Adding Bootstrap to Phoenix using NPM

1.js

// 1. Add Bootstrap as a dependency in package.json

{
  ...
  "dependencies": {
    ...
    "bootstrap": "~3.3.6"
  }
}

// 2. Modify brunch-config.js

exports.config = {
 ...
  npm: {
    enabled: true,
    // Whitelist the npm deps to be pulled in as front-end assets.
    // All other deps in package.json will be excluded from the bundle.
    whitelist: ["phoenix", "phoenix_html"],
    styles: {bootstrap: ["dist/css/bootstrap.css"]}
  }
};

You can add import "bootstrap" to your app.js.
Make sure to include jQuery as npm dependency and add this to the brunch-config:

npm: {
      globals: {
          $: 'jquery',
          jQuery: 'jquery'
      }
}