Code refactor as per peer review.

Author: yu-tian113

scripts/rollup/packaging.js

@@ -168,39 +168,44 @@ function copyNodePackageTemplate(packageName) {
   }
   fs.mkdirSync(to);
   const packageJson = resolve(`${from}/package.json`);
-  const whitelistedFiles = fs.existsSync(packageJson)
-    ? require(packageJson).files
-    : null;
+  const whitelistedFiles = (fs.existsSync(packageJson) &&
+    require(packageJson).files) || [];
   const npmRootFiles = fs.readdirSync(npmFrom);
   // Get all entry points in the package root
   // Exceptions: *.fb.js
   const packageRootEntries = glob.sync('!(*.fb).js', {
     cwd: from,
   });
   packageRootEntries.forEach(entry => {
-    // Terminate the build if any entry point in the package root does not have an equivalent in ./npm.
+    // Terminate the build if any entry point in the package root
+    // does not have an equivalent in ./npm.
     if (!npmRootFiles.includes(entry)) {
       console.error(
         `Entry point ${entry} in package ${packageName} does not have an equivalent in ./npm`
       );
       process.exit(1);
     }
   });
-  if (whitelistedFiles && whitelistedFiles.length > 0) {
-    let asyncCopyProxies = [];
-    asyncCopyProxies = npmRootFiles.reduce((proxies, file) => {
-      if (
-        whitelistedFiles.includes(file) ||
-        whitelistedFiles.includes(`${file}/`)
-      ) {
-        proxies.push(
-          asyncCopyTo(resolve(`${npmFrom}/${file}`), `${to}/${file}`)
-        );
-      }
-      return proxies;
-    }, asyncCopyProxies);
+  if (whitelistedFiles.length > 0) {
+    // Looping through files and folders in ./npm root,
+    // if whitelisted in package.json, copy to build package root,
+    // return an array of the promises generated by async copy.
+    const promisesForForwardingModules = npmRootFiles.reduce(
+      (promises, file) => {
+        if (
+          whitelistedFiles.includes(file) ||
+          whitelistedFiles.includes(`${file}/`)
+        ) {
+          promises.push(
+            asyncCopyTo(resolve(`${npmFrom}/${file}`), `${to}/${file}`)
+          );
+        }
+        return promises;
+      },
+      []
+    );
     return Promise.all([
-      ...asyncCopyProxies,
+      ...promisesForForwardingModules,
       asyncCopyTo(resolve(`${from}/package.json`), `${to}/package.json`),
       asyncCopyTo(resolve(`${from}/README.md`), `${to}/README.md`),
       asyncCopyTo(resolve('./LICENSE'), `${to}/LICENSE`),