Compiling and bundling JavaScript, the painless way

Michele Riva

Michele Riva

Senior Software Architect @NearForm

Google Developer Expert

Microsoft MVP

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Real-World Next.js

Build scalable, high performances and modern web applications using Next.js, the React framework for production

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Compiling and bundling JavaScript

is (often) a pain*

*but it shouldn't

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A bit of terminology: compiling

to change a computer program into a machine language

Cambridge Dictionary

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A bit of terminology: compiling transpiling

to translate a source code into a different language source code

Me, myself

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A bit of terminology: bundling

to encapsulate code and resources into a single executable file

Me, myself

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Why do we want to transpile our code?

To make it compatible with different platforms
To write our scripts in different languages
To adopt new language features
To run large-scale refactors

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Scala.js

https://www.scala-js.org

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ReasonML

ReScript

F# (via Fable)

Gleam

Elm

Kotlin

Nim, Haxe, C/C++ (via Emscripten), ClojureScript, Dart, PureScript, Haskell (via GHCJS)

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LLVM

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function toUpper(x) {
  return x.toUpperCase();
}

function addExclamationMarks(x) {
  return x + '!!!'
}

function scream(input) {
  return input
    |> toUpper
    |> addExclamationMarks
    |> console.log
}

scream('Hello, Serbia');
// HELLO, SERBIA!!!

"use strict";

function toUpper(x) {
  return x.toUpperCase();
}

function addExclamationMarks(x) {
  return x + '!!!';
}

function scream(input) {
  var _ref, _ref2, _input;

  return _ref = (
    _ref2 = (_input = input, toUpper(_input)),
    addExclamationMarks(_ref2)
  ), console.log(_ref);
}

scream('Hello, Serbia');
// HELLO, SERBIA!!!

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import { VFC } from 'react';

enum UserType {
  ADMIN     = "admin",
  EDITOR    = "editor",
  USER      = "user",
  ANONYMOUS = "guest"
}

type MyProps = {
  userType: UserType;
}

const MyComponent: VFC<MyProps> = ({ userType }) => {
  return (
    <div>
      User is of type: {props.userType}
    </div>
  )
}

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
var UserType;

(function (UserType) {
  UserType["ADMIN"] = "admin";
  UserType["EDITOR"] = "editor";
  UserType["USER"] = "user";
  UserType["ANONYMOUS"] = "guest";
})(UserType || (UserType = {}));

const MyComponent = ({ userType }) => {
  return /*#__PURE__*/ React.createElement(
    "div",
    null,
    "User is of type: ",
    props.userType
  );
};

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Why do we want to bundle our code?

To create a single executable file
To serve a single JS file over the net

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ReadArticle.jsx

NewArticle.jsx

AuthorProfile.jsx

BundledPage.js

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in depths

transpiling

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Language-to-language

ClojureScript

(defn simple-component []
  [:div
   [:p "I am a component!"]
   [:p.someclass
    "I have " [:strong "bold"]
    [:span {:style {:color "red"}} " and red "] "text."]])

cljs.user.simple_component = (function cljs$user$simple_component(){
  return new cljs.core.PersistentVector(null, 3, 5, cljs.core.PersistentVector.EMPTY_NODE,
  [new cljs.core.Keyword(null,"div","div",(1057191632)),
  new cljs.core.PersistentVector(null, 2, 5, cljs.core.PersistentVector.EMPTY_NODE,
  [new cljs.core.Keyword(null,"p","p",(151049309)),"I am a component!"], null),
  new cljs.core.PersistentVector(null, 5, 5, cljs.core.PersistentVector.EMPTY_NODE,
  [new cljs.core.Keyword(null,"p.someclass","p.someclass",(-1904646929)),
  "I have ",new cljs.core.PersistentVector(null, 2, 5, cljs.core.PersistentVector.EMPTY_NODE, 
  [new cljs.core.Keyword(null,"strong","strong",(269529000)),"bold"], null),
  new cljs.core.PersistentVector(null, 3, 5, cljs.core.PersistentVector.EMPTY_NODE, 
  [new cljs.core.Keyword(null,"span","span",(1394872991)),
  new cljs.core.PersistentArrayMap(null, 1, [new cljs.core.Keyword(null,"style","style",(-496642736)),
  new cljs.core.PersistentArrayMap(null, 1, [
  new cljs.core.Keyword(null,"color","color",(1011675173)),"red"]
  , null)], null)," and red "], null),"text."], null)], null);
});

https://reagent-project.github.io

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[@bs.config {jsx: 3}];

module Greeting = {
  [@react.component]
  let make = () => {
    <button> {React.string("Hello!")} </button>
  };
};

