diff --git a/scripts/build.ts b/scripts/build.ts
index 3fd596c8a..1e5630d46 100644
--- a/scripts/build.ts
+++ b/scripts/build.ts
@@ -20,6 +20,7 @@ import {
   logError,
   cleanDistFolder,
   parseArgs,
+  flatten,
 } from "./utils";
 import { ScriptOpts, NormalizedOpts } from "./types";
 import * as fs from "fs-extra";
@@ -267,10 +268,10 @@ function createConfigItems(type: any, items: any[]) {
 function mergeConfigItems(type: any, ...configItemsToMerge: any[]) {
   const mergedItems: any[] = [];
 
-  configItemsToMerge.forEach(configItemToMerge => {
+  configItemsToMerge.forEach((configItemToMerge) => {
     configItemToMerge.forEach((item: any) => {
       const itemToMergeWithIndex = mergedItems.findIndex(
-        mergedItem => mergedItem.file.resolved === item.file.resolved
+        (mergedItem) => mergedItem.file.resolved === item.file.resolved
       );
 
       if (itemToMergeWithIndex === -1) {
@@ -307,13 +308,17 @@ if (process.env.NODE_ENV === 'production') {
 export async function createBuildConfigs(
   opts: NormalizedOpts
 ): Promise<RollupOptions[]> {
-  const allInputs = opts.input.flatMap((input: string) =>
-    createAllFormats(opts, input).map((options: ScriptOpts, index: number) => ({
-      ...options,
-      // We want to know if this is the first run for each entryfile
-      // for certain plugins (e.g. css)
-      writeMeta: index === 0,
-    }))
+  const allInputs = flatten(
+    flatten(opts.input as any).map((input: string) =>
+      createAllFormats(opts, input).map(
+        (options: ScriptOpts, index: number) => ({
+          ...options,
+          // We want to know if this is the first run for each entryfile
+          // for certain plugins (e.g. css)
+          writeMeta: index === 0,
+        })
+      )
+    )
   );
 
   return await Promise.all(
diff --git a/scripts/utils.ts b/scripts/utils.ts
index 456b2e1fa..3c9eae8e7 100644
--- a/scripts/utils.ts
+++ b/scripts/utils.ts
@@ -61,13 +61,12 @@ export async function isDir(name: string) {
 export async function getInputs(
   entries?: string | string[]
 ): Promise<string[]> {
-  return ([] as any[])
-    .concat(
-      entries && entries.length
-        ? entries
-        : (await isDir(resolveApp("src"))) && (await jsOrTs("src/index"))
-    )
-    .flatMap(file => glob(file));
+  const inputs = ([] as any[]).concat(
+    entries && entries.length
+      ? entries
+      : (await isDir(resolveApp("src"))) && (await jsOrTs("src/index"))
+  );
+  return flatten(inputs).map((file) => glob(file));
 }
 
 export function getPackageName(opts: any) {
@@ -93,8 +92,9 @@ export async function createProgressEstimator() {
 
 export function logError(err: any) {
   const error = err.error || err;
-  const description = `${error.name ? error.name + ": " : ""}${error.message ||
-    error}`;
+  const description = `${error.name ? error.name + ": " : ""}${
+    error.message || error
+  }`;
   const message = error.plugin
     ? error.plugin === "rpt2"
       ? `(typescript) ${description}`
@@ -127,3 +127,7 @@ export function parseArgs() {
   let { _, ...args } = mri(process.argv.slice(2));
   return args;
 }
+
+export function flatten(arr: any[][]): any[] {
+  return arr.reduce((flat, next) => flat.concat(next), []);
+}