Modernize Babel build tooling and share preset config

.chronus/changes/feature-babel-preset-types-2026-7-21.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@alloy-js/babel-preset"
+---
+
+`@alloy-js/babel-preset` now ships with TypeScript type declarations, so consumers can import the preset without `@ts-expect-error`.

.chronus/changes/feature-rollup-plugin-api-2026-7-21.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@alloy-js/rollup-plugin"
+---
+
+The Rollup/Vite plugin now accepts `addSourceInfo` and `sourceMaps` options and automatically configures esbuild for Alloy (`jsx: "preserve"` plus the `source` resolve conditions). Consumers no longer need to set these by hand in their Vite/Vitest config.

.chronus/changes/internal-share-babel-config-2026-7-21.md

@@ -0,0 +1,17 @@
+---
+changeKind: internal
+packages:
+  - "@alloy-js/cli"
+  - "@alloy-js/core"
+  - "@alloy-js/csharp"
+  - "@alloy-js/go"
+  - "@alloy-js/java"
+  - "@alloy-js/json"
+  - "@alloy-js/markdown"
+  - "@alloy-js/msbuild"
+  - "@alloy-js/python"
+  - "@alloy-js/typescript"
+  - "@alloy-js/typespec"
+---
+
+Rely on the Rollup/Vite plugin to configure esbuild, removing the duplicated esbuild/resolve boilerplate from each package's `vitest.config.ts`.

packages/babel-preset-alloy/package.json

@@ -1,21 +1,52 @@
 {
   "name": "@alloy-js/babel-preset",
   "version": "0.3.0",
-  "description": "",
+  "type": "module",
+  "description": "Babel preset for Alloy's JSX/TSX transformation",
+  "homepage": "https://github.com/alloy-framework/alloy",
+  "license": "MIT",
+  "author": "Microsoft",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "source": "./src/index.ts",
+      "default": "./dist/index.js"
+    }
+  },
+  "publishConfig": {
+    "exports": {
+      ".": {
+        "types": "./dist/index.d.ts",
+        "default": "./dist/index.js"
+      }
+    }
+  },
+  "files": [
+    "dist"
+  ],
   "repository": {
     "type": "git",
     "url": "git+https://github.com/alloy-framework/alloy.git"
   },
-  "main": "index.js",
+  "engines": {
+    "node": ">=22.0.0"
+  },
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "watch": "tsc -p ./tsconfig.build.json --watch",
+    "build": "tsc -p ./tsconfig.build.json",
+    "clean": "rimraf dist/ .temp/"
   },
   "dependencies": {
     "@alloy-js/babel-plugin": "workspace:~",
     "@alloy-js/babel-plugin-jsx-dom-expressions": "workspace:~"
   },
-  "keywords": [],
-  "author": "",
-  "license": "MIT",
-  "type": "module"
+  "devDependencies": {
+    "@babel/core": "catalog:",
+    "@types/node": "catalog:",
+    "rimraf": "catalog:",
+    "typescript": "catalog:"
+  },
+  "bugs": "https://github.com/alloy-framework/alloy/issues"
 }

