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Reduce Docusaurus JS bundle Pt1 - Why is Brainfuck in my JS bundle

· 3 min read
Roger Jiang

In this series, we continue to fix the poorly optimized client js bundles that are needlessly large and significantly add to total blocking time.

I discovered a quirk with prism-react-renderer, where if you do not add to the optional languages config - there is apparently no code-splitting for the base language support - and it ends up injecting every language it supports (regardless of whether it appears in your site) into the final production bundle. Just search for "fuck" in your bundle - and if "brainfuck" exists - you know you got the same problem!

TL;DR - You must add ONE valid prism.languages:string[] to themeConfig (EVEN if you don't need it!)​

initial docusaurus.config.js - looks fine, makes sense - but for some unclear reason, a ton of pointless boilerplate config settings for prism-renderer end up in the final bundle. Including obscure novelty languages such as BrainFuck of all things..!

themeConfig:{
  prism: {
    theme: require("prism-react-renderer/themes/github").default,
    darkTheme: require("prism-react-renderer/themes/dracula").default,
  },
}
Code I extracted from production main.js bundle - containing an overwhelming number of languages that were not used on this site.
e.exports&&(e.exports={core:{meta:{path:"components/prism-core.js",option:"mandatory"},core:"Core"},themes:{meta:{path:"themes/{id}.css",link:"index.html?theme={id}",exclusive:!0},prism:{title:"Default",option:"default"},"prism-dark":"Dark","prism-funky":"Funky","prism-okaidia":{title:"Okaidia",owner:"ocodia"},"prism-twilight":{title:"Twilight",owner:"remybach"},"prism-coy":{title:"Coy",owner:"tshedor"},"prism-solarizedlight":{title:"Solarized Light",owner:"hectormatos2011 "},"prism-tomorrow":{title:"Tomorrow Night",owner:"Rosey"}},languages:{meta:{path:"components/prism-{id}",noCSS:!0,examplesPath:"examples/prism-{id}",addCheckAll:!0},markup:{title:"Markup",alias:["html","xml","svg","mathml","ssml","atom","rss"],aliasTitles:{html:"HTML",xml:"XML",svg:"SVG",mathml:"MathML",ssml:"SSML",atom:"Atom",rss:"RSS"},option:"default"},css:{title:"CSS",option:"default",modify:"markup"},clike:{title:"C-like",option:"default"},javascript:{title:"JavaScript",require:"clike",modify:"markup",optional:"regex",alias:"js",option:"default"},abap:{title:"ABAP",owner:"dellagustin"},abnf:{title:"ABNF",owner:"RunDevelopment"},actionscript:{title:"ActionScript",require:"javascript",modify:"markup",owner:"Golmote"},ada:{title:"Ada",owner:"Lucretia"},agda:{title:"Agda",owner:"xy-ren"},al:{title:"AL",owner:"RunDevelopment"},antlr4:{title:"ANTLR4",alias:"g4",owner:"RunDevelopment"},apacheconf:{title:"Apache Configuration",owner:"GuiTeK"},apex:{title:"Apex",require:["clike","sql"],owner:"RunDevelopment"},apl:{title:"APL",owner:"ngn"},applescript:{title:"AppleScript",owner:"Golmote"},aql:{title:"AQL",owner:"RunDevelopment"},arduino:{title:"Arduino",require:"cpp",alias:"ino",owner:"dkern"},arff:{title:"ARFF",owner:"Golmote"},armasm:{title:"ARM Assembly",alias:"arm-asm",owner:"RunDevelopment"},arturo:{title:"Arturo",alias:"art",optional:["bash","css","javascript","markup","markdown","sql"],owner:"drkameleon"},asciidoc:{alias:"adoc",title:"AsciiDoc",owner:"Golmote"},aspnet:{title:"ASP.NET (C#)",require:["markup","csharp"],owner:"nauzilus"},asm6502:{title:"6502 Assembly",owner:"kzurawel"},asmatmel:{title:"Atmel AVR Assembly",owner:"cerkit"},autohotkey:{title:"AutoHotkey",owner:"aviaryan"},autoit:{title:"AutoIt",owner:"Golmote"},avisynth:{title:"AviSynth",alias:"avs",owner:"Zinfidel"},"avro-idl":{title:"Avro IDL",alias:"avdl",owner:"RunDevelopment"},awk:{title:"AWK",alias:"gawk",aliasTitles:{gawk:"GAWK"},owner:"RunDevelopment"},bash:{title:"Bash",alias:["sh","shell"],aliasTitles:{sh:"Shell",shell:"Shell"},owner:"zeitgeist87"},basic:{title:"BASIC",owner:"Golmote"},batch:{title:"Batch",owner:"Golmote"},bbcode:{title:"BBcode",alias:"shortcode",aliasTitles:{shortcode:"Shortcode"},owner:"RunDevelopment"},bbj:{title:"BBj",owner:"hyyan"},bicep:{title:"Bicep",owner:"johnnyreilly"},birb:{title:"Birb",require:"clike",owner:"Calamity210"},bison:{title:"Bison",require:"c",owner:"Golmote"},bnf:{title:"BNF",alias:"rbnf",aliasTitles:{rbnf:"RBNF"},owner:"RunDevelopment"},bqn:{title:"BQN",owner:"yewscion"},brainfuck:{title:"Brainfuck",owner:"Golmote"},brightscript:{title:"BrightScript",owner:"RunDevelopment"},bro:{title:"Bro",owner:"wayward710"},bsl:{title:"BSL (1C:Enterprise)",alias:"oscript",aliasTitles:{oscript:"OneScript"},owner:"Diversus23"},c:{title:"C",require:"clike",owner:"zeitgeist87"},csharp:{title:"C#",require:"clike",alias:["cs","dotnet"],owner:"mvalipour"},cpp:{title:"C++",require:"c",owner:"zeitgeist87"},cfscript:{title:"CFScript",require:"clike",alias:"cfc",owner:"mjclemente"},chaiscript:{title:"ChaiScript",require:["clike","cpp"],owner:"RunDevelopment"},cil:{title:"CIL",owner:"sbrl"},cilkc:{title:"Cilk/C",require:"c",alias:"cilk-c",owner:"OpenCilk"},cilkcpp:{title:"Cilk/C++",require:"cpp",alias:["cilk-cpp","cilk"],owner:"OpenCilk"},clojure:{title:"Clojure",owner:"troglotit"},cmake:{title:"CMake",owner:"mjrogozinski"},cobol:{title:"COBOL",owner:"RunDevelopment"},coffeescript:{title:"CoffeeScript",require:"javascript",alias:"coffee",owner:"R-osey"},concurnas:{title:"Concurnas",alias:"conc",owner:"jasontatton"},csp:{title:"Content-Security-Policy",owner:"ScottHelme"},cooklang:{title:"Cooklang",owner:"ahue"},coq:{title:"Coq",owner:"RunDevelopment"},crystal:{title:"Crystal",require:"ruby",owner:"MakeNowJust"},"css-extras":{title:"CSS 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(Graphviz)",alias:"gv",optional:"markup",owner:"RunDevelopment"},ebnf:{title:"EBNF",owner:"RunDevelopment"},editorconfig:{title:"EditorConfig",owner:"osipxd"},eiffel:{title:"Eiffel",owner:"Conaclos"},ejs:{title:"EJS",require:["javascript","markup-templating"],owner:"RunDevelopment",alias:"eta",aliasTitles:{eta:"Eta"}},elixir:{title:"Elixir",owner:"Golmote"},elm:{title:"Elm",owner:"zwilias"},etlua:{title:"Embedded Lua templating",require:["lua","markup-templating"],owner:"RunDevelopment"},erb:{title:"ERB",require:["ruby","markup-templating"],owner:"Golmote"},erlang:{title:"Erlang",owner:"Golmote"},"excel-formula":{title:"Excel Formula",alias:["xlsx","xls"],owner:"RunDevelopment"},fsharp:{title:"F#",require:"clike",owner:"simonreynolds7"},factor:{title:"Factor",owner:"catb0t"},false:{title:"False",owner:"edukisto"},"firestore-security-rules":{title:"Firestore security 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Configure",owner:"dev-itsheng"},keyman:{title:"Keyman",owner:"mcdurdin"},kotlin:{title:"Kotlin",alias:["kt","kts"],aliasTitles:{kts:"Kotlin Script"},require:"clike",owner:"Golmote"},kumir:{title:"KuMir (КуМир)",alias:"kum",owner:"edukisto"},kusto:{title:"Kusto",owner:"RunDevelopment"},latex:{title:"LaTeX",alias:["tex","context"],aliasTitles:{tex:"TeX",context:"ConTeXt"},owner:"japborst"},latte:{title:"Latte",require:["clike","markup-templating","php"],owner:"nette"},less:{title:"Less",require:"css",optional:"css-extras",owner:"Golmote"},lilypond:{title:"LilyPond",require:"scheme",alias:"ly",owner:"RunDevelopment"},liquid:{title:"Liquid",require:"markup-templating",owner:"cinhtau"},lisp:{title:"Lisp",alias:["emacs","elisp","emacs-lisp"],owner:"JuanCaicedo"},livescript:{title:"LiveScript",owner:"Golmote"},llvm:{title:"LLVM IR",owner:"porglezomp"},log:{title:"Log 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Notebook"},owner:"msollami"},wren:{title:"Wren",owner:"clsource"},xeora:{title:"Xeora",require:"markup",alias:"xeoracube",aliasTitles:{xeoracube:"XeoraCube"},owner:"freakmaxi"},"xml-doc":{title:"XML doc (.net)",require:"markup",modify:["csharp","fsharp","vbnet"],owner:"RunDevelopment"},xojo:{title:"Xojo (REALbasic)",owner:"Golmote"},xquery:{title:"XQuery",require:"markup",owner:"Golmote"},yaml:{title:"YAML",alias:"yml",owner:"hason"},yang:{title:"YANG",owner:"RunDevelopment"},zig:{title:"Zig",owner:"RunDevelopment"}}

Adding a random language, empty string or empty array will not work​

Prism needs to resolve the specific module to the specified language - and it appears that ONLY upon doing that, does it NOT result in pushing every language config to the production bundle.

Undefined or empty array is same as default, results in same massive bloat.

If you add in a random non-existent language (or one not pre-defined to prism) the build will throw upon failing to resolve that language's module
Error: Cannot find module './prism-dsfkjhsdlkjfhsldkf'
    at Array.forEach (<anonymous>)
The build will throw if supplied an empty string in an array or empty array for languages
[ERROR] ValidationError: "prism.additionalLanguages[0]" is not allowed to be emp
ty

Final bundle reduced by 25KB​

after adding in one random additional language
prism: {
  additionalLanguages: ["latex"],
  ...
}

So adding just one minor line, to add an extra language you probably don't need, cuts down the bundle by just over 25KB!

There's definitely some flawed code-splitting logic somewhere in the code, but there's no need to dig further for now.

On that note, there's also some flawed module loading logic by Docusaurus - there's no need to load Prism/CodeBlock libraries when fetching bundles for the homepage (which doesn't have any CodeBlock elements). This is expecially true for any main bundles that directly contribute to TBT lighthouse scores - which impact the perceived loading speed for users visiting your docusaurus site.