Build A Library With esbuild

How to bundle ESM, IIFE or CommonJS libraries with esbuild.

May 24, 2021

#javascript #esbuild #tutorial #typescript

Photo by Colin Watts on Unsplash

I recently developed plugins and, migrated all the utilities of DeckDeckGo to build these with esbuild.

If you are looking to do the same, hope this tutorial helps you get started!


esbuild is “an extremely fast JavaScript bundler” made by Evan Wallace. It is its tagline and, according my tests, the least we can say is that it is true. It is blazing fast ⚡️.

Sometimes while migrating my libraries, I even found myself waiting for the end of a build because I did not notice that it was already finished. I assumed it would still need some more time, old habits die hard I guess 😅.

In addition, other things which make me really like this new bundler are its clean, flexible API and, its documentation. It is easy to follow and, clear.


To get started, let’s create a new empty project.

mkdir mylib && cd mylib && npm init --yes

You can use esbuild to bundle libraries from vanilla JavaScript source files but, at least in this tutorial, we are going to use TypeScript too. That’s why, in addition to the bundler, we also install it and rimraf, to remove the output folder before any new build.

npm i esbuild typescript rimraf --save-dev

At the root of our project, we create a ./tsconfig.json file to indicates that the directory is the root of a TypeScript project.

{ "compilerOptions": { "declaration": true, "target": "esnext", "lib": ["esnext", "dom"], "strict": true, "noImplicitAny": false, "esModuleInterop": true, "moduleResolution": "node", "outDir": "lib" } }

In the above configuration, I set esnext to target the most recent ES standard and, also set declaration to true . esbuild not generating the declarations, we are going to generate these using the tsc command.

We update our ./package.json with a script to build our library and, we define a new types entry which should point to the types declarations.

{ "name": "mylib", "version": "1.0.0", "description": "", "main": "lib/index.js", "types": "lib/index.d.ts", "scripts": { "ts-types": " tsc --emitDeclarationOnly --outDir lib", "build": "rimraf lib && node ./esbuild.js && npm run ts-types" }, "keywords": [], "author": "", "license": "ISC", "devDependencies": { "esbuild": "^0.12.1", "typescript": "^4.2.4" } }

The configuration and, build using esbuild is going to be developed in following chapters in a dedicated file, ./esbuild.js . That’s why, at this point, we leave it empty.

touch esbuild.js

It is worth to notice that the esbuild commands can be inlined inside the scripts tag of the package.json but, I personally like to handle it separately.

That’s useful when the scripts evolve or, when multiple miscellaneous builds and, steps are performed.

Finally, we add some source code to be compiled, such as following sample function, in a main new entry point file ./src/index.ts .

export const add = (a: number, b: number): number => a + b;

ECMAScript module

The esm format stands for "ECMAScript module". It assumes the environment supports import and export syntax (documentation).

To bundle such a modern library, we can add the following configuration to our ./esbuild.js :

const esbuild = require("esbuild"); esbuild .build({ entryPoints: ["src/index.ts"], outdir: "lib", bundle: true, sourcemap: true, minify: true, splitting: true, format: "esm", target: ["esnext"] }) .catch(() => process.exit(1));

That’s already it 🥳.

If we run npm run build in our project, the library is bundled to esm , a source map is generated and, the TypeScript declarations as well.

“But, what’s happening here” you may ask yourself? Therefore, here some context:

In the script we first require esbuild and, with the help of the method .build , we run an async compilation. It is also possible to perform this step synchronously and, get the results (I will develop this in a following chapter).

To perform the operation, we set following options:

  • entryPoints and ourdir defines which files need to be bundled to which output
  • bundle means to inline any imported dependencies into the file itself. This process is recursive so dependencies of dependencies (and so on) will also be inlined (documentation). In other words, if you have got import in your entryPoints , bundle will resolve these to add their code in the results.
  • sourcemap if set to true , generates source map files next to your JavaScript outcome
  • minify makes the code smaller (documentation)
  • splitting is a work in progress (at the time I write these lines) which improves the code sharing between multiple endpoints (see documentation)
  • format is set to esm as it is the goal in this chapter (documentation)
  • target defines which types of JavaScript we want to output. In our case, only the most recent version (documentation)

Module Field

In above chapter we are generating an esm library. If you aim to use this tutorial to create an effective bundle, I suggest adding a module entry in our package.json . Even though not officially documented, this entry is useful for such types of library (see Stackoverflow).

