diff --git a/gulpfile.babel.js b/gulpfile.babel.js
index fa18ef9..68ca2a0 100644
--- a/gulpfile.babel.js
+++ b/gulpfile.babel.js
@@ -33,7 +33,7 @@ if (isStandalone) {
   webpackConfig.pop()
 }
 
-let watchOpts = {
+const watchOpts = {
   readDelay: 500,
   verbose: true,
   read: false
@@ -41,12 +41,12 @@ let watchOpts = {
 
 let packageVersion = require('./package.json').version
 
-let webpackDefines = new webpack.DefinePlugin({
+const webpackDefines = new webpack.DefinePlugin({
   FIMFIC2EPUB_VERSION: JSON.stringify(packageVersion)
 })
 
 // No need to bloat the build with a list of all tlds...
-let replaceTlds = new webpack.NormalModuleReplacementPlugin(/^tlds$/, '../../src/false')
+const replaceTlds = new webpack.NormalModuleReplacementPlugin(/^tlds$/, '../../src/false')
 
 webpackConfig.forEach((c) => {
   c.plugins.push(webpackDefines)
@@ -89,8 +89,8 @@ function webpackTask () {
 }
 
 function convertFontAwesomeVars (contents) {
-  let vars = {}
-  let matchVar = /\$fa-var-(.*?): "\\(.*?)";/g
+  const vars = {}
+  const matchVar = /\$fa-var-(.*?): "\\(.*?)";/g
   let ma
   for (;(ma = matchVar.exec(contents));) {
     vars[ma[1]] = String.fromCharCode(parseInt(ma[2], 16))
diff --git a/package.json b/package.json
index d474bc7..b0dda38 100644
--- a/package.json
+++ b/package.json
@@ -9,7 +9,7 @@
     "url": "https://github.com/daniel-j/fimfic2epub.git"
   },
   "scripts": {
-    "build": "NODE_ENV=production gulp",
+    "build": "gulp",
     "dev": "NODE_ENV=development gulp"
   },
   "bin": {
diff --git a/src/FimFic2Epub.js b/src/FimFic2Epub.js
index f2df4d7..55076a2 100644
--- a/src/FimFic2Epub.js
+++ b/src/FimFic2Epub.js
@@ -31,9 +31,9 @@ const trimWhitespace = /^\s*(<br\s*\/?\s*>)+|(<br\s*\/?\s*>)+\s*$/ig
 class FimFic2Epub extends EventEmitter {
   static getStoryId (id) {
     if (isNaN(id)) {
-      let url = URL(id)
+      const url = URL(id)
       if (url.hostname === 'www.fimfiction.net' || url.hostname === 'fimfiction.net') {
-        let m = url.pathname.match(/^\/story\/(\d+)/)
+        const m = url.pathname.match(/^\/story\/(\d+)/)
         if (m) {
           id = m[1]
         }
@@ -49,7 +49,7 @@ class FimFic2Epub extends EventEmitter {
   static fetchStoryInfo (storyId, raw = false) {
     return new Promise((resolve, reject) => {
       storyId = FimFic2Epub.getStoryId(storyId)
-      let url = '/api/story.php?story=' + storyId
+      const url = '/api/story.php?story=' + storyId
       fetch(url).then((content) => {
         let data
         try {
@@ -63,7 +63,7 @@ class FimFic2Epub extends EventEmitter {
           reject(new Error(data.error + ' (id: ' + storyId + ')'))
           return
         }
-        let story = data.story
+        const story = data.story
         if (raw) {
           resolve(story)
           return
@@ -223,20 +223,20 @@ class FimFic2Epub extends EventEmitter {
 
     this.progress(0, 0, 'Fetching chapters...')
 
-    let chapterCount = this.storyInfo.chapters.length
-    let url = 'https://fimfiction.net/story/download/' + this.storyInfo.id + '/html'
+    const chapterCount = this.storyInfo.chapters.length
+    const url = 'https://fimfiction.net/story/download/' + this.storyInfo.id + '/html'
 
     this.pcache.chapters = fetch(url).then((html) => {
       let p = Promise.resolve()
-      let matchChapter = /<article class="chapter">[\s\S]*?<\/header>([\s\S]*?)<\/article>/g
+      const matchChapter = /<article class="chapter">[\s\S]*?<\/header>([\s\S]*?)<\/article>/g
       for (let ma, i = 0; (ma = matchChapter.exec(html)); i++) {
         const ch = this.storyInfo.chapters[i]
         let chapterContent = ma[1]
         chapterContent = chapterContent.replace(/<footer>[\s\S]*?<\/footer>/g, '').trim()
 
-        let authorNotesPos = chapterContent.indexOf('<aside ')
+        const authorNotesPos = chapterContent.indexOf('<aside ')
         let notesContent = ''
-        let notesFirst = authorNotesPos === 0
+        const notesFirst = authorNotesPos === 0
         if (authorNotesPos !== -1) {
           chapterContent = chapterContent.replace(/<aside class="authors-note">([\s\S]*?)<\/aside>/, (match, content, pos) => {
             content = content.replace(/<header><h1>.*?<\/h1><\/header>/, '')
@@ -290,14 +290,14 @@ class FimFic2Epub extends EventEmitter {
       return Promise.resolve()
     }
 
-    let checksums = new Map()
+    const checksums = new Map()
 
     this.progress(0, 0, 'Fetching remote files...')
     this.pcache.remoteResources = new Promise((resolve, reject) => {
-      let iter = this.remoteResources.entries()
+      const iter = this.remoteResources.entries()
       let completeCount = 0
 
-      let next = (r) => {
+      const next = (r) => {
         completeCount++
         if (r.data) {
           this.progress(0, completeCount / this.remoteResources.size, 'Fetched remote file ' + completeCount + ' / ' + this.remoteResources.size)
@@ -307,7 +307,7 @@ class FimFic2Epub extends EventEmitter {
         recursive()
       }
 
