diff --git a/test/fixtures/sass-data-prepend/index.js b/test/fixtures/sass-data-prepend/index.js
index 423b033c..72dcf9c2 100644
--- a/test/fixtures/sass-data-prepend/index.js
+++ b/test/fixtures/sass-data-prepend/index.js
@@ -1 +1 @@
-import './style.scss';
+import './style.scss'
diff --git a/test/index.test.js b/test/index.test.js
index 2f5cb76d..b6d3ea2e 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -56,35 +56,36 @@ function snapshot({
   options = {}
 }) {
   test(title, async () => {
-    let res
+    let result
     try {
-      res = await write({
+      result = await write({
         input,
         outDir,
         options
       })
-    } catch (err) {
-      const frame = err.codeFrame || err.snippet
+    } catch (error) {
+      const frame = error.codeFrame || error.snippet
       if (frame) {
-        throw new Error(frame + err.message)
+        throw new Error(frame + error.message)
       }
-      throw err
+
+      throw error
     }
 
-    expect(await res.jsCode()).toMatchSnapshot('js code')
+    expect(await result.jsCode()).toMatchSnapshot('js code')
 
     if (options.extract) {
-      expect(await res.hasCssFile()).toBe(true)
-      expect(await res.cssCode()).toMatchSnapshot('css code')
+      expect(await result.hasCssFile()).toBe(true)
+      expect(await result.cssCode()).toMatchSnapshot('css code')
     }
 
     const sourceMap = options && options.sourceMap
     if (sourceMap === 'inline') {
-      expect(await res.hasCssMapFile()).toBe(false)
+      expect(await result.hasCssMapFile()).toBe(false)
     } else if (sourceMap === true) {
-      expect(await res.hasCssMapFile()).toBe(Boolean(options.extract))
+      expect(await result.hasCssMapFile()).toBe(Boolean(options.extract))
       if (options.extract) {
-        expect(await res.cssMap()).toMatchSnapshot('css map')
+        expect(await result.cssMap()).toMatchSnapshot('css map')
       }
     }
   })
@@ -349,7 +350,7 @@ snapshotMany('sass', [
 ])
 
 test('onExtract', async () => {
-  const res = await write({
+  const result = await write({
     input: 'simple/index.js',
     outDir: 'onExtract',
     options: {
@@ -359,8 +360,8 @@ test('onExtract', async () => {
       }
     }
   })
-  expect(await res.jsCode()).toMatchSnapshot()
-  expect(await res.hasCssFile()).toBe(false)
+  expect(await result.jsCode()).toMatchSnapshot()
+  expect(await result.hasCssFile()).toBe(false)
 })
 
 test('augmentChunkHash', async () => {
@@ -381,6 +382,7 @@ test('augmentChunkHash', async () => {
     })
     outputFiles.push(output[0])
   }
+
   const [fooOne, fooTwo, barOne] = outputFiles
 
   const fooHash = fooOne.fileName.split('.')[1]
diff --git a/types/index.d.ts b/types/index.d.ts
index a9041139..17d4d30c 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -1,44 +1,49 @@
 import { Plugin } from 'rollup'
 import { CreateFilter } from 'rollup-pluginutils'
 
-type FunctionType<T = any, U = any> = (...args: T[]) => U;
+type FunctionType<T = any, U = any> = (...args: readonly T[]) => U;
 
-type onExtract = (asset: {
-  code: any
-  map: any
-  codeFileName: string
-  mapFileName: string
-}) => boolean
+type onExtract = (asset: Readonly<{
+	code: any;
+	map: any;
+	codeFileName: string;
+	mapFileName: string;
+}>) => boolean;
 
 export type PostCSSPluginConf = {
-  inject?: boolean | { [key: string]: any } | ((cssVariableName: string, id: string) => string)
-  extract?: boolean | string
-  onExtract?: onExtract
-  modules?: boolean | { [key: string]: any }
-  extensions?: string[]
-  plugins?: any[]
-  autoModules?: boolean
-  namedExports?: boolean | ((id: string) => boolean)
-  minimize?: boolean | any
-  parser?: string | FunctionType
-  stringifier?: string | FunctionType
-  syntax?: string | FunctionType
-  exec?: boolean
-  config?: boolean | {
-    path: string
-    ctx: any
-  };
-  to?: string
-  name?: any[] | any[][]
-  loaders?: any[]
-  onImport?: (id: string) => void
-  use?: string[] | { [key in 'sass' | 'stylus' | 'less']: any }
-  /**
+	inject?:
+	| boolean
+	| { [key: string]: any }
+	| ((cssVariableName: string, id: string) => string);
+	extract?: boolean | string;
+	onExtract?: onExtract;
+	modules?: boolean | { [key: string]: any };
+	extensions?: string[];
+	plugins?: any[];
+	autoModules?: boolean;
+	namedExports?: boolean | ((id: string) => boolean);
+	minimize?: boolean | any;
+	parser?: string | FunctionType;
+	stringifier?: string | FunctionType;
+	syntax?: string | FunctionType;
+	exec?: boolean;
+	config?:
+	| boolean
+	| {
+		path: string;
+		ctx: any;
+	};
+	to?: string;
+	name?: any[] | any[][];
+	loaders?: any[];
+	onImport?: (id: string) => void;
+	use?: string[] | { [key in 'sass' | 'stylus' | 'less']: any };
+	/**
    * @default: false
    **/
-  sourceMap?: boolean | 'inline'
-  include?: Parameters<CreateFilter>[0]
-  exclude?: Parameters<CreateFilter>[1]
-}
+	sourceMap?: boolean | 'inline';
+	include?: Parameters<CreateFilter>[0];
+	exclude?: Parameters<CreateFilter>[1];
+};
 
-export default function (options: PostCSSPluginConf): Plugin
+export default function (options: Readonly<PostCSSPluginConf>): Plugin