JS library compilation to browser, esm and cjs using esbuild

This is a quick post, public but mostly for myself about how to do simple JS library compilation and bundling with esbuild.

I have a few JS libraries that are shipped via npm, and can be used with an ESM import, CommonJS require, or a browser script element. I've found it pretty tricky in the past working out how to compile to all of these correctly from my ESM source, and the internet hasn't been very helpful either.

What I want is a setup that will produce all three of those with no huge legwork, handle bundling, sourcemaps, minification for me, convert modern syntax to es6, and have a dev mode that will re-compile on source change.

Esbuild is ideal for this circumstance as it has a simple API, all of those features and is extremely fast! I'm not saying this is the ideal setup but it works pretty well for me.

build.js

This is the build script, called with node build.js for a production build and node build.js --watch for dev mode. Pop these in your package.json scripts if you like.

You define the formats you want and the esbuild options within buildAll, I find this works well in most instances. This will get src/index.js and src/script.js (for browsers, imports index and runs it) and output to the dist/ directory.

import esbuild from 'esbuild'

buildAll()

async function buildAll() {
	return Promise.all([
		build('script', {
			entryPoints: ['src/script.js'],
			platform: 'browser',
			minify: true,
			target: ['es6'],
		}),
		build('esm', {
			entryPoints: ['src/index.js'],
			platform: 'neutral'
		}),
		build('cjs', {
			entryPoints: ['src/index.js'],
			target: ['node10.4'],
			platform: 'node',
		}),
	])
}

async function build(name, options) {
	const path = `${name}.js`
	console.log(`Building ${name}`)

	if (process.argv.includes('--watch')) {
		let ctx = await esbuild.context({
			outfile: `./dist/${path}`,
			bundle: true,
			logLevel: 'info',
			sourcemap: true,
			...options,
      minify: false
		})
		await ctx.watch()
	}
	else {
		return esbuild.build({
			outfile: `./dist/${path}`,
			bundle: true,
			...options,
		})
	}
}

package.json

To deploy this via npm and potentially load the browser version from a CDN like JSDelivr, we need to set up our package.json correctly. The following snippet is what I've found works well for modern versions of node. exports defines which version of the file to use based on how it's imported, Node will change which version is used based on whether we require or import. We set main to the script.js file so it's picked up by all major CDNs I've tried.

{
  "main": "./dist/script.js",
	"exports": {
		"require": "./dist/cjs.js",
		"import": "./dist/esm.js",
		"default": "./dist/esm.js"
	},
}