feat: conform to handler function signature (#10)

* feat: conform to handler function signature

* chore: improve Promise detection

* chore: return original value from wrapped function if not Promise

package.json

@@ -49,7 +49,9 @@
   "directories": {
     "test": "test"
   },
-  "dependencies": {},
+  "dependencies": {
+    "is-promise": "^2.2.2"
+  },
   "devDependencies": {
     "@commitlint/cli": "^11.0.0",
     "@commitlint/config-conventional": "^11.0.0",

src/lib/builder_functions.js

@@ -1,3 +1,5 @@
+const isPromise = require('is-promise')
+
 const { HTTP_STATUS_METHOD_NOT_ALLOWED, HTTP_STATUS_OK } = require('./consts')
 
 const augmentResponse = (response) => {
@@ -12,19 +14,24 @@ const augmentResponse = (response) => {
 }
 
 // eslint-disable-next-line promise/prefer-await-to-callbacks
-const wrapHandler = (handler) => async (event, context, callback) => {
+const wrapHandler = (handler) => (event, context, callback) => {
   if (event.httpMethod !== 'GET' && event.httpMethod !== 'HEAD') {
-    return {
+    return Promise.resolve({
       body: 'Method Not Allowed',
       statusCode: HTTP_STATUS_METHOD_NOT_ALLOWED,
-    }
+    })
   }
 
   // eslint-disable-next-line promise/prefer-await-to-callbacks
   const wrappedCallback = (error, response) => callback(error, augmentResponse(response))
-  const response = await handler(event, context, wrappedCallback)
+  const execution = handler(event, context, wrappedCallback)
+
+  if (isPromise(execution)) {
+    // eslint-disable-next-line promise/prefer-await-to-then
+    return execution.then(augmentResponse)
+  }
 
-  return augmentResponse(response)
+  return execution
 }
 
 module.exports = { builderFunction: wrapHandler }

test/builder_functions.js

@@ -132,3 +132,21 @@ test('Returns a 405 error for requests using the PATCH method', async (t) => {
 
   t.deepEqual(response, { body: 'Method Not Allowed', statusCode: 405 })
 })
+
+test('Preserves errors thrown inside the wrapped handler', async (t) => {
+  const error = new Error('Uh-oh!')
+
+  error.someProperty = ':thumbsdown:'
+
+  const myHandler = async () => {
+    const asyncTask = new Promise((resolve) => {
+      setTimeout(resolve, 0)
+    })
+
+    await asyncTask
+
+    throw error
+  }
+
+  await t.throwsAsync(invokeLambda(builderFunction(myHandler)), { is: error })
+})