fix: module resolution returns early and accurate error messages on failure (#534)

Author: garethgeorge

src/loader.ts

@@ -93,7 +93,11 @@ export async function getUserFunction(
   try {
     const functionModulePath = getFunctionModulePath(codeLocation);
     if (functionModulePath === null) {
-      console.error('Provided code is not a loadable module.');
+      console.error(
+        `Provided code location '${codeLocation}' is not a loadable module.` +
+          '\nDid you specify the correct location for the module defining ' +
+          'your function?'
+      );
       return null;
     }
 
@@ -174,16 +178,17 @@ export async function getUserFunction(
  * @return Resolved path or null.
  */
 function getFunctionModulePath(codeLocation: string): string | null {
-  let path: string | null = null;
   try {
-    path = require.resolve(codeLocation);
+    return require.resolve(codeLocation);
   } catch (ex) {
-    try {
-      // TODO: Decide if we want to keep this fallback.
-      path = require.resolve(codeLocation + '/function.js');
-    } catch (ex) {
-      return path;
-    }
+    // Ignore exception, this means the function was not found here.
+  }
+
+  try {
+    return require.resolve(codeLocation + '/function.js');
+  } catch (ex) {
+    // Ignore exception, this means the function was not found here.
   }
-  return path;
+
+  return null;
 }

test/loader.ts

@@ -93,11 +93,17 @@ describe('loading function', () => {
     }
   }
 
-  it('fails to load a function that does not exist', async () => {
-    FunctionRegistry.http('function', () => {
-      return 'PASS';
-    });
+  it('fails to load a module that does not exist', async () => {
+    const loadedFunction = await loader.getUserFunction(
+      process.cwd() + '/test/data/does_not_exist',
+      'functionDoesNotExist',
+      'http'
+    );
 
+    assert.strictEqual(loadedFunction, null);
+  });
+
+  it('fails to load a function that does not exist', async () => {
     const loadedFunction = await loader.getUserFunction(
       process.cwd() + '/test/data/with_main',
       'functionDoesNotExist',