-      let recursive = () => {
+      const recursive = () => {
         let r = iter.next().value
         if (!r) {
           if (completeCount === this.remoteResources.size) {
@@ -315,7 +315,7 @@ class FimFic2Epub extends EventEmitter {
           }
           return
         }
-        let url = r[0]
+        const url = r[0]
         r = r[1]
         if (r.data) {
           next(r)
@@ -335,9 +335,9 @@ class FimFic2Epub extends EventEmitter {
             }
           }
           if (info) {
-            let checksum = crc32(isNode ? data : new Uint8Array(data))
+            const checksum = crc32(isNode ? data : new Uint8Array(data))
             if (checksums.has(checksum)) {
-              let sameFile = this.remoteResources.get(checksums.get(checksum))
+              const sameFile = this.remoteResources.get(checksums.get(checksum))
               r.dest = sameFile.dest
               r.filename = sameFile.dest
               r.type = sameFile.type
@@ -348,11 +348,11 @@ class FimFic2Epub extends EventEmitter {
                 data = await utils.webp2png(isNode ? data : new Uint8Array(data))
                 info = fileType(data)
               }
-              let type = info.mime
+              const type = info.mime
               r.type = type
-              let isImage = type.startsWith('image/')
-              let folder = isImage ? 'Images' : 'Misc'
-              let dest = folder + '/*.' + info.ext
+              const isImage = type.startsWith('image/')
+              const folder = isImage ? 'Images' : 'Misc'
+              const dest = folder + '/*.' + info.ext
               r.dest = dest.replace('*', r.filename)
               r.data = data
             }
@@ -393,8 +393,8 @@ class FimFic2Epub extends EventEmitter {
     this.notesHtml.length = 0
 
     for (let i = 0; i < this.chapters.length; i++) {
-      let ch = this.storyInfo.chapters[i]
-      let chapter = this.chapters[i]
+      const ch = this.storyInfo.chapters[i]
+      const chapter = this.chapters[i]
       let content = chapter.content
 
       if (this.options.typogrify) {
@@ -486,8 +486,8 @@ class FimFic2Epub extends EventEmitter {
     this.zip.file('OEBPS/Styles/navstyle.css', Buffer.from(navstyleCss, 'utf8'))
 
     for (let i = 0; i < this.chapters.length; i++) {
-      let filename = 'OEBPS/Text/chapter_' + zeroFill(3, i + 1) + '.xhtml'
-      let html = this.chaptersHtml[i]
+      const filename = 'OEBPS/Text/chapter_' + zeroFill(3, i + 1) + '.xhtml'
+      const html = this.chaptersHtml[i]
       this.zip.file(filename, Buffer.from(html, 'utf8'))
     }
 
@@ -496,8 +496,8 @@ class FimFic2Epub extends EventEmitter {
 
       for (let i = 0; i < this.chapters.length; i++) {
         if (!this.chapters[i].notes) continue
-        let filename = 'OEBPS/Text/note_' + zeroFill(3, i + 1) + '.xhtml'
-        let html = this.notesHtml[i]
+        const filename = 'OEBPS/Text/note_' + zeroFill(3, i + 1) + '.xhtml'
+        const html = this.notesHtml[i]
         this.zip.file(filename, Buffer.from(html, 'utf8'))
       }
     }
@@ -511,7 +511,7 @@ class FimFic2Epub extends EventEmitter {
       (paragraphsCss[this.options.paragraphStyle] || '')
       , 'utf8'))
 
-    let remoteDestCache = new Set()
+    const remoteDestCache = new Set()
     this.remoteResources.forEach((r) => {
       if (r.dest && !remoteDestCache.has(r.dest)) {
         this.zip.file('OEBPS/' + r.dest, r.data)
@@ -541,7 +541,7 @@ class FimFic2Epub extends EventEmitter {
         compression: 'DEFLATE',
         compressionOptions: { level: 9 }
       }, (metadata) => { // onUpdate
-        let currentPercent = Math.round(metadata.percent / 10) * 10
+        const currentPercent = Math.round(metadata.percent / 10) * 10
         if (lastPercent !== currentPercent) {
           lastPercent = currentPercent
           this.progress(0, currentPercent / 100, 'Compressing...')
@@ -575,7 +575,7 @@ class FimFic2Epub extends EventEmitter {
         compressionOptions: { level: 9 }
       }, (metadata) => {
         if (onUpdate) onUpdate(metadata)
-        let currentPercent = Math.round(metadata.percent / 20) * 20
+        const currentPercent = Math.round(metadata.percent / 20) * 20
         if (lastPercent !== currentPercent) {
           lastPercent = currentPercent
           this.progress(0, currentPercent / 100, 'Compressing...')
@@ -587,13 +587,15 @@ class FimFic2Epub extends EventEmitter {
     this.storyInfo.title = title.trim()
     this.filename = FimFic2Epub.getFilename(this.storyInfo)
   }
+
   setAuthorName (name) {
     this.storyInfo.author.name = name.trim()
     this.filename = FimFic2Epub.getFilename(this.storyInfo)
   }
+
   setCoverImage (buffer) {
     buffer = isNode ? buffer : Buffer.from(new Uint8Array(buffer))
-    let info = fileType(buffer)
+    const info = fileType(buffer)
     if (!info || !info.mime.startsWith('image/')) {
       throw new Error('Invalid image')
     }
@@ -620,21 +622,21 @@ class FimFic2Epub extends EventEmitter {
 
   findRemoteResources (prefix, where, html) {
     let remoteCounter = 1
-    let matchUrl = /<img.*?src="([^">]*\/([^">]*?))".*?>/g
-    let emoticonUrl = /static\.fimfiction\.net\/images\/emoticons\/([a-z_]*)\.[a-z]*$/
+    const matchUrl = /<img.*?src="([^">]*\/([^">]*?))".*?>/g
+    const emoticonUrl = /static\.fimfiction\.net\/images\/emoticons\/([a-z_]*)\.[a-z]*$/
 
     for (let ma; (ma = matchUrl.exec(html));) {
-      let url = ma[1]
-      let cleanurl = entities.decode(url)
+      const url = ma[1]
+      const cleanurl = entities.decode(url)
       if (this.remoteResources.has(cleanurl)) {
-        let r = this.remoteResources.get(cleanurl)
+        const r = this.remoteResources.get(cleanurl)
         if (r.where.indexOf(where) === -1) {
           r.where.push(where)
         }
         continue
       }
       let filename = prefix + '_' + remoteCounter
-      let emoticon = url.match(emoticonUrl)
+      const emoticon = url.match(emoticonUrl)
       if (emoticon) {
         filename = 'emoticon_' + emoticon[1]
       }
@@ -644,7 +646,7 @@ class FimFic2Epub extends EventEmitter {
   }
 
   async findIcons () {
-    let matchIcon = /<i class="fa fa-fw fa-(.*?)"/g
+    const matchIcon = /<i class="fa fa-fw fa-(.*?)"/g
     this.usedIcons.clear()
 
     const scan = (html) => {
@@ -667,10 +669,10 @@ class FimFic2Epub extends EventEmitter {
       return
     }
 
