Netlify Function Handler: Use promise rather than callback

Use the promise approach of next-aws-lambda rather than the callback
approach, because it makes the code easier to read and puts it in the
correct order of execution.

Author: FinnWoelm

lib/templates/netlifyFunction.js

@@ -6,26 +6,7 @@ const compat = require("./compat");
 // Load the NextJS page
 const page = require("./nextJsPage");
 
-// next-aws-lambda is made for AWS. There are some minor differences between
-// Netlify and AWS which we resolve here.
-const callbackHandler = (callback) =>
-  // The callbackHandler wraps the callback
-  (argument, response) => {
-    // Convert header values to string. Netlify does not support integers as
-    // header values. See: https://github.com/netlify/cli/issues/451
-    Object.keys(response.multiValueHeaders).forEach((key) => {
-      response.multiValueHeaders[key] = response.multiValueHeaders[
-        key
-      ].map((value) => String(value));
-    });
-
-    response.multiValueHeaders["Cache-Control"] = ["no-cache"];
-
-    // Invoke callback
-    callback(argument, response);
-  };
-
-exports.handler = (event, context, callback) => {
+exports.handler = async (event, context, callback) => {
   // x-forwarded-host is undefined on Netlify for proxied apps that need it
   // fixes https://github.com/netlify/next-on-netlify/issues/46
   if (!event.multiValueHeaders.hasOwnProperty("x-forwarded-host")) {
@@ -37,15 +18,24 @@ exports.handler = (event, context, callback) => {
   console.log("[request]", path);
 
   // Render the page
-  compat(page)(
+  const response = await compat(page)(
     {
       ...event,
       // Required. Otherwise, compat() will complain
       requestContext: {},
     },
-    context,
-    // Wrap the Netlify callback, so that we can resolve differences between
-    // Netlify and AWS (which next-aws-lambda optimizes for)
-    callbackHandler(callback)
+    context
   );
+
+  // Convert header values to string. Netlify does not support integers as
+  // header values. See: https://github.com/netlify/cli/issues/451
+  Object.keys(response.multiValueHeaders).forEach((key) => {
+    response.multiValueHeaders[key] = response.multiValueHeaders[
+      key
+    ].map((value) => String(value));
+  });
+
+  response.multiValueHeaders["Cache-Control"] = ["no-cache"];
+
+  callback(null, response);
 };