According to MDN, “A polyfill is a piece of code (usually JavaScript on the Web) used to provide modern functionality on older browsers that do not natively support it.”

Now, let me explain it in my own words. When we write JavaScript code that uses certain built-in prototype functions, sometimes it throws an error in some browsers — something like “______() function is not available.” This happens because that particular browser doesn’t yet support the function we've used. In such cases, we write our own function that mimics the behavior of the missing built-in function. This custom implementation is what we call a polyfill.

What will happen if we not use polyfills?

If we don’t use polyfills, our website might break on older versions of browsers that don’t support certain modern features. This can result in a poor user experience and potentially losing valuable viewers or customers — which can be a big loss, especially for a business. To avoid this, we write fallback code (a polyfill) in our JavaScript files that replicates the missing functionality. This way, even if the browser doesn't support the feature, our website still works as expected.

🧠 Why Polyfills Are Not Used Exclusively?

🔧 1. Native Implementations Are More Powerful

• Native API implementations (i.e., those built directly into the browser) are written in low-level languages like C++.

• They are optimized by the browser’s engine for speed, memory efficiency, and complete feature support.

• On the other hand, polyfills are written in JavaScript and can’t access lower-level browser functionalities — they're limited by what JavaScript can do.

✅ Example:

• A polyfill for Object.create() may allow you to create an object with a specific prototype, but it won’t include every internal behavior the native version has.

• The native version can do more behind the scenes, like handling internal [[Prototype]] chains more efficiently.

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⚡ 2. Performance Matters

• Polyfills are extra JavaScript code, which:

  • Increases file size (affecting load time)

  • Requires extra CPU processing to run

• Native features are already built-in, so they don’t require additional resources or processing once the browser is loaded.

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📦 3. Limited Functionality in Polyfills

• A polyfill can only mimic the behavior of a feature.

• If the feature requires something that JavaScript alone cannot do (like access certain browser internals or hardware optimizations), the polyfill will just approximate the behavior — not replicate it 100%.

Example

We used a map function in our code

const numbers = [1, 2, 3, 4];

const doubled = numbers.map(num => num * 2);

console.log(doubled); // Output: [2, 4, 6, 8]

But a Browser Shows an error stating -”TypeError: numbers.map is not a function”

Now in this case we have to Exclusively type a polyfill for map function

if (!Array.prototype.map) {
  Array.prototype.map = function(callback, thisArg) {
    if (this == null) {
      throw new TypeError('Array.prototype.map called on null or undefined');
    }

    const arr = Object(this);
    const len = arr.length >>> 0; // Convert to unsigned 32-bit int
    const result = new Array(len);

    for (let i = 0; i < len; i++) {
      if (i in arr) {
        result[i] = callback.call(thisArg, arr[i], i, arr);
      }
    }

    return result;
  };
}

Summary

A polyfill is a piece of JavaScript code that replicates modern browser features in older browsers that don’t support them natively. This ensures consistent functionality across all user devices, regardless of browser version.

Imagine using a modern JavaScript method like Array.prototype.map(). If an older browser doesn’t recognize it, your code might break. That’s where a polyfill steps in — it defines that missing function manually so your website doesn’t crash and users still have a smooth experience.

Why not rely on polyfills?

Although polyfills are useful, they aren't a perfect substitute:

• Native browser implementations are faster, more efficient, and offer complete functionality.

• Polyfills are written in JavaScript and can only approximate the behavior, often lacking in performance and edge-case handling.

• They also increase file size and execution time, which can affect page speed and user experience.

Final Thought

Polyfills are a backup plan — not a replacement. Always prefer native APIs for performance and completeness, and use polyfills only when you need to support older browsers. This way, your website stays accessible without compromising speed or modern standards.