don't use readFileSync

index.ts

@@ -1,6 +1,6 @@
 /* eslint-env node */
 
-import {readFileSync} from 'node:fs';
+import {readFile} from 'node:fs/promises';
 import {basename, dirname, extname, join, relative, resolve} from 'node:path';
 
 import {type Plugin, type RollupOptions} from 'rollup';
@@ -21,7 +21,7 @@ import {getAssetEntrypoints, getScriptEntrypoints} from './entrypoints';
  * Javascript entry point
  * @param options.sourcemap Controls inclusion of sourcemaps in the output
  */
-export function buildConfig ({
+export async function buildConfig ({
 	manifest: manifestPathRelative,
 	outDir,
 	plugins = [],
@@ -31,14 +31,14 @@ export function buildConfig ({
 	outDir: string;
 	plugins: Plugin[];
 	sourcemap: boolean | 'inline' | 'hidden';
-}): RollupOptions[] {
+}): Promise<RollupOptions[]> {
 	const manifestPath = resolve(process.cwd(), manifestPathRelative);
 	const manifestDirname = dirname(manifestPath);
 
 	// Load the manifest
 	let manifestContent: chrome.runtime.Manifest;
 	try {
-		manifestContent = JSON.parse(readFileSync(manifestPath, {encoding: 'utf-8'}));
+		manifestContent = JSON.parse(await readFile(manifestPath, {encoding: 'utf-8'}));
 	} catch (error) {
 		throw new Error('Failed to load manifest');
 	}
@@ -106,8 +106,8 @@ export function buildConfig ({
 				{
 					name: '_manifest-asset-processing',
 					// emit other assets
-					buildStart () {
-						assets.forEach(({path, replacePath}) => {
+					async buildStart () {
+						await Promise.all(assets.map(async ({path, replacePath}) => {
 							// Figure out where the asset will live in output
 							const outPath = ensureUniquePath(path);
 
@@ -118,9 +118,9 @@ export function buildConfig ({
 							this.emitFile({
 								type: 'asset',
 								fileName: outPath,
-								source: readFileSync(join(manifestDirname, path)),
+								source: await readFile(join(manifestDirname, path)),
 							});
-						});
+						}));
 					},
 
 					// hacks to make sure the manifest is emitted as bare JSON