diff --git a/lib/config.js b/lib/config.js
index fd4aa753..df110b5d 100644
--- a/lib/config.js
+++ b/lib/config.js
@@ -11,10 +11,13 @@ module.exports = (nextConfig = {}) => {
     assetPrefix,
     pageExtensions: ['js', 'jsx', 'md', 'mdx'],
 
-    publicRuntimeConfig: Object.assign({
-      assetPrefix,
-      production: NODE_ENV === 'production'
-    }, nextConfig.publicRuntimeConfig),
+    publicRuntimeConfig: Object.assign(
+      {
+        assetPrefix,
+        production: NODE_ENV === 'production'
+      },
+      nextConfig.publicRuntimeConfig
+    ),
 
     webpack(config, options) {
       if (!options.defaultLoaders) {
@@ -23,7 +26,7 @@ module.exports = (nextConfig = {}) => {
         )
       }
 
-      const {dev, isServer} = options
+      const {dev} = options
 
       // only attempt to sync locally and in CI
       if (dev && !configured) {
@@ -37,10 +40,7 @@ module.exports = (nextConfig = {}) => {
 
       config.module.rules.push({
         test: /\.mdx?$/,
-        use: [
-          options.defaultLoaders.babel,
-          require.resolve('./mdx-loader')
-        ]
+        use: [options.defaultLoaders.babel, require.resolve('./mdx-loader')]
       })
 
       configured = true
diff --git a/lib/mdx-loader.js b/lib/mdx-loader.js
index bb8335d6..8072d69f 100644
--- a/lib/mdx-loader.js
+++ b/lib/mdx-loader.js
@@ -24,9 +24,7 @@ module.exports = async function(source) {
         emoji,
         [textr, {plugins: [typographicBase]}]
       ],
-      hastPlugins: [
-        rehypePrism
-      ],
+      hastPlugins: [rehypePrism],
       compilers: [mdxExportJSONByDefault('frontMatter', data)]
     },
     getOptions(this),
@@ -39,11 +37,12 @@ module.exports = async function(source) {
     return callback(err)
   }
 
-  let code = `
+  return callback(
+    null,
+    `
 import React from 'react'
 import {MDXTag} from '@mdx-js/tag'
 ${result}
 `
-
-  return callback(null, code)
+  )
 }
diff --git a/lib/rehype-prism.js b/lib/rehype-prism.js
index 1cf431fc..4290825b 100644
--- a/lib/rehype-prism.js
+++ b/lib/rehype-prism.js
@@ -27,7 +27,7 @@ module.exports = options => {
     let result = node
     const source = nodeToString(node)
     try {
-      parent.properties.className = (parent.properties.className || []).concat('language-' + lang)
+      parent.properties.className = (parent.properties.className || []).concat(`language-${lang}`)
       result = refractor.highlight(source, lang)
     } catch (err) {
       if (/Unknown language/.test(err.message)) {
@@ -52,8 +52,8 @@ function getLanguage(node, aliases) {
 
   for (const classListItem of className) {
     if (classListItem.slice(0, 9) === 'language-') {
-      let language = classListItem.slice(9).replace(/{.*/, '')
-      let alias = aliases[language]
+      const language = classListItem.slice(9).replace(/{.*/, '')
+      const alias = aliases[language]
       return alias || language
     }
   }
diff --git a/lib/rename.js b/lib/rename.js
index a9d697a8..603f7448 100644
--- a/lib/rename.js
+++ b/lib/rename.js
@@ -2,7 +2,7 @@ module.exports = function rename(fn, options = {}) {
   const {log = noop} = options
   return (files, metal, done) => {
     for (const [key, file] of Object.entries(files)) {
-      let dest = fn(file, key, files, metal)
+      const dest = fn(file, key, files, metal)
       if (dest === true) {
         log(`[rename] keep: ${key}`)
       } else if (dest && dest !== key) {