-    let glyphs = [...this.usedIcons].map((name) => {
+    const glyphs = [...this.usedIcons].map((name) => {
       return fontAwesomeCodes[name].charCodeAt(0)
     })
-    let fontFile = require('font-awesome/fonts/fontawesome-webfont.ttf')
+    const fontFile = require('font-awesome/fonts/fontawesome-webfont.ttf')
     this.iconsFont = await subsetFont(fontFile, glyphs, { local: isNode })
   }
 
@@ -692,7 +694,7 @@ class FimFic2Epub extends EventEmitter {
       return Promise.resolve(this.coverImage)
     }
     this.coverImage = null
-    let url = this.coverUrl || this.storyInfo.full_image
+    const url = this.coverUrl || this.storyInfo.full_image
     if (!url) {
       console.warn('Story has no image. Generating one...')
       let canvas
@@ -703,7 +705,7 @@ class FimFic2Epub extends EventEmitter {
         canvas.width = 1080
         canvas.height = 1440
       }
-      let ctx = canvas.getContext('2d')
+      const ctx = canvas.getContext('2d')
       ctx.fillStyle = 'white'
       ctx.fillRect(0, 0, canvas.width, canvas.height)
 
@@ -714,8 +716,8 @@ class FimFic2Epub extends EventEmitter {
       ctx.strokeRect(20, 20, canvas.width - 40, canvas.height - 40)
       ctx.strokeRect(12, 12, canvas.width - 24, canvas.height - 24)
 
-      let title = this.storyInfo.title
-      let author = this.storyInfo.author.name
+      const title = this.storyInfo.title
+      const author = this.storyInfo.author.name
 
       let fontSize = 150
       let width
@@ -744,14 +746,14 @@ class FimFic2Epub extends EventEmitter {
 
     this.pcache.coverImage = fetchRemote(url, 'arraybuffer').then((data) => {
       data = isNode ? data : new Uint8Array(data)
-      let info = fileType(data)
+      const info = fileType(data)
       if (info) {
-        let type = info.mime
-        let isImage = type.startsWith('image/')
+        const type = info.mime
+        const isImage = type.startsWith('image/')
         if (!isImage) {
           return null
         }
-        let filename = 'Images/cover.' + info.ext
+        const filename = 'Images/cover.' + info.ext
         this.coverFilename = filename
         this.coverType = type
 
@@ -771,7 +773,7 @@ class FimFic2Epub extends EventEmitter {
 
   fetchTitlePage () {
     let viewMature = true
-    let isStoryMature = this.storyInfo.content_rating === 2
+    const isStoryMature = this.storyInfo.content_rating === 2
     if (!isNode) {
       viewMature = document.cookie.split('; ').includes('view_mature=true')
       if (!viewMature && isStoryMature) {
@@ -796,19 +798,19 @@ class FimFic2Epub extends EventEmitter {
     startTagsPos += html.substring(startTagsPos).indexOf('<ul class="story-tags">') + 23
     let tagsHtml = html.substring(startTagsPos)
 
-    let endTagsPos = tagsHtml.indexOf('</ul>')
+    const endTagsPos = tagsHtml.indexOf('</ul>')
     tagsHtml = tagsHtml.substring(0, endTagsPos)
 
-    let tags = []
+    const tags = []
     let c
     tags.byImage = {}
     this.subjects.length = 0
     this.subjects.push('Fimfiction')
     this.subjects.push(this.storyInfo.content_rating_text)
 
-    let matchTag = /<a href="([^"]*?)" class="([^"]*?)" title="[^"]*?" data-tag="([^"]*?)".*?>(.*?)<\/a>/g
+    const matchTag = /<a href="([^"]*?)" class="([^"]*?)" title="[^"]*?" data-tag="([^"]*?)".*?>(.*?)<\/a>/g
     for (;(c = matchTag.exec(tagsHtml));) {
-      let cat = {
+      const cat = {
         url: 'https://fimfiction.net' + c[1],
         className: 'story-tag ' + c[2],
         name: entities.decode(c[4]),
@@ -831,12 +833,12 @@ class FimFic2Epub extends EventEmitter {
       html = html.substring(html.indexOf('<hr />') + 6)
     }
 
-    let endDescPos = html.indexOf('</span>\n')
-    let description = html.substring(0, endDescPos).trim()
+    const endDescPos = html.indexOf('</span>\n')
+    const description = html.substring(0, endDescPos).trim()
     this.description = description
 
     html = html.substring(endDescPos + 7)
-    let extraPos = html.indexOf('<div class="extra_story_data">')
+    const extraPos = html.indexOf('<div class="extra_story_data">')
     html = html.substring(extraPos + 30)
 
     ma = html.match(/<span class="approved-date">.*?data-time="(.*?)".*?<\/span>/)
@@ -848,7 +850,7 @@ class FimFic2Epub extends EventEmitter {
   }
 
   parseChapterPage (html) {
-    let trimWhitespace = /^\s*(<br\s*\/?\s*>)+|(<br\s*\/?\s*>)+\s*$/ig
+    const trimWhitespace = /^\s*(<br\s*\/?\s*>)+|(<br\s*\/?\s*>)+\s*$/ig
 
     let authorNotesPos = html.indexOf('<div class="authors-note"')
     let authorNotes = ''
@@ -860,10 +862,10 @@ class FimFic2Epub extends EventEmitter {
       authorNotes = authorNotes.replace(trimWhitespace, '')
     }
 
-    let chapterPos = html.indexOf('<div class="bbcode">')
+    const chapterPos = html.indexOf('<div class="bbcode">')
     let chapter = html.substring(chapterPos + 20)
 
-    let pos = chapter.indexOf('\t\t</div>\n\t</div>\t\t\n\t\t\t\t\t</div>\n')
+    const pos = chapter.indexOf('\t\t</div>\n\t</div>\t\t\n\t\t\t\t\t</div>\n')
 
     chapter = chapter.substring(0, pos).trim()
 
