Snowpack config for Phoenix and PostCSS

dev.exs

config :my_app, MyAppWeb.Endpoint,
  http: [port: 4000],
  debug_errors: true,
  code_reloader: true,
  check_origin: false,
  watchers: [
    node: [
      "node_modules/.bin/npm-run-all",
      "watch",
      cd: Path.expand("../assets", __DIR__)
    ]
  ]

endpoint.ex

defmodule MyAppWeb.Endpoint do
  # ...
  plug Plug.Static,
    at: "/",
    from: :my_app,
    gzip: false,
    only: ~w(css fonts images js favicon.ico robots.txt _snowpack)
  # ...
end

package.json

{
  "repository": {},
  "description": " ",
  "license": "MIT",
  "scripts": {
    "deploy": "NODE_ENV=production snowpack build",
    "watch": "NODE_ENV=development snowpack build"
  },
  "dependencies": {
    "nprogress": "^0.2.0",
    "phoenix": "file:../deps/phoenix",
    "phoenix_html": "file:../deps/phoenix_html",
    "phoenix_live_view": "file:../deps/phoenix_live_view"
  },
  "devDependencies": {
    "@snowpack/plugin-postcss": "^1.1.0",
    "autoprefixer": "^10.2.0",
    "npm-run-all": "^4.1.5",
    "postcss": "^8.2.2",
    "postcss-cli": "^8.3.1",
    "snowpack": "^3.0.5",
    "tailwindcss": "^2.0.2"
  }
}

snowpack.config.js

const glob = require("glob");

const isProduction = process.env.NODE_ENV == "production"

const vendorJs = glob.sync("vendor/**")
const jsFiles = vendorJs.concat(["js/app.js"])

/** @type { import("snowpack").SnowpackUserConfig } */
module.exports = {
  mount: {
    "js": "/js",
    [vendorJs]: "/js",
    "css": "/css",
    "static": "/"
  },
  optimize: {
    entrypoints: jsFiles,
    bundle: isProduction,
    splitting: true,
    treeshake: isProduction,
    minify: isProduction,
    target: "es2017"
  },
  plugins: [
    "@snowpack/plugin-postcss"
  ],
  packageOptions: {
    knownEntrypoints: jsFiles,
  },
  buildOptions: {
    out: "../priv/static",
    watch: !isProduction,
    sourcemap: !isProduction,
  }
}