feat(vercel): ESM support and Node.js 16

.changeset/breezy-grapes-smash.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+You can now select a Node.js version for your serverless functions

.changeset/tall-ties-juggle.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+The output of serverless now is ESM instead of CJS

packages/adapter-vercel/index.d.ts

@@ -4,6 +4,7 @@ type Options = {
 	edge?: boolean;
 	external?: string[];
 	split?: boolean;
+	nodeVersion?: '16.x' | '14.x' | '12.x';
 };
 
 declare function plugin(options?: Options): Adapter;

packages/adapter-vercel/index.js

@@ -81,16 +81,24 @@ const redirects = {
 };
 
 /** @type {import('.')} **/
-export default function ({ external = [], edge, split } = {}) {
+export default function ({ external = [], edge, split, nodeVersion } = {}) {
 	return {
 		name: '@sveltejs/adapter-vercel',
 
 		async adapt(builder) {
 			if (process.env.ENABLE_VC_BUILD) {
-				await v3(builder, external, edge, split);
+				if (edge && nodeVersion) {
+					throw new Error(
+						'`nodeVersion` option can only be used with serverless functions, not edge functions'
+					);
+				}
+
+				await v3(builder, external, edge, split, nodeVersion || '16.x');
 			} else {
-				if (edge || split) {
-					throw new Error('edge and split options can only be used with ENABLE_VC_BUILD');
+				if (edge || split || nodeVersion) {
+					throw new Error(
+						'`edge`, `split` and `nodeVersion` options can only be used with ENABLE_VC_BUILD'
+					);
 				}
 
 				await v1(builder, external);
@@ -198,8 +206,9 @@ async function v1(builder, external) {
  * @param {string[]} external
  * @param {boolean} edge
  * @param {boolean} split
+ * @param {'16.x' | '14.x' | '12.x'} nodeVersion
  */
-async function v3(builder, external, edge, split) {
+async function v3(builder, external, edge, split, nodeVersion) {
 	const dir = '.vercel/output';
 
 	const tmp = builder.getBuildDirectory('vercel-tmp');
@@ -263,23 +272,24 @@ async function v3(builder, external, edge, split) {
 		await esbuild.build({
 			entryPoints: [`${tmp}/serverless.js`],
 			outfile: `${dirs.functions}/${name}.func/index.js`,
-			target: 'node14',
+			target:
+				nodeVersion === '12.x' ? 'node12.22' : nodeVersion === '14.x' ? 'node14.19' : 'node16.15',
 			bundle: true,
 			platform: 'node',
-			format: 'cjs',
+			format: 'esm',
 			external
 		});
 
 		write(
 			`${dirs.functions}/${name}.func/.vc-config.json`,
 			JSON.stringify({
-				runtime: 'nodejs14.x',
+				runtime: `nodejs${nodeVersion}`,
 				handler: 'index.js',
 				launcherType: 'Nodejs'
 			})
 		);
 
-		write(`${dirs.functions}/${name}.func/package.json`, JSON.stringify({ type: 'commonjs' }));
+		write(`${dirs.functions}/${name}.func/package.json`, JSON.stringify({ type: 'module' }));
 
 		routes.push({ src: pattern, dest: `/${name}` });
 	}