@@ -877,9 +879,9 @@ class FimFic2Epub extends EventEmitter {
     if (!this.options.includeExternal) {
       this.remoteResources.forEach((r, url) => {
         if (r.originalUrl && r.where) {
-          let ourl = new RegExp(escapeStringRegexp(r.originalUrl), 'g')
+          const ourl = new RegExp(escapeStringRegexp(r.originalUrl), 'g')
           for (var i = 0; i < r.where.length; i++) {
-            let w = r.where[i]
+            const w = r.where[i]
             if (typeof w === 'number') {
               if (ourl.test(this.chapters[w])) {
                 this.storyInfo.chapters[w].remote = true
@@ -895,10 +897,10 @@ class FimFic2Epub extends EventEmitter {
     } else {
       this.remoteResources.forEach((r, url) => {
         if (r.dest && r.originalUrl && r.where) {
-          let dest = '../' + r.dest
-          let ourl = new RegExp(escapeStringRegexp(r.originalUrl), 'g')
+          const dest = '../' + r.dest
+          const ourl = new RegExp(escapeStringRegexp(r.originalUrl), 'g')
           for (var i = 0; i < r.where.length; i++) {
-            let w = r.where[i]
+            const w = r.where[i]
             if (typeof w === 'object' && w.chapter !== undefined && this.chaptersHtml[w.chapter]) {
               this.chaptersHtml[w.chapter] = this.chaptersHtml[w.chapter].replace(ourl, dest)
             } else if (typeof w === 'object' && w.note !== undefined && this.notesHtml[w.note]) {
diff --git a/src/cleanMarkup.js b/src/cleanMarkup.js
index 15a2218..da49440 100644
--- a/src/cleanMarkup.js
+++ b/src/cleanMarkup.js
@@ -44,12 +44,12 @@ export async function cleanMarkup (html) {
   html = html.replace('<blockquote style="margin: 10px 0px; box-sizing:border-box; -moz-box-sizing:border-box;margin-left:25px; padding: 15px;background-color: #F7F7F7;border: 1px solid #AAA;width: 50%;float:right;box-shadow: 5px 5px 0px #EEE;">', '<blockquote class="right_insert">')
 
   // add alt attributes to images that don't have them
-  let imageEmbed = /<img src="(.*?)" \/>/g
+  const imageEmbed = /<img src="(.*?)" \/>/g
   html = await replaceAsync(html, imageEmbed, (match, src) => render(m('img', { src: entities.decode(src), alt: 'Image' }), { strict: true }))
 
   // Fix links pointing to pages on fimfiction
   // Example: <a href="/user/djazz" rel="nofollow">djazz</a>
-  let matchLink = /(<a .?href=")(.+?)(".+?>)/g
+  const matchLink = /(<a .?href=")(.+?)(".+?>)/g
   html = html.replace(matchLink, (match, head, url, tail) => {
     if (url.substring(0, 1) !== '#' && url.substring(0, 2) !== '//' && url.substring(0, 4) !== 'http' && url.substring(0, 1) === '/') {
       url = 'https://fimfiction.net' + url
@@ -62,14 +62,14 @@ export async function cleanMarkup (html) {
   const query = new Map()
   let completeCount = 0
 
-  let matchYouTube = /<p><a class="embed" href="https:\/\/www\.youtube\.com\/watch\?v=(.*?)">.*?<\/a><\/p>/g
+  const matchYouTube = /<p><a class="embed" href="https:\/\/www\.youtube\.com\/watch\?v=(.*?)">.*?<\/a><\/p>/g
   for (let ma; (ma = matchYouTube.exec(html));) {
-    let youtubeId = ma[1].match(/^[^&]+/)[0]
+    const youtubeId = ma[1].match(/^[^&]+/)[0]
     cache.set(youtubeId, null)
     query.set(entities.decode(ma[1]), youtubeId)
   }
 
-  let matchSoundCloud = /<p><a class="embed" href="(https:\/\/soundcloud\.com\/.*?)">.*?<\/a><\/p>/g
+  const matchSoundCloud = /<p><a class="embed" href="(https:\/\/soundcloud\.com\/.*?)">.*?<\/a><\/p>/g
   html = await replaceAsync(html, matchSoundCloud, (match, url) => {
     return render(m('.soundcloud.leftalign', [
       'SoundCloud: ', m('a', { href: entities.decode(url), rel: 'nofollow' }, url.replace('https://soundcloud.com/', '').replace(/[-_]/g, ' ').replace('/', ' - ').replace(/ {2}/g, ' '))
@@ -104,12 +104,12 @@ export async function cleanMarkup (html) {
 
   function replaceYouTube (match, queryString) {
     queryString = entities.decode(queryString)
-    let youtubeId = query.get(queryString)
+    const youtubeId = query.get(queryString)
     let thumbnail = 'https://img.youtube.com/vi/' + youtubeId + '/hqdefault.jpg'
-    let youtubeUrl = 'https://youtube.com/watch?v=' + queryString
+    const youtubeUrl = 'https://youtube.com/watch?v=' + queryString
     let title = 'Youtube Video'
     let caption = ''
-    let data = cache.get(youtubeId)
+    const data = cache.get(youtubeId)
 
     if (data) {
       thumbnail = (data.thumbnails.standard || data.thumbnails.high || data.thumbnails.medium || data.thumbnails.default).url
@@ -138,7 +138,7 @@ export function fixDoubleSpacing (html) {
 
 export function fixParagraphIndent (html) {
   // from FimFictionConverter by Nyerguds
-  let fixIndent = 2
+  const fixIndent = 2
   if (fixIndent > 0) {
     // only trigger indenting when finding as many whitespace characters in a row as indicated by the FixIndent setting.
 
diff --git a/src/cli.js b/src/cli.js
index 92a3411..70a4ce4 100755
--- a/src/cli.js
+++ b/src/cli.js
@@ -39,7 +39,6 @@ if (outputStdout) {
 const mock = require('mithril/test-utils/browserMock')(global)
 global.requestAnimationFrame = mock.requestAnimationFrame
 
