fix(api-server): lambdaLoader: improve handler import logic...

For ESModule and CommonJS compatibility.
When using a bundler like Rollup, the handler function is exported as default.

Author: rj-au

packages/api-server/src/plugins/lambdaLoader.ts

@@ -28,7 +28,26 @@ export const setLambdaFunctions = async (foundFunctions: string[]) => {
     const ts = Date.now()
     const routeName = path.basename(fnPath).replace('.js', '')
 
-    const { handler } = await import(`file://${fnPath}`)
+    const fnImport = await import(`file://${fnPath}`)
+    const handler: Handler = (() => {
+      if ('handler' in fnImport) {
+        // ESModule export of handler - when using `export const handler = ...` - most common case
+        return fnImport.handler
+      }
+      if ('default' in fnImport) {
+        if ('handler' in fnImport.default) {
+          // CommonJS export of handler - when using `module.exports.handler = ...` or `export default { handler: ... }`
+          // This is less common, but required for bundling tools that export a default object, like esbuild or rollup
+          return fnImport.default.handler
+        }
+        // ESModule export of default function - fallback
+        return fnImport.default
+      }
+      throw new Error(
+        `Function "${routeName}" at "${fnPath}" does not export a "handler" function.`,
+      )
+    })()
+
     LAMBDA_FUNCTIONS[routeName] = handler
     if (!handler) {
       console.warn(