ReactDOMRe.renderToElementWithId(<Greeting />, "preview");

// Generated by BUCKLESCRIPT, PLEASE EDIT WITH CARE
'use strict';

var React = require("react");
var ReactDOMRe = require("./stdlib/reactDOMRe.js");

function _none_$Greeting(Props) {
  return React.createElement("button", undefined, "Hello!");
}

var Greeting = {
  make: _none_$Greeting
};

ReactDOMRe.renderToElementWithId(React.createElement(_none_$Greeting, { }), "preview");

exports.Greeting = Greeting;
/*  Not a pure module */

https://reasonml.github.io

Language-to-language

ReasonML

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Language-to-language

TypeScript

import { VFC } from 'react';

enum UserType {
  ADMIN     = "admin",
  EDITOR    = "editor",
  USER      = "user",
  ANONYMOUS = "guest"
}

type MyProps = {
  userType: UserType;
}

const MyComponent: VFC<MyProps> = ({ userType }) => {
  return (
    <div>
      User is of type: {props.userType}
    </div>
  )
}

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
var UserType;

(function (UserType) {
  UserType["ADMIN"] = "admin";
  UserType["EDITOR"] = "editor";
  UserType["USER"] = "user";
  UserType["ANONYMOUS"] = "guest";
})(UserType || (UserType = {}));

const MyComponent = ({ userType }) => {
  return /*#__PURE__*/ React.createElement(
    "div",
    null,
    "User is of type: ",
    props.userType
  );
};

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Every language has its own transpiler

TypeScript TSC

Fable

ClojureScript

BuckleScript

JavaScript (Babel)

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Problem #1

transpilation time

Really fast

Average

Fast

Slow on large codebases

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Problem #2

optimized output

Beautifully optimized

Awful

Beautifully optimized

Quite optimized

Well optimized*

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Let's focus on the most popular ones

Quite slow, quite optimized

Quite fast, well optimized*

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Bundling time grows

Quite slow, quite optimized

Quite fast, well optimized

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How does a transpiler work?

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Parsing

Transformation

Codegen

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Parsing

Step 1

Tokenization

var foo = 10

var
foo
=
10

input

tokens

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Parsing

Step 2

Syntactic Analysis

var foo = 10

input

parse tree
(aka concrete syntax tree)

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Parsing

Step 2

Syntactic Analysis

var foo = 10

input

       variableDeclaration
               |
               |
       vairiableDeclarator
          /         \
         /           \
   Identifier   NumericLiteral

abstract syntax tree (AST)

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{
   "body":[
      {
         "type":"VariableDeclaration",
         "declarations":[
            {
               "type":"VariableDeclarator",
               "id":{
                  "type":"Identifier",
                  "name":"foo",
                  "loc":{
                     "identifierName":"foo"
                  }
               },
               "init":{
                  "type":"NumericLiteral",
                  "extra":{
                     "rawValue":10,
                     "raw":"10"
                  },
                  "value":10
               }
            }
         ],
         "kind":"var"
      }
   ]
}

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export default function () {
  return {
    visitor: {
      VariableDeclaration(path) {
        console.log({ VariableDeclaration: path.node });
        // { type: "VariableDeclaration", kind: "var", ... }
      },
      Identifier(path) {
        console.log({ Identifier: path.node });
        // { type: "Identifier", name: "foo", ... }
      },
      NumericLiteral(path) {
        console.log({ NumericLiteral: path.node });
        // { type: "NumericLiteral", value: 10, ... }
      }
    }
  };
}

Traversing the AST

Babel example implementing the visitor pattern

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export default function() {  
  return {
    visitor: {
      VariableDeclaration(path) {
        if (path.node.kind === "var") {
          path.node.kind = "let"
        }
      }
    }
  };
}