-
 const htmlToText = require('./utils').htmlToText
 const FimFic2Epub = require('./FimFic2Epub').default
 const fs = require('fs')
diff --git a/src/constants.js b/src/constants.js
index 84b01f5..a13231a 100644
--- a/src/constants.js
+++ b/src/constants.js
@@ -10,13 +10,13 @@ export const NS = {
 }
 
 export const tidyOptions = {
-  'indent': 'auto',
+  indent: 'auto',
   'numeric-entities': 'yes',
   'output-xhtml': 'yes',
   'alt-text': 'Image',
-  'wrap': '0',
-  'quiet': 'yes',
-  'newline': 'LF',
+  wrap: '0',
+  quiet: 'yes',
+  newline: 'LF',
   'tidy-mark': 'no',
   'show-body-only': 'auto'
 }
diff --git a/src/eventPage.js b/src/eventPage.js
index 5052363..3c05607 100644
--- a/src/eventPage.js
+++ b/src/eventPage.js
@@ -4,7 +4,7 @@ import fetch from './fetch'
 
 if (typeof safari !== 'undefined') {
   safari.application.addEventListener('message', function (ev) {
-    let url = ev.message
+    const url = ev.message
     fetch(url, 'arraybuffer').then((buffer) => {
       console.log('Fetched ' + url)
       ev.target.page.dispatchMessage('remote', {
@@ -14,12 +14,12 @@ if (typeof safari !== 'undefined') {
     })
   }, false)
 } else {
-  let onMessage = chrome.extension.onMessage ? chrome.extension.onMessage : chrome.runtime.onMessage
+  const onMessage = chrome.extension.onMessage ? chrome.extension.onMessage : chrome.runtime.onMessage
 
   onMessage.addListener(function (request, sender, sendResponse) {
     if (typeof request === 'string') {
       fetch(request, 'blob').then((blob) => {
-        let ourl = URL.createObjectURL(blob)
+        const ourl = URL.createObjectURL(blob)
         console.log('Fetched', request)
         sendResponse(ourl)
       })
diff --git a/src/fetch.js b/src/fetch.js
index 968dbdd..9d41b46 100644
--- a/src/fetch.js
+++ b/src/fetch.js
@@ -11,9 +11,9 @@ function fetchNode (url, responseType) {
       url: url,
       encoding: responseType ? null : 'utf8',
       headers: {
-        'referer': 'https://fimfiction.net/',
-        'cookie': 'view_mature=true',
-        'accept': '*/*'
+        referer: 'https://fimfiction.net/',
+        cookie: 'view_mature=true',
+        accept: '*/*'
       }
     }, (error, response, body) => {
       if (error) {
@@ -46,7 +46,7 @@ export default function fetch (url, responseType) {
         cache: 'default',
         redirect: 'follow',
         headers: {
-          'accept': '*/*' // Fix for not getting webp images from Fimfiction
+          accept: '*/*' // Fix for not getting webp images from Fimfiction
         },
         referrer: window.location.origin
       }).then((response) => {
@@ -61,7 +61,7 @@ export default function fetch (url, responseType) {
         reject(new Error('Error fetching ' + url + ' (' + err + ')'))
       })
     } else {
-      let x = new XMLHttpRequest()
+      const x = new XMLHttpRequest()
       x.withCredentials = true
       x.setRequestHeader('accept', '*/*') // Fix for not getting webp images from Fimfiction
       x.open('get', url, true)
diff --git a/src/fetchRemote.js b/src/fetchRemote.js
index d1de989..91781b3 100644
--- a/src/fetchRemote.js
+++ b/src/fetchRemote.js
@@ -7,25 +7,25 @@ const safariQueue = {}
 
 // messaging with the safari extension global page
 function safariHandler (ev) {
-  let type = ev.message.type
-  let url = ev.message.input
-  let data = ev.message.output // arraybuffer
+  const type = ev.message.type
+  const url = ev.message.input
+  const data = ev.message.output // arraybuffer
   if (!safariQueue[url]) {
     // console.error("Unable to get callback for " + url, JSON.stringify(safariQueue))
     return
   }
-  let cb = safariQueue[url].cb
-  let responseType = safariQueue[url].responseType
+  const cb = safariQueue[url].cb
+  const responseType = safariQueue[url].responseType
   console.log(url, cb, responseType, data)
   delete safariQueue[url]
 
   if (responseType === 'blob') {
-    let blob = new Blob([data], { type: type })
+    const blob = new Blob([data], { type: type })
     cb(blob, type)
   } else {
     if (!responseType) {
-      let blob = new Blob([data], { type: type })
-      let fr = new FileReader()
+      const blob = new Blob([data], { type: type })
+      const fr = new FileReader()
       fr.onloadend = function () {
         cb(fr.result, type)
       }
diff --git a/src/kepubify.js b/src/kepubify.js
index 9166e1d..a74ac68 100644
--- a/src/kepubify.js
+++ b/src/kepubify.js
@@ -46,7 +46,7 @@ function textToSentences (text) {
     .split(/\s*\0/)
 
   for (let i = 0; i < tokenSentences.length; i++) {
-    let s = tokenSentences[i]
+    const s = tokenSentences[i]
     if (s.trim().length === 0) {
       if (i - 1 >= 0) tokenSentences[i - 1] += s
       tokenSentences.splice(i, 1)
@@ -61,15 +61,15 @@ function textToSentences (text) {
 function fixupTree (node, parent) {
   if (node.tag !== '#') {
     if (node.text && !node.tag.match(specialTags)) {
-      let el = et.Element('#')
+      const el = et.Element('#')
       el.text = node.text
       node._children.unshift(el)
       delete node.text
     }
     if (node.tail) {
-      let el = et.Element('#')
+      const el = et.Element('#')
       el.text = node.tail
-      let pos = parent._children.indexOf(node) + 1
+      const pos = parent._children.indexOf(node) + 1
       parent._children.splice(pos, 0, el)
       delete node.tail
     }
@@ -84,11 +84,11 @@ function addSpansToNode (node, parent, state) {
   if (node.tag === '#') {
     state.segment++
 
-    let sentences = textToSentences(node.text)
+    const sentences = textToSentences(node.text)
     let pos
 
     sentences.forEach((sentence) => {
-      let span = createSpan(state.paragraph, state.segment++)
+      const span = createSpan(state.paragraph, state.segment++)
       span.text = sentence
 
       // insert the span before the text node
diff --git a/src/main.js b/src/main.js
index ccc5f8c..a947bfe 100644
--- a/src/main.js
+++ b/src/main.js
@@ -8,15 +8,15 @@ import { saveAs } from 'file-saver'
 import autosize from 'autosize'
 import { htmlToText } from './utils'
 
-m.withAttr = function(attrName, callback, context) {
-	return function(e) {
-		callback.call(context || this, attrName in e.currentTarget ? e.currentTarget[attrName] : e.currentTarget.getAttribute(attrName))
-	}
+m.withAttr = function (attrName, callback, context) {
+  return function (e) {
+    callback.call(context || this, attrName in e.currentTarget ? e.currentTarget[attrName] : e.currentTarget.getAttribute(attrName))
+  }
 }
 
 function blobToDataURL (blob) {
   return new Promise((resolve, reject) => {
-    let fr = new FileReader()
+    const fr = new FileReader()
     fr.onloadend = function (e) { resolve(fr.result) }
     fr.readAsDataURL(blob)
   })
@@ -24,7 +24,7 @@ function blobToDataURL (blob) {
 
 function blobToArrayBuffer (blob) {
   return new Promise((resolve, reject) => {
-    let fr = new FileReader()
+    const fr = new FileReader()
     fr.onloadend = function (e) { resolve(fr.result) }
     fr.readAsArrayBuffer(blob)
   })
@@ -37,23 +37,23 @@ try {
   pageStoryId = document.location.pathname.match(/^\/story\/(\d*)/)[1]
 } catch (e) {}
 