"main": "lib/index.js", "module": "lib/index.js", "types": "lib/index.d.ts",


The iife format stands for "immediately-invoked function expression" and is intended to be run in the browser (documentation).

If you rather like or, are in need to create library which is immediately available, it can be done by removing from the previous configuration both format and splitting .

The iife format is the default format unless we set platform to node (as in next chapter). splitting is only available for esm modules.

const esbuild = require("esbuild"); esbuild .build({ entryPoints: ["src/index.ts"], outdir: "lib", bundle: true, sourcemap: true, minify: true, target: ["esnext"] }) .catch(() => process.exit(1));

CommonJS — Node

The cjs format stands for "CommonJS" and is intended to be run in node (documentation).

If your library aims to be used in a Node or, in a non-browser environment, it can be bundled for such purpose with a related platform option.

const esbuild = require("esbuild"); esbuild .build({ entryPoints: ["src/index.ts"], outdir: "lib", bundle: true, sourcemap: true, minify: true, platform: "node", target: ["node10.4"] }) .catch(() => process.exit(1));

In this configuration we define node as platform and, set target to Node version 10 (documentation).

Synchronous Build

Above builds are asynchronous. You might want to run a synchronous builds to either get information on the process or, do something with the output without writing it to the filesystem.

This can be achieved by replacing the method .build with .buildSync .


To get to know if there were errors, or warnings, we can call buildSync . It will return an object which contains such information.

const esbuild = require("esbuild"); const results = esbuild.buildSync({ entryPoints: ["src/index.ts"], outdir: "lib", bundle: true, sourcemap: true, minify: true, splitting: true, format: "esm", target: ["esnext"] }); console.log(results); // Output: // { errors: [], warnings: [] }

In Memory Results

To get the files that would have been written as in-memory buffers, we can leverage the option write(documentation). For each entryPoints, esbuild will answer with a related outputFiles entry in an array of results.

For example, if we would like to inline our script in an HTML file, we would be able to get these results and, parse it manually to the output of our choice.

const esbuild = require("esbuild"); const { readFile, writeFile, mkdir } = require("fs").promises; (async () => { await mkdir("./lib"); const script = esbuild.buildSync({ entryPoints: ["src/index.ts"], bundle: true, minify: true, format: "esm", target: ["esnext"], write: false }); const html = await readFile("src/index.html", "utf8"); await writeFile( "lib/index.html", `<script type="module">${script.outputFiles[0].text}</script>${html}` ); })();

A bit out of the scope of this article but, to demonstrate how flexible esbuild is, we would be able to install a html minifier to get even more fancier.

npm i html-minifier-terser --save-dev

Once added to our project, we would be able to minify the resulting HTML and, are almost already ready to turn this small library in a build pipeline for modern application 😱.

const esbuild = require('esbuild'); const {readFile, writeFile, mkdir} = require('fs').promises; const minify = require('html-minifier-terser').minify; (async () => { await mkdir('./lib'); const script = esbuild .buildSync({ entryPoints: ['src/index.ts'], bundle: true, minify: true, format: 'esm', target: ['esnext'], write: false }); const html = await readFile('src/index.html', 'utf8');¨ const minifyOptions = { collapseWhitespace: true, keepClosingSlash: true, removeComments: true, removeRedundantAttributes: true, removeScriptTypeAttributes: true, removeStyleLinkTypeAttributes: true, useShortDoctype: true, minifyCSS: true }; await writeFile( 'lib/index.html', `<script>${script.outputFiles[0].text}</script>${await minify(html, minifyOptions)}` ); })();


esbuild is slick 🤙.

To infinity and beyond!