rules_js//js:defs.bzl", "js_run_binary")
+
+def baseline_browserslist(name, baseline, out, **kwargs):
+    """Generates a `browserslist` configuration from a Baseline date.
+
+    Args:
+        name: Name of this target.
+        baseline: A string date in "YYYY-MM-DD" format of the Baseline widely
+            available browser set to use in the generated `browserslist`.
+        out: Name of the output browserslist file. Prefer using `.browserslistrc`
+            for the output as the `browserslist` package seems to not like files
+            with a different name, even when explicitly provided.
+
+    See: https://web.dev/baseline
+    """
+
+    js_run_binary(
+        name = name,
+        srcs = [],
+        tool = Label(":baseline_browserslist"),
+        stdout = out,
+        args = [baseline],
+        **kwargs
+    )

packages/angular/build/tools/baseline_browserslist/generate_browserslist.mts

@@ -0,0 +1,41 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { getCompatibleVersions } from 'baseline-browser-mapping';
+
+// Map `baseline-browser-mapping` browsers to `browserslist` browsers.
+const browsers = new Map(
+  Object.entries({
+    chrome: 'Chrome',
+    chrome_android: 'ChromeAndroid',
+    edge: 'Edge',
+    firefox: 'Firefox',
+    firefox_android: 'FirefoxAndroid',
+    safari: 'Safari',
+    safari_ios: 'iOS',
+  }),
+);
+
+/**
+ * Generates the `browserslist` configuration for the given Baseline date.
+ *
+ * @param date The Baseline "widely available" date to generate a `browserslist`
+ *     configuration for. Uses `YYYY-MM-DD` format.
+ * @returns The `browserslist` configuration file content.
+ */
+export function generateBrowserslist(date: string): string {
+  // Generate a `browserslist` configuration.
+  return getCompatibleVersions({
+    widelyAvailableOnDate: date,
+    includeDownstreamBrowsers: false,
+  })
+    .map(({ browser, version }) => ({ browser: browsers.get(browser), version }))
+    .filter(({ browser }) => Boolean(browser))
+    .map(({ browser, version }) => `${browser} >= ${version}`)
+    .join('\n');
+}

packages/angular/build/tools/baseline_browserslist/generate_browserslist_spec.mts

@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { generateBrowserslist } from './generate_browserslist.mjs';
+
+describe('generate_browserslist', () => {
+  describe('generateBrowserslist', () => {
+    it('generates a `browserslist` file', () => {
+      expect(generateBrowserslist('2025-03-31').trim()).toBe(
+        `
+Chrome >= 107
+ChromeAndroid >= 107
+Edge >= 107
+Firefox >= 104
+FirefoxAndroid >= 104
+Safari >= 16
+iOS >= 16
+      `.trim(),
+      );
+    });
+  });
+});

packages/angular/build/tools/baseline_browserslist/index.mts

@@ -0,0 +1,15 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { generateBrowserslist } from './generate_browserslist.mjs';
+
+const [baselineDate] = process.argv.slice(2);
+const browserslist = generateBrowserslist(baselineDate);
+
+// eslint-disable-next-line no-console
+console.log(browserslist);

packages/angular/build/tools/baseline_browserslist/tsconfig-build.json

@@ -0,0 +1,6 @@
+{
+  "extends": "../../../../../tsconfig-build-esm.json",
+  "compilerOptions": {
+    "types": ["node"]
+  }
+}