To support legacy browsers to use modern Web APIs at the same time, you need to fill in missing features through polyfills. 1. Select well-maintained polyfill libraries such as core-js, whatwg-fetch, etc.; 2. Only the required polyfill is introduced according to the target browser; 3. Use feature detection or Polyfill.io to load on demand to reduce bandwidth waste; 4. Automatically inject polyfills on demand in building tools such as Webpack or Vite; 5. Always test in real or simulated old browser environments to ensure compatibility; 6. Common APIs that require polyfill include Promise, fetch, Array.from, Object.assign, Array.includes, URLSearchParams, etc.; ultimately, the balance between modern user experience and old browser support is achieved.

Supporting older browsers while using modern web APIs is a common challenge in frontend development. Polyfills are the go-to solution — they "fill in" missing functionality so your code runs consistently across environments. Here's how to effectively polyfill modern web APIs for older browsers.

What Is a Polyfill and Why You Need One

A polyfill is a piece of code that provides modern functionality on older browsers that don't natively support it. Unlike translation (eg, with Babel), which rewrites syntax, polyfills implement missing APIs — like fetch , Promise , Array.from , or Object.assign .

For example, if you use fetch() in Internet Explorer 11, it will throw an error because fetch isn't available. A polyfill adds that function so your app doesn't break.

Key point: Polyfills don't fix syntax errors. Use Babel or similar tools to handle modern JavaScript syntax (like arrow functions or async/await ).

Choose the Right Polyfills

Not all polyfills are created equal. Here's how to pick the best ones:

• Use well-maintained, community-backed libraries
  Popular choices include:

  • core-js : Covers ES5, ES6, and even some upcoming features.
  • whatwg-fetch : Adds fetch support.
  • promise-polyfill : For Promise support.
  • regenerator-runtime : Needed for async/await if using Babel.

• Avoid full libraries when possible
  Instead of including all of core-js , import only what you need:

   import 'core-js/features/promise';
  import 'core-js/features/array/from';

• Check browser compatibility first
  Use Can I use to see which APIs are missing in your target browsers.

Load Polyfills Conditionally (Don't Waste Bandwidth)

Polyfilling every browser slows down modern users. Instead, load polyfills only when needed.

Use feature detection instead of browser sniffing:

 <script>
  if (!window.Promise || !(&#39;fetch&#39; in window) || !Array.from) {
    document.write(&#39;<script src="polyfills.js"><\/script>&#39;);
  }
</script>

Or, use a service like Polyfill.io , which serves only the polyfills needed by the user's browser:

 <script src="http://ipnx.cn/link/7ce6aa193902742535f640eff746a494v3/polyfill.min.js?features=fetch,Promise,Array.from,Object.assign"></script>

This is efficient: a modern Chrome user gets nothing; an IE11 user gets exactly what's required.

Integrate with Build Tools

If you're using a bundler like Webpack or Vite, you can manage polyfills more precisely.

Webpack Example

Install required polyfills:

 npm install core-js regenerator-runtime

In your entry file (eg, index.js ):

 import 'core-js/stable';
import 'regenerator-runtime/runtime';

This gives broad coverage. For finer control, configure Babel with @babel/preset-env and set your target browsers in browserslist :

 // package.json
"browserslist": [
  "defaults",
  "IE 11"
]

With useBuiltIns: 'usage' in Babel config, it will automatically import only the polyfills your code actually uses.

Test in Target Browsers

Never assume polyfills work perfectly. Always test in real (or emulated) older environments:

• Use BrowserStack or Sauce Labs for IE and older Edge.
• Try VirtualBox with Windows VMs for local testing.
• Watch for subtle bugs — some polyfills don't 100% match native behavior (eg, Array.from with sparse arrays).

Also, check console errors and network requests — a missing fetch polyfill might cause silent failures.

Common APIs That Need Polyfills

Here's a quick reference:

Polyfilling modern APIs for older browsers doesn't have to be messy. Use targeted polyfills, load them conditionally, and integrate with your build process. The result? Cleaner code and broader compatibility without punishing modern users.

Basically: detect, import only what you need, and test. That's the smart way to support legacy browsers.

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