Transforming

Babel example

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Codegen

Babel example

input

output

var foo = 10
const bar = true

let foo = 10;
const bar = true;

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ESLint

jscodeshift

Prettier

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ESLint

export default function(context) {
  return {
    TemplateLiteral(node) {
      context.report({
        node,
        message: 'Do not use template literals',

        fix(fixer) {
          if (node.expressions.length) {
            // Can't auto-fix template literal with expressions
            return;
          }
          
          return [
            fixer.replaceTextRange([node.start, node.start + 1], '"'),
            fixer.replaceTextRange([node.end - 1, node.end], '"'),
          ];
        },
      });
    }
  };
};

Example code taken from astexplorer.net

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ESLint

const myString = `Hello, World!`;

const myString = "Hello, World!";

input

output

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ttypescript (https://github.com/cevek/ttypescript)

export default function (program) {
  const checker = program.getTypeChecker();
  return (context) => {
    return (sourceFile) => {
      const visitor = (node) => {
        // This branch evaluates '2 + 2' like expressions and replaces the node with the result (in this case '4')
        if (ts.isBinaryExpression(node)) {
          if (ts.isNumericLiteral(node.left) && ts.isNumericLiteral(node.right)) {
            // We could parse `node.text` as a number, or we can use the typechecker to get type info for nodes
            const lhs = checker.getTypeAtLocation(node.left);
            const rhs = checker.getTypeAtLocation(node.right);

            switch (node.operatorToken.kind) {
              case ts.SyntaxKind.PlusToken:
                return context.factory.createNumericLiteral(lhs.value + rhs.value);
            }
          }
        }
        // 
        if (ts.isIdentifier(node) && node.text === 'printTips' || node.text === 'tips') {
            return context.factory.createIdentifier(node.text.split('').reverse().join(''));
        }
        return ts.visitEachChild(node, visitor, context);
      };
      return ts.visitNode(sourceFile, visitor);
    };
  };
};

Example code taken from https://github.com/cevek/ttypescript#program

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ttypescript (https://github.com/cevek/ttypescript)

input

output

const mySum = 10 + 20;

const mySum = 30;

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export const parser = "babel";

export default function transformer(file, api) {
  const j = api.jscodeshift;

  return j(file.source)
    .find(j.Identifier)
    .forEach((path) => {
      if (j(path).get().value.name.includes("oldName")) {
        j(path).replaceWith(
          j.identifier(path.node.name.replace("oldName", "newName"))
        );
      }
    })
    .toSource();
}

jscodeshift

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jscodeshift

const oldNameFactory = () => {/* */};

const newNameFactory = () => {/* */};

input

output

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Desired output

input

output

import MyHeader from 'components/MyHeader';

export function MyApp(props) {
  return (
    <div>
      <MyHeader {...props.headerProps} />
      <p> Hello, {props.name}!</p> 
    </div>
  )
}

export function MyApp(props) {
  return (
    <div>
      <p> Hello, {props.name}!</p> 
    </div>
  )
}

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export const parser = 'babel'

export default function transformer(file, api) {
  const j = api.jscodeshift;

  const withoutElement = j(file.source)
    .find(j.JSXElement)
    .forEach(function (path) {
      if (path.value.openingElement.name.name === "MyHeader") {
        path.prune();
      }
    })
    .toSource();

  const withoutImport = j(withoutElement)
    .find(j.ImportDefaultSpecifier)
    .forEach(function (path) {
      if (path.value.local.name === "MyHeader") {
        path.parentPath.parentPath.prune();
      }
    })
    .toSource();

  return withoutImport;
};

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const user = {
  data: {
    name: {
      first: "Michele",
      last: "Riva"
    }
  }
}

const middleName = user.data
  && user.data.name
  && user.data.name.middle
  || "No middle name";

console.log(middleName);

Node 12.x

No middle name

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const user = {
  data: {
    name: {
      first: "Michele",
      last: "Riva"
    }
  }
}

const middleName = user.data?.name?.middle ?? "No middle name";

console.log(middleName);

Node 12.x

SyntaxError: Unexpected token '.'