-let logoUrl = chrome.extension.getURL('fimfic2epub-logo.png')
+const logoUrl = chrome.extension.getURL('fimfic2epub-logo.png')
 let ffc
-let stories = document.querySelectorAll('.story_container')
+const stories = document.querySelectorAll('.story_container')
 
 stories.forEach((story) => {
-  let id = story.dataset.story
+  const id = story.dataset.story
   function epubClick (e) {
     e.preventDefault()
     openStory(id)
   }
 
-  let epubButtons = story.querySelectorAll('.drop-down ul li a[title="Download Story (.epub)"]')
+  const epubButtons = story.querySelectorAll('.drop-down ul li a[title="Download Story (.epub)"]')
   if (epubButtons.length === 0) return
   for (let i = 0; i < epubButtons.length; i++) {
     epubButtons[i].addEventListener('click', epubClick, false)
   }
-  let logo = new Image()
+  const logo = new Image()
   logo.className = 'fimfic2epub-logo'
   logo.title = 'Download EPUB with fimfic2epub'
   logo.src = logoUrl
@@ -61,17 +61,17 @@ stories.forEach((story) => {
   logo.addEventListener('click', epubClick, false)
 })
 
-let cards = document.querySelectorAll('.story-card-container')
+const cards = document.querySelectorAll('.story-card-container')
 cards.forEach((card) => {
   let id
-  let classes = card.className.split(' ')
+  const classes = card.className.split(' ')
   for (let i = 0; i < classes.length && !id; i++) {
-    let c = classes[i]
+    const c = classes[i]
     id = c.substring(21)
   }
   if (!id) return
-  let flip = card.querySelector('a.card-flip')
-  let epubButton = card.querySelector('a[title="Download .ePub"]')
+  const flip = card.querySelector('a.card-flip')
+  const epubButton = card.querySelector('a[title="Download .ePub"]')
   if (!epubButton) return
   epubButton.addEventListener('click', function (e) {
     e.preventDefault()
@@ -84,7 +84,7 @@ const dialogContainer = document.createElement('div')
 dialogContainer.id = 'epubDialogContainer'
 document.body.appendChild(dialogContainer)
 
-let checkbox = {
+const checkbox = {
   view: function ({ attrs, children }) {
     return m('label.toggleable-switch', [
       m('input', Object.assign({
@@ -113,10 +113,10 @@ function redraw (arg) {
   }
 }
 
-let ffcProgress = prop(0)
-let ffcStatus = prop('')
+const ffcProgress = prop(0)
+const ffcStatus = prop('')
 
-let dialog = {
+const dialog = {
   oninit () {
     const ctrl = this
 
@@ -173,7 +173,7 @@ let dialog = {
     }
 
     this.setCoverFile = (e) => {
-      let el = e.dom || e.target
+      const el = e.dom || e.target
       if (el.target) {
         this.coverUrl('')
       }
@@ -182,7 +182,7 @@ let dialog = {
 
     this.setSubjects = function () {
       // 'this' is the textarea
-      let set = new Set()
+      const set = new Set()
       ctrl.subjects(this.value.split('\n').map((s) => s.trim()).filter((s) => {
         if (!s) return false
         if (set.has(s)) return false
@@ -200,16 +200,16 @@ let dialog = {
     }
 
     this.ondown = (e) => {
-      let rect = this.el().firstChild.getBoundingClientRect()
-      let offset = { x: e.pageX - rect.left - document.documentElement.scrollLeft, y: e.pageY - rect.top - document.documentElement.scrollTop }
+      const rect = this.el().firstChild.getBoundingClientRect()
+      const offset = { x: e.pageX - rect.left - document.documentElement.scrollLeft, y: e.pageY - rect.top - document.documentElement.scrollTop }
       this.dragging(true)
-      let onmove = (e) => {
+      const onmove = (e) => {
         e.preventDefault()
         if (this.dragging()) {
           this.move(e.pageX - offset.x, e.pageY - offset.y)
         }
       }
-      let onup = () => {
+      const onup = () => {
         this.dragging(false)
         window.removeEventListener('mousemove', onmove)
         window.removeEventListener('mouseup', onup)
@@ -219,8 +219,8 @@ let dialog = {
     }
 
     this.move = (xpos, ypos) => {
-      let bc = document.querySelector('.body_container')
-      let rect = this.el().firstChild.getBoundingClientRect()
+      const bc = document.querySelector('.body_container')
+      const rect = this.el().firstChild.getBoundingClientRect()
       this.xpos(Math.max(0, Math.min(xpos, bc.offsetWidth - rect.width)))
       this.ypos(Math.max(0, Math.min(ypos, bc.offsetHeight - rect.height)))
       this.el().style.left = this.xpos() + 'px'
@@ -228,7 +228,7 @@ let dialog = {
     }
     this.center = () => {
       if (this.dragging()) return
-      let rect = this.el().firstChild.getBoundingClientRect()
+      const rect = this.el().firstChild.getBoundingClientRect()
       this.move(
         Math.max(document.documentElement.scrollLeft, (window.innerWidth / 2) - (rect.width / 2) + document.documentElement.scrollLeft),
         Math.max(document.documentElement.scrollTop, 100 + document.documentElement.scrollTop)
@@ -242,7 +242,7 @@ let dialog = {
   },
 
