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The Immutable Webapp

One of the defining features of an Immutable Web App is that the configuration of the web app is extracted from the code.

We’ve achieved this in the past by extracting the configuration to a environments/config.json, then prior to a build or local serve, we require the correct enviroment’s JSON config into a script that appends all the keys to a property on the window object.

An example script template.

const stringify = require('json-stringify-pretty-compact');

module.exports = configObject => {
  const safeJsonConfig = stringify(configObject).replace(/</g,'\\u003c'); // Sanitise the JSON

  return `;(function(window) {\nwindow.__ENV__ = ${safeJsonConfig};\n})(this);`;

Loading the config.js script before the body tag ensures the variables are available for when the app loads.

<script src="./config.js"></script>
  <div id="app-root"></div>

In production, to prevent caching the environment variables, we would append a hash to the config.js file and update the index.html as required.

This allows us to create a deployable static artifact and use the index.html as the application manifest.

Prior to deploying to the environment one would run a deploy script which would unpackage the application, and update the index.html file for the required environment.

Immutable Web App’s does propose a more elegant solution.

Written on Feb 4, 2019.