fix: build .mjs functions with esbuild

src/lib/functions/runtimes/js/builders/zisi.js

@@ -68,16 +68,18 @@ const getTargetDirectory = async ({ errorExit }) => {
 }
 
 module.exports = async ({ config, directory, errorExit, func, projectRoot }) => {
-  const isTSFunction = path.extname(func.mainFile) === '.ts'
   const functionsConfig = addFunctionsConfigDefaults(
     normalizeFunctionsConfig({ functionsConfig: config.functions, projectRoot }),
   )
 
+  // We must use esbuild for certain file extensions.
+  const mustUseEsbuild = ['.mjs', '.ts'].includes(path.extname(func.mainFile))
+
   // TODO: Resolve functions config globs so that we can check for the bundler
   // on a per-function basis.
   const isUsingEsbuild = functionsConfig['*'].nodeBundler === 'esbuild_zisi'
 
-  if (!isTSFunction && !isUsingEsbuild) {
+  if (!mustUseEsbuild && !isUsingEsbuild) {
     return false
   }
 

tests/serving-functions.test.js

@@ -624,6 +624,41 @@ export { handler }
       })
     })
   })
+
+  test(testName('Serves functions with a `.mjs` extension', args), async (t) => {
+    await withSiteBuilder('function-mjs', async (builder) => {
+      const bundlerConfig = args.includes('esbuild') ? { node_bundler: 'esbuild' } : {}
+
+      await builder
+        .withNetlifyToml({
+          config: {
+            build: { publish: 'public' },
+            functions: { directory: 'functions' },
+            ...bundlerConfig,
+          },
+        })
+        .withContentFile({
+          path: 'functions/hello.mjs',
+          content: `
+  const handler = async () => {
+    return {
+      statusCode: 200,
+      body: 'Hello, world!'
+    }
+  }
+
+  export { handler }
+            `,
+        })
+        .buildAsync()
+
+      await withDevServer({ cwd: builder.directory, args }, async ({ port, outputBuffer }) => {
+        await tryAndLogOutput(async () => {
+          t.is(await got(`http://localhost:${port}/.netlify/functions/hello`).text(), 'Hello, world!')
+        }, outputBuffer)
+      })
+    })
+  })
 })
 
 test('Serves functions that dynamically load files included in the `functions.included_files` config property', async (t) => {