add initializeFunction lifecycle hook

src/index.ts

@@ -18,7 +18,7 @@ import * as UserFunction from "./utils/UserFunction";
 
 LogPatch.patchConsole();
 
-export function run(appRoot: string, handler: string): void {
+export async function run(appRoot: string, handler: string): Promise<void> {
   if (!process.env.AWS_LAMBDA_RUNTIME_API) {
     throw new Error("Missing Runtime API Server configuration.");
   }
@@ -50,7 +50,10 @@ export function run(appRoot: string, handler: string): void {
   BeforeExitListener.reset();
   process.on("beforeExit", BeforeExitListener.invoke);
 
-  const handlerFunc = UserFunction.load(appRoot, handler) as HandlerFunction;
+  const handlerFunc = (await UserFunction.load(
+    appRoot,
+    handler
+  )) as HandlerFunction;
   const runtime = new Runtime(client, handlerFunc, errorCallbacks);
 
   runtime.scheduleIteration();

src/utils/UserFunction.ts

@@ -110,6 +110,20 @@ function _loadUserApp(
   }
 }
 
+async function _initializeFunction(userApp: any): Promise<void> {
+  try {
+     await userApp.initializeFunction();
+  } catch (e) {
+      if (e instanceof TypeError) {
+          // initializeFunction lifecycle hook not implemented
+          return;
+      }
+      else {
+          throw e;
+      }
+  }
+}
+
 function _throwIfInvalidHandler(fullHandlerString: string): void {
   if (fullHandlerString.includes(RELATIVE_PATH_SUBSTRING)) {
     throw new MalformedHandlerName(
@@ -137,10 +151,10 @@ function _throwIfInvalidHandler(fullHandlerString: string): void {
  *       for traversing up the filesystem '..')
  *   Errors for scenarios known by the runtime, will be wrapped by Runtime.* errors.
  */
-export const load = function (
+export const load = async function (
   appRoot: string,
   fullHandlerString: string
-): HandlerFunction {
+): Promise<HandlerFunction> {
   _throwIfInvalidHandler(fullHandlerString);
 
   const [moduleRoot, moduleAndHandler] = _moduleRootAndHandler(
@@ -149,6 +163,9 @@ export const load = function (
   const [module, handlerPath] = _splitHandlerString(moduleAndHandler);
 
   const userApp = _loadUserApp(appRoot, moduleRoot, module);
+
+  await _initializeFunction(userApp);
+
   const handlerFunc = _resolveHandler(userApp, handlerPath);
 
   if (!handlerFunc) {