packages/babel-preset-alloy/src/index.ts

@@ -0,0 +1,73 @@
+import alloyTransform from "@alloy-js/babel-plugin";
+import jsxTransform from "@alloy-js/babel-plugin-jsx-dom-expressions";
+import type { PluginItem } from "@babel/core";
+
+/** Options accepted by the Alloy Babel preset. */
+export interface AlloyPresetOptions {
+  /** Module name providing the Alloy runtime. @default "@alloy-js/core" */
+  alloyModuleName?: string;
+  /** Module name providing the JSX runtime. @default "@alloy-js/core/jsx-runtime" */
+  moduleName?: string;
+  /** Code generation mode passed to the JSX transform. @default "universal" */
+  generate?: string;
+  /** Whether to wrap conditional expressions. @default true */
+  wrapConditionals?: boolean;
+  /** Preserve JSX whitespace. @default true */
+  preserveWhitespace?: boolean;
+  /** Opt into the legacy whitespace handling (not recommended). */
+  legacyWhitespace?: boolean;
+  /**
+   * Emit source location information for components. When omitted, this is
+   * inferred from `dev` or from `BABEL_ENV`/`NODE_ENV`.
+   */
+  addSourceInfo?: boolean;
+  /** Whether this is a development build. Used to infer `addSourceInfo`. */
+  dev?: boolean;
+}
+
+/**
+ * Alloy Babel preset. Transforms Alloy's JSX syntax into calls against the
+ * Alloy runtime.
+ */
+export default function alloyPreset(
+  _context: unknown,
+  options: AlloyPresetOptions = {},
+): { plugins: PluginItem[] } {
+  const envMode = process.env.BABEL_ENV ?? process.env.NODE_ENV;
+  const inferredDev = envMode === undefined ? true : envMode !== "production";
+  const defaultOptions = {
+    alloyModuleName: "@alloy-js/core",
+    moduleName: "@alloy-js/core/jsx-runtime",
+    generate: "universal",
+    wrapConditionals: true,
+    preserveWhitespace: true,
+  };
+
+  const jsxOptions = {
+    ...defaultOptions,
+    ...options,
+  } as Record<string, unknown>;
+  if (options.addSourceInfo === undefined) {
+    if (options.dev !== undefined) {
+      jsxOptions.addSourceInfo = options.dev;
+    } else {
+      jsxOptions.addSourceInfo = inferredDev;
+    }
+  }
+
+  // `@babel/core`'s `PluginItem` type does not model the Alloy plugins' custom
+  // option objects, so assert comparability here.
+  const plugins = [
+    [
+      alloyTransform,
+      {
+        alloyModuleName:
+          options.alloyModuleName ?? defaultOptions.alloyModuleName,
+        legacyWhitespace: options.legacyWhitespace,
+      },
+    ],
+    [jsxTransform, jsxOptions],
+  ] as PluginItem[];
+
+  return { plugins };
+}

packages/babel-preset-alloy/src/vendored-plugins.d.ts

@@ -0,0 +1,7 @@
+// `@alloy-js/babel-plugin-jsx-dom-expressions` ships as plain JS without type
+// declarations, so provide a minimal ambient declaration for it here.
+declare module "@alloy-js/babel-plugin-jsx-dom-expressions" {
+  import type { PluginObj } from "@babel/core";
+  const plugin: (...args: unknown[]) => PluginObj;
+  export default plugin;
+}

packages/babel-preset-alloy/tsconfig.build.json

@@ -0,0 +1,10 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "dist"
+  },
+  "references": [],
+  "include": ["src"],
+  "exclude": ["**/*.test.*", "test/**/*"]
+}

packages/babel-preset-alloy/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "declaration": true,
+    "outDir": "dist"
+  },
+  "include": ["src/**/*.ts", "src/**/*.d.ts"],
+  "exclude": ["node_modules", "dist"]
+}

packages/cli/src/babel.ts

@@ -1,7 +1,5 @@
 import { mkdir, writeFile } from "node:fs/promises";
 
-// eslint-disable-next-line @typescript-eslint/ban-ts-comment
-// @ts-expect-error
 import alloyPreset from "@alloy-js/babel-preset";
 import * as babel from "@babel/core";
 import typescriptPreset from "@babel/preset-typescript";

packages/core/test/babel-e2e.test.ts

@@ -10,7 +10,6 @@
  *    intrinsics returning AlloyNodes eagerly).
  */
 
-// @ts-expect-error — preset has no types
 import alloyPreset from "@alloy-js/babel-preset";
 import { transformSync } from "@babel/core";
 import typescriptPreset from "@babel/preset-typescript";

packages/core/test/output-e2e.test.ts

@@ -10,7 +10,6 @@
  *  - `For` works under the runtime emitting multiple files.
  */
 
-// @ts-expect-error — preset has no types
 import alloyPreset from "@alloy-js/babel-preset";
 import { transformSync } from "@babel/core";
 import typescriptPreset from "@babel/preset-typescript";

packages/core/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],

packages/csharp/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],

packages/go/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],

packages/java/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],

packages/json/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],

packages/markdown/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],

packages/msbuild/vitest.config.ts

@@ -2,18 +2,6 @@ import alloyPlugin from "@alloy-js/rollup-plugin";
 import { defineConfig } from "vitest/config";
 
 export default defineConfig({
-  resolve: {
-    conditions: ["source"],
-  },
-  ssr: {
-    resolve: {
-      conditions: ["source"],
-    },
-  },
-  esbuild: {
-    jsx: "preserve",
-    sourcemap: "both",
-  },
   test: {
     exclude: ["**/dist/**", "**/node_modules/**"],
     setupFiles: ["./test/vitest.setup.ts"],