Asset Fingerprint Webpack Rails

A webpack plugin to fingerprint your JS for consumption by Rails

Install

npm install asset-fingerprint-webpack-rails --save-dev

Usage

Note: You will likely want to avoid running the fingerprinting in anything but a production build. Otherwise, if you have file watching setup for instance, it will rebuild the fingerprinting each time a file is updated, requiring you to bounce the Rails server to pickup the new fingerprint. One way to avoid this is by passing an option to your command for dev vs prod: (webpack for dev without fingerprinting, and webpack --env.fingerprint for prod build).

This plugin requires you to set your initializer directory path relative to your webpack config, such as: config/initializers as the first argument. The second optional argument determines whether or not the fingerprinting should take place. By default, this is set to true.

const AssetFingerprintPlugin = require('asset-fingerprint-webpack-rails');
new AssetFingerprintPlugin('config/initializers', true);

It uses the hash values from the webpack stats object. This is the value that will be output into a new initializer called asset_fingerprint.rb, as: ASSET_FINGERPRINT=XXXXXXXX. You should setup your webpack.config.js to use this hash when building your output file (see sample config below).

It is also recommended that you use a plugin such as clean-webpack-plugin in conjunction with this plugin to clean out your output directory on each build, if you wish to avoid a collection of old files.

Option 1

Use a local variable in webpack.config.js to pass into the plugin to inform it whether not not it should fingerprint, calling webpack without any args. plugin defualts to fingerprint=true with no args

// webpack.config.js

...
/**
 * require the plugin
 */
const AssetFingerprintPlugin = require('asset-fingerprint-webpack-rails');
const needsFingerprint = someLocalBoolean;
const initializerDirectory = 'config/initializers';
/**
 * add it to your plugins and conditionally use the hash in the filename
 */
module.exports = {
  entry: "./entry.js",
  output: {
    path: __dirname,
    filename: needsFingerprint ? "bundle-[hash].js" : "bundle.js"
  },
  plugins: [
    new AssetFingerprintPlugin(initializerDirectory, needsFingerprint)
  ]
};

Option 2

Setup your webpack.config.js to conditionally return an object based on an argument passed into the webpack command, like webpack --env.fingerprint

start webpack with an option: webpack --env.fingerprint

Then in webpack.config.js

// webpack.config.js

/**
 * require the plugin
 */
const AssetFingerprintPlugin = require('asset-fingerprint-webpack-rails');

/**
 * return a configuration object based on the option passed in, only using the plugin when it exists
 * and conditionally using the hash in the filename
 */
module.exports = function(fingerprint) {
  if (fingerprint) {
    return {
      entry: "./entry.js",
      output: {
        path: __dirname,
        filename: "bundle-[hash].js"
      },
      plugins: [
        new AssetFingerprintPlugin(initializerDirectory)
      ]
    }
  } else {
    return {
      entry: "./entry.js",
      output: {
        path: __dirname,
        filename: "bundle.js"
      }
    }
  }
};

Integration with Rails

Now that our asset_fingerprint.rb file is setup, we can conditionally use it in our views by setting up a helper method in application_helper.rb:

  def asset_with_fingerprint(asset_name)
    Rails.env.production? ? "#{asset_name}-#{ASSET_FINGERPRINT}" : asset_name
  end

And in your layout:

  = javascript_include_tag asset_with_fingerprint('path_of_bundled_js_output/bundle')

That's it! Now if you use a cleaner as suggested, along with file watching, you'll be able to run this in development and have bundle.js rebuild without fingerprinting, and your Rails app with use that directly. Once you build for production, your Rails app and your bundle.js will use a fingerprinted version of the file.

Managing multiple webpack builds

If your build setup launches webpack multiple times a new hash will be generated with each run. This can cause problems when using the default arguments. The last run will overwrite any previously written hash value. An additional parameter can be passed to tell the plugin to append a new named value that has the key from the subsequent run.

Passing a name in the third paramenter will generate a new line in asset_fingerprint.rb having the name assigned to the new hash. For example:

AssetFingerprintPlugin(rubyConfigInitPath, true, 'CUSTOM_ASSET_FINGERPRINT')

will generate a new line in the file like this:

ASSET_FINGERPRINT = '0556f9ab7cbd607d10ed'
CUSTOM_ASSET_FINGERPRINT = '921dcffe35e5f2740ef5'

The format for the fingerprint name must uppercase separating words with underscore i.e. "CUSTOM_ASSET_FINGERPRINT". The name also needs to end with "_FINGERPRINT".

Credit

Concept adapted from @samullen - http://samuelmullen.com/articles/replacing-the-rails-asset-pipeline-with-webpack-and-yarn/