Asset Management

Beside producing HTML, most web applications have various assets (JavaScript, CSS, images, fonts and so on).

From Phoenix v1.6, new applications use esbuild to prepare assets via the Elixir esbuild wrapper. This direct integration with esbuild means that newly generated applications do not have dependencies on Node.js or an external build system (e.g. Webpack).

Your JavaScript is typically placed at "assets/js/app.js" and esbuild will extract it to "priv/static/assets/app.js". In development, this is done automatically via the esbuild watcher. In production, this is done by running mix assets.deploy.

esbuild can also handle your CSS files. For this, there is typically an import "../css/app.css" at the top of your "assets/js/app.js". We will explore alternatives below.

Finally, all other assets, that usually don't have to be preprocessed, go directly to "priv/static".

Third-party JS packages

If you want to import JavaScript dependencies, you have two options to add them to your application:

  1. Vendor those dependencies inside your project and import them in your "assets/js/app.js" using a relative path:

    import topbar from "../vendor/topbar"
  2. Call npm install topbar --save inside your assets directory and esbuild will be able to automatically pick them up:

    import topbar from "topbar"


esbuild has basic support for CSS. If you import a .css file at the top of your main .js file, esbuild will also bundle it, and write it to the same directory as your final app.js. That's what Phoenix does by default:

import "../css/app.css"

However, if you want to use a CSS framework, you will need to use a separate tool. Here are some options to do so:

Don't forget to remove the import "../css/app.css" from your JavaScript file when doing so.

Images, fonts, and external files

If you reference an external file in your CSS or JavaScript files, esbuild will attempt to validate and manage them, unless told otherwise.

For example, imagine you want to reference priv/static/images/bg.png, served at /images/bg.png, from your CSS file:

body {
  background-image: url(/images/bg.png);

The above may fail with the following message:

error: Could not resolve "/images/bg.png" (mark it as external to exclude it from the bundle)

Given the images are already managed by Phoenix, you need to mark all resources from /images (and also /fonts) as external, as the error message says. This is what Phoenix does by default for new apps since v1.6.1+. In your config/config.exs, you will find:

args: ~w(js/app.js --bundle --target=es2017 --outdir=../priv/static/assets --external:/fonts/* --external:/images/*),

If you need to reference other directories, you need to update the arguments above accordingly. Note running mix phx.digest will create digested files for all of the assets in priv/static, so your images and fonts are still cache-busted.

Esbuild plugins

Phoenix's default configuration of esbuild (via the Elixir wrapper) does not allow you to use esbuild plugins. If you want to use an esbuild plugin, for example to compile SASS files to CSS, you can replace the default build system with a custom build script.

The following is an example of a custom build using esbuild via Node.JS. First of all, you'll need to install Node.js in development and make it available for your production build step.

Then you'll need to add esbuild to your Node.js packages and the Phoenix packages. Inside the assets directory, run:

$ npm install esbuild --save-dev
$ npm install ../deps/phoenix ../deps/phoenix_html ../deps/phoenix_live_view --save

or, for Yarn:

$ yarn add --dev esbuild
$ yarn add ../deps/phoenix ../deps/phoenix_html ../deps/phoenix_live_view

Next, add a custom Javascript build script. We'll call the example assets/build.js:

const esbuild = require('esbuild')

const args = process.argv.slice(2)
const watch = args.includes('--watch')
const deploy = args.includes('--deploy')

const loader = {
  // Add loaders for images/fonts/etc, e.g. { '.svg': 'file' }

const plugins = [
  // Add and configure plugins here

let opts = {
  entryPoints: ['js/app.js'],
  bundle: true,
  target: 'es2017',
  outdir: '../priv/static/assets',
  logLevel: 'info',

if (watch) {
  opts = {
    sourcemap: 'inline'

if (deploy) {
  opts = {
    minify: true

const promise =

if (watch) {
  promise.then(_result => {
    process.stdin.on('close', () => {


This script covers following use cases:

  • node build.js: builds for development & testing (useful on CI)
  • node build.js --watch: like above, but watches for changes continuously
  • node build.js --deploy: builds minified assets for production

Modify config/dev.exs so that the script runs whenever you change files, replacing the existing :esbuild configuration under watchers:

config :hello, HelloWeb.Endpoint,
  watchers: [
    node: ["build.js", "--watch", cd: Path.expand("../assets", __DIR__)]

Modify the aliases task in mix.exs to install npm packages during mix setup and use the new esbuild on mix assets.deploy:

  defp aliases do
      setup: ["deps.get", "ecto.setup", "cmd --cd assets npm install"],
      "assets.deploy": ["cmd --cd assets node build.js --deploy", "phx.digest"]

Finally, remove the esbuild configuration from config/config.exs and remove the dependency from the deps function in your mix.exs, and you are done!

Removing esbuild

If you are writing an API, or for some other reason you do not need to serve any assets, you can disable asset management completely.

  1. Remove the esbuild configuration in config/config.exs and config/dev.exs,
  2. Remove the assets.deploy task defined in mix.exs,
  3. Remove the esbuild dependency from mix.exs,
  4. Unlock the esbuild dependency:
$ mix deps.unlock esbuild