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What is differential loading in Angular?

Differential loading in Angular is a performance optimization technique where the Angular CLI generates two sets of JavaScript bundles during the build process:

What is differential loading in Angular?

  1. Modern bundle – Uses ES2015+ syntax for modern browsers.

  2. Legacy bundle – Uses ES5 syntax for older browsers (like IE11).

These bundles are automatically served based on the capabilities of the browser, allowing modern browsers to load smaller, faster code while still supporting older ones.

🔍 How It Works

When you run:

ng build --prod

Angular produces:

  • ES2015+ bundles (e.g., main-es2015.js)

  • ES5 bundles (e.g., main-es5.js)

The index.html will include <script> tags like:

<script type="module" src="main-es2015.js"></script>
<script nomodule src="main-es5.js"></script>

  • type="module" is recognized by modern browsers → they load the ES2015+ bundle.

  • nomodule is ignored by modern browsers → used by older browsers to load ES5.

✅ Benefits of Differential Loading

FeatureBenefit
🏎 Faster loadsSmaller ES2015+ bundles = better performance on modern browsers
🌐 Broader supportES5 fallback ensures older browsers (e.g., IE11) still work
📦 Reduced bloatModern browsers skip the heavy ES5 bundles

⚙️ Enabling Differential Loading

  • Angular 8+ introduced it.

  • Angular 12+ enabled it by default for production builds.

  • No manual configuration needed—just run ng build --prod.

🚫 Angular 13+ Update

As of Angular 13:

  • Angular dropped support for IE11, so differential loading is disabled by default.

  • Only modern JavaScript (ES2015+) is emitted unless you specifically opt in via custom configurations.

Summary

Differential Loading = Angular’s way of optimizing performance by serving modern JavaScript to capable browsers and a backward-compatible version to older ones.

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