   view (vnode) {
-    let ctrl = vnode.state
+    const ctrl = vnode.state
     return m('.drop-down-pop-up-container', { oncreate: ctrl.onOpen.bind(ctrl) }, m('.drop-down-pop-up', { style: { 'min-width': '720px' } }, [
       m('h1', { onmousedown: ctrl.ondown }, m('i.fa.fa-book'), 'Export to EPUB (v' + FIMFIC2EPUB_VERSION + ')', m('a.close_button', { onclick: closeDialog })),
       m('.drop-down-pop-up-content', [
@@ -295,7 +295,7 @@ let dialog = {
           ffcProgress() >= 0 ? m('.rating_container',
             m('.rating-bar', { style: { background: 'rgba(0, 0, 0, 0.2)', 'margin-right': '5px' } }, m('.like-bar', { style: { width: Math.max(0, ffcProgress()) * 100 + '%' } })),
             ' ',
-            ffcProgress() >= 0 && ffcProgress() < 1 ? [ m('i.fa.fa-spin.fa-spinner'), m.trust('&nbsp;&nbsp;') ] : null,
+            ffcProgress() >= 0 && ffcProgress() < 1 ? [m('i.fa.fa-spin.fa-spinner'), m.trust('&nbsp;&nbsp;')] : null,
             ffcStatus()
           ) : null,
           m('div', { style: 'clear: both' })
diff --git a/src/subsetFont.js b/src/subsetFont.js
index 0dff316..0788db0 100644
--- a/src/subsetFont.js
+++ b/src/subsetFont.js
@@ -7,8 +7,8 @@ import fileType from 'file-type'
 
 async function subsetFont (fontPath, glyphs, options = {}) {
   let data
-  let fontdata = Buffer.from(fontPath, 'binary')
-  let type = fileType(fontdata)
+  const fontdata = Buffer.from(fontPath, 'binary')
+  const type = fileType(fontdata)
   if (type && type.mime === 'font/ttf') {
     data = fontdata.buffer
   } else {
diff --git a/src/templates.js b/src/templates.js
index 6947235..a590ba4 100644
--- a/src/templates.js
+++ b/src/templates.js
@@ -18,7 +18,7 @@ function nth (d) {
 
 export function prettyDate (d) {
   // format: 27th Oct 2011
-  let months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
+  const months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
   return d.getDate() + nth(d) + ' ' + months[d.getMonth()].substring(0, 3) + ' ' + d.getFullYear()
 }
 
@@ -28,7 +28,7 @@ function chapterBars (chapters, currentChapter = -1, highlightCurrent = false) {
   if (chapters.length <= 1) return null
   let windowSize = 50
   let wordCounts = []
-  let highestWordCount = chapters.reduce((max, ch) => {
+  const highestWordCount = chapters.reduce((max, ch) => {
     wordCounts.push(ch.realWordCount)
     if (ch.realWordCount > max) return ch.realWordCount
     return max
@@ -42,9 +42,9 @@ function chapterBars (chapters, currentChapter = -1, highlightCurrent = false) {
     currentChapter -= start
   }
   wordCounts = wordCounts.map((c) => c / highestWordCount)
-  let barWidth = 9
-  let barSpacing = 2
-  let rowSpacing = 9
+  const barWidth = 9
+  const barSpacing = 2
+  const rowSpacing = 9
   const barCount = Math.min(wordCounts.length, windowSize)
   const rows = Math.floor(wordCounts.length / barCount) + 1
   const rowHeight = 30 + rowSpacing
@@ -68,9 +68,9 @@ function chapterBars (chapters, currentChapter = -1, highlightCurrent = false) {
 }
 
 export function createChapter (ffc, ch, isNotesChapter) {
-  let { content, notes, notesFirst, title, link, linkNotes, index, showHeadings, showDuration, showWordCount } = ch
+  const { content, notes, notesFirst, title, link, linkNotes, index, showHeadings, showDuration, showWordCount } = ch
 
-  let sections = [
+  const sections = [
     m.trust(content || ''),
     notes ? m('div#author_notes', { className: notesFirst ? 'top' : 'bottom' }, [
       m('p', m('b', 'Author\'s Note:')),
@@ -124,7 +124,7 @@ export function createChapter (ffc, ch, isNotesChapter) {
 function sortSpineItems (items) {
   let count = items.length
   for (let i = 0; i < count; i++) {
-    let item = items[i]
+    const item = items[i]
     if (!item) {
       continue
     }
@@ -140,9 +140,9 @@ function sortSpineItems (items) {
 }
 
 export function createOpf (ffc) {
-  let remotes = []
+  const remotes = []
   // let remoteCounter = 0
-  let remoteCache = new Set()
+  const remoteCache = new Set()
   ffc.remoteResources.forEach((r, url) => {
     // remoteCounter++
     if (!ffc.options.includeExternal) {
@@ -176,18 +176,18 @@ export function createOpf (ffc) {
     remotes.push(m('item', { id: r.filename, href: r.dest, 'media-type': r.type }))
   })
 
-  let manifestChapters = ffc.storyInfo.chapters.map((ch, num) =>
+  const manifestChapters = ffc.storyInfo.chapters.map((ch, num) =>
     m('item', { id: 'chapter_' + zeroFill(3, num + 1), href: 'Text/chapter_' + zeroFill(3, num + 1) + '.xhtml', 'media-type': 'application/xhtml+xml', properties: ((ch.remote ? 'remote-resources' : '') + (ffc.options.addChapterBars ? ' svg' : '')).trim() || null })
   )
-  let spineChapters = ffc.storyInfo.chapters.map((ch, num) =>
+  const spineChapters = ffc.storyInfo.chapters.map((ch, num) =>
     m('itemref', { idref: 'chapter_' + zeroFill(3, num + 1) })
   )
-  let manifestNotes = []
-  let spineNotes = []
+  const manifestNotes = []
+  const spineNotes = []
   if (ffc.options.includeAuthorNotes && ffc.options.useAuthorNotesIndex && ffc.hasAuthorNotes) {
     spineNotes.push(m('itemref', { idref: 'notesnav' }))
     ffc.chaptersWithNotes.forEach((num) => {
-      let id = 'note_' + zeroFill(3, num + 1)
+      const id = 'note_' + zeroFill(3, num + 1)
       manifestNotes.push(m('item', { id: id, href: 'Text/' + id + '.xhtml', 'media-type': 'application/xhtml+xml' }))
       spineNotes.push(m('itemref', { idref: id }))
     })
@@ -219,8 +219,8 @@ export function createOpf (ffc) {
       m('manifest', [
         ffc.coverImage ? m('item', { id: 'cover', href: ffc.coverFilename, 'media-type': ffc.coverType, properties: 'cover-image' }) : null,
         m('item', { id: 'ncx', href: 'toc.ncx', 'media-type': 'application/x-dtbncx+xml' }),
-        m('item', { id: 'nav', 'href': 'nav.xhtml', 'media-type': 'application/xhtml+xml', properties: 'nav' + (ffc.options.addChapterBars && ffc.storyInfo.chapters.length > 1 ? ' svg' : '') }),
-        ffc.options.includeAuthorNotes && ffc.options.useAuthorNotesIndex && ffc.hasAuthorNotes ? m('item', { id: 'notesnav', 'href': 'notesnav.xhtml', 'media-type': 'application/xhtml+xml' }) : null,
+        m('item', { id: 'nav', href: 'nav.xhtml', 'media-type': 'application/xhtml+xml', properties: 'nav' + (ffc.options.addChapterBars && ffc.storyInfo.chapters.length > 1 ? ' svg' : '') }),
+        ffc.options.includeAuthorNotes && ffc.options.useAuthorNotesIndex && ffc.hasAuthorNotes ? m('item', { id: 'notesnav', href: 'notesnav.xhtml', 'media-type': 'application/xhtml+xml' }) : null,
 