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const user = {
  data: {
    name: {
      first: "Michele",
      last: "Riva"
    }
  }
}

const middleName = user.data?.name?.middle ?? "No middle name";

console.log(middleName);

Node >14.x

"No middle name"

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import get from 'lodash/get';

const user = {
  data: {
    name: {
      first: "Michele",
      last: "Riva"
    }
  }
}

const middleName = get(user, "data.name.middle", "No middle name");

console.log(middleName);

"No middle name"

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import reverse from 'lodash/reverse';
import isString from 'lodash/isString';
import isFunction from 'lodash/isFunction';
import isNull from 'lodash/isNull';
import split from 'lodash/split';
import filter from 'lodash/filter';
import map from 'lodash/map';
import keys from 'lodash/keys';

// and so on...

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import reverse from 'lodash/reverse';
// Array.reverse()

import isString from 'lodash/isString';
// typeof

import isFunction from 'lodash/isFunction';
// typeof

import isNull from 'lodash/isNull';
// typeof

import split from 'lodash/split';
// Array.split()

import filter from 'lodash/filter';
// Array.filter

import map from 'lodash/map';
// Array.map

import keys from 'lodash/keys';
// Object.keys

// and so on...

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What if we have thousands of files to refactor?

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in depths

bundling

WebPack

Parcel

Rollup

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Hard

Easier

Easiest

WebPack

Parcel

Rollup

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Hard

Easier

Easiest

Slowest

Slower

Fast

WebPack

Parcel

Rollup

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king of configurations

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king of configurations

Is it still worth it?

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ESBuild

SWC

Vite

Snowpack

is there any better alternative?

(rest in pepperoni)

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esbuild src/myEntry.js --bundle --sourcemap --minify --outfile=dist/mybundle.js

ESBuild

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ES2019

ES2020

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "jsx": false,
      "dynamicImport": false,
      "privateMethod": false,
      "functionBind": false,
      "exportDefaultFrom": false,
      "exportNamespaceFrom": false,
      "decorators": false,
      "decoratorsBeforeExport": false,
      "topLevelAwait": false,
      "importMeta": false
    },
    "transform": null,
    "target": "es5",
    "loose": false,
    "externalHelpers": false,
    // Requires v1.2.50 or upper and requires target to be es2016 or upper.
    "keepClassNames": false
  }
}

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SWC can run in a browser thanks to WASM

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Vite

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Vite

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Vite

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// ESM
export default function greet() {
  return "Hello Serbia!";
}

// ESM
import foo, { bar } from "foobar";

// CJS
module.exports = function greet() {
  return "Hello Serbia!";
}

// CJS
const foo, { bar } = require("foobar");

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Vite

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rest in pepperoni

Snowpack

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Snowpack

rest in pepperoni

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Snowpack

rest in pepperoni

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/*
 * Skypack CDN - canvas-confetti@1.4.0
 *
 * Learn more:
 *   📙 Package Documentation: https://www.skypack.dev/view/canvas-confetti
 *   📘 Skypack Documentation: https://www.skypack.dev/docs
 *
 * Pinned URL: (Optimized for Production)
 *   ▶️ Normal: https://cdn.skypack.dev/pin/canvas-confetti@v1.4.0-POmgSMO0U5q84otJfYlN/mode=imports/optimized/canvas-confetti.js
 *   ⏩ Minified: https://cdn.skypack.dev/pin/canvas-confetti@v1.4.0-POmgSMO0U5q84otJfYlN/mode=imports,min/optimized/canvas-confetti.js
 *
 */

// Browser-Optimized Imports (Don't directly import the URLs below in your application!)
export * from '/-/canvas-confetti@v1.4.0-POmgSMO0U5q84otJfYlN/dist=es2020,mode=imports/optimized/canvas-confetti.js';
export {default} from '/-/canvas-confetti@v1.4.0-POmgSMO0U5q84otJfYlN/dist=es2020,mode=imports/optimized/canvas-confetti.js';

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The future is no-bundle

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The future is no-bundle*

*maybe

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The future is bright

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https://kdy1.dev/posts/2022/1/tsc-go

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@MicheleRiva

@MicheleRivaCode

/in/MicheleRiva95

www.micheleriva.dev