         m('item', { id: 'style', href: 'Styles/style.css', 'media-type': 'text/css' }),
         m('item', { id: 'coverstyle', href: 'Styles/coverstyle.css', 'media-type': 'text/css' }),
@@ -255,7 +255,7 @@ export function createOpf (ffc) {
 
 function navPoints (list) {
   let playOrder = 1
-  let arr = []
+  const arr = []
   for (let i = 0; i < list.length; i++) {
     if (!list[i]) continue
     arr.push(m('navPoint', { id: 'navPoint-' + (i + 1), playOrder: playOrder++ }, [
@@ -290,7 +290,7 @@ export function createNcx (ffc) {
 }
 
 export function createNav (ffc) {
-  let list = [
+  const list = [
     m('li', m('a', { href: 'Text/cover.xhtml' }, 'Cover')),
     m('li', m('a', { href: 'Text/title.xhtml' }, 'Title Page')),
     ffc.storyInfo.chapters.length > 1 || (ffc.options.includeAuthorNotes && ffc.options.useAuthorNotesIndex && ffc.hasAuthorNotes) ? m('li', m('a', { href: 'nav.xhtml' }, 'Contents')) : null
@@ -299,7 +299,7 @@ export function createNav (ffc) {
       m('a', { href: 'Text/chapter_' + zeroFill(3, num + 1) + '.xhtml' }, ch.title)
     ])
   ))
-  let prettyList = ffc.storyInfo.chapters.map((ch, num) =>
+  const prettyList = ffc.storyInfo.chapters.map((ch, num) =>
     m('li.item', [
       m('.floatbox', m('span.wordcount', ch.realWordCount.toLocaleString('en-GB'))),
       m('a', { href: 'Text/chapter_' + zeroFill(3, num + 1) + '.xhtml' }, ch.title),
@@ -334,8 +334,8 @@ export function createNav (ffc) {
 }
 
 export function createNotesNav (ffc) {
-  let list = ffc.chaptersWithNotes.map((num) => {
-    let ch = ffc.storyInfo.chapters[num]
+  const list = ffc.chaptersWithNotes.map((num) => {
+    const ch = ffc.storyInfo.chapters[num]
     return m('.item', m('a.leftalign', { href: 'Text/note_' + zeroFill(3, num + 1) + '.xhtml' }, ch.title))
   })
 
@@ -362,7 +362,7 @@ export function createNotesNav (ffc) {
 export function createCoverPage (ffc) {
   let body
 
-  let { width, height } = ffc.coverImageDimensions
+  const { width, height } = ffc.coverImageDimensions
 
   if (ffc.coverImage) {
     body = m('svg#cover', { xmlns: NS.SVG, 'xmlns:xlink': NS.XLINK, version: '1.1', viewBox: '0 0 ' + width + ' ' + height },
@@ -402,7 +402,7 @@ function infoBox (heading, data, title) {
 
 function calcReadingTime (ffc, wordCount = 0) {
   const wpm = ffc.options.wordsPerMinute
-  let time = (wordCount || ffc.totalWordCount) / wpm
+  const time = (wordCount || ffc.totalWordCount) / wpm
   let value = 0
   let unit = ''
   if (time < 1) {
diff --git a/src/utils.js b/src/utils.js
index 4b6c940..8670bc8 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -9,13 +9,13 @@ import { unicode } from './constants'
 export function replaceAsync (str, re, callback) {
   // http://es5.github.io/#x15.5.4.11
   str = String(str)
-  let parts = []
+  const parts = []
   let i = 0
   if (Object.prototype.toString.call(re) === '[object RegExp]') {
     if (re.global) { re.lastIndex = i }
     let m
     while ((m = re.exec(str))) {
-      let args = m.concat([m.index, m.input])
+      const args = m.concat([m.index, m.input])
       parts.push(str.slice(i, m.index), callback.apply(null, args))
       i = re.lastIndex
       if (!re.global) { break } // for non-global regexes only take the first match
@@ -45,22 +45,22 @@ export function webp2png (data) {
       webpdecoder = new libwebp.WebPDecoder()
     }
 
-    let frame = WebPRiffParser(data, 0).frames[0]
-    let width = [0]
-    let height = [0]
-    let decodedData = webpdecoder.WebPDecodeRGBA(
+    const frame = WebPRiffParser(data, 0).frames[0]
+    const width = [0]
+    const height = [0]
+    const decodedData = webpdecoder.WebPDecodeRGBA(
       data,
-      frame['src_off'], frame['src_size'],
+      frame.src_off, frame.src_size,
       width, height
     )
 
-    let png = new PNGPacker({})
-    let buffers = []
+    const png = new PNGPacker({})
+    const buffers = []
     png.on('data', (chunk) => {
       buffers.push(chunk)
     })
     png.once('end', () => {
-      let pngData = Buffer.concat(buffers)
+      const pngData = Buffer.concat(buffers)
       resolve(pngData)
     })
     png.pack(decodedData, width[0], height[0])
@@ -122,7 +122,7 @@ export async function readingEase (text, wakeupInterval = Infinity, progresscb)
   let lastTime = Date.now()
 
   for (let i = 0; i < tokenSentences.length; i++) {
-    let now = Date.now()
+    const now = Date.now()
     if (lastTime + wakeupInterval < now) {
       lastTime = now
       if (typeof progresscb === 'function') {
diff --git a/webpack.config.js b/webpack.config.js
index 9cf0a30..27cb356 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -2,7 +2,7 @@
 import path from 'path'
 import nodeExternals from 'webpack-node-externals'
 
-let inProduction = process.env.NODE_ENV === 'production' || process.argv.indexOf('-p') !== -1
+const inProduction = process.env.NODE_ENV === 'production' || process.argv.indexOf('-p') !== -1
 
 const bundleExtensionConfig = {
   entry: {