What polyfills are: a technical guide with a focus on javaScript

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In the context of modern web development, the evolution of specifications often proceeds faster than uniform adoption by browsers and runtimes. This misalignment creates compatibility issues that, historically, have been resolved through the use of polyfills.

This article provides a technical explanation of polyfills: first at a general conceptual level, then with a specific focus on JavaScript, including practical examples and an overview of the most commonly used polyfills in contemporary web projects.

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Definition of a Polyfill

A polyfill is a runtime implementation of a feature defined by an official specification but not available (or not fully implemented) in a given execution environment.

📝 Memo

A polyfill is a piece of code that implements a missing feature in an environment that does not natively support it.

From a technical standpoint, a polyfill:

  • checks for the existence of a feature (feature detection)
  • if missing, provides an alternative implementation
  • exposes the same interface defined by the standard

The goal is to ensure compliance with specifications and allow the developer to write modern code without worrying excessively about compatibility.

Why are they called "polyfills"?

The term originated as a metaphor: just as spackling paste (Polyfills) fills holes in walls, a polyfill "fills the holes" in missing platform implementations.


Polyfills, Transpilers, and Runtime Compatibility

It is important to clearly distinguish polyfills from other compatibility tools:

  • Transpilers (e.g., Babel)
    • transform source code into a syntactically compatible version (e.g., let → var)
    • act during the build phase
    • do not add missing APIs at runtime
  • Polyfills
    • operate at runtime
    • add or extend global objects (Array, Promise, window, etc.)
    • implement missing standard APIs
  • Shims
    • a generic term, often used as a synonym, but less rigorous

In modern environments, compatibility is almost always the result of a combination of transpiling + polyfilling.


Technical Reasons for Using Polyfills

Using polyfills is justified when:

  • the project must support legacy browsers
  • recent ECMAScript or Web APIs are used
  • you want to keep code adherent to standards
  • the cost of manual rewriting exceeds the cost of the polyfill
  • adopting new APIs early

From an architectural perspective, polyfills allow for the reduction of branching and conditional code related to the browser.


Polyfills in JavaScript

JavaScript represents the primary use case for polyfills for several reasons:

  • the continuous evolution of the ECMAScript standard
  • the progressive introduction of Web APIs
  • the need for backward compatibility

A JavaScript polyfill generally follows this pattern:

  • checks if a feature exists
  • if it doesn't, defines it

if (!someFeature) {
  // specification-compliant definition
}

The check must be based on feature detection, not user agent sniffing.


Example: Array.prototype.includes Polyfill

if (!Array.prototype.includes) {
  Object.defineProperty(Array.prototype, 'includes', {
    value: function (searchElement, fromIndex) {
      const len = this.length >>> 0;
      let i = fromIndex | 0;
      if (len === 0) return false;
      if (i < 0) i = Math.max(len + i, 0);
      while (i < len) {
        if (this[i] === searchElement || (Number.isNaN(this[i]) && Number.isNaN(searchElement))) {
          return true;
        }
        i++;
      }
      return false;
    }
  });
}

This example shows how a real polyfill should:

  • respect corner cases defined by the standard
  • avoid overwriting native implementations
  • use Object.defineProperty to control enumerability and configuration


Promise Polyfill

Before the full adoption of Promise, many browsers did not provide a native implementation.

new Promise((resolve) => {
  setTimeout(() => resolve('OK'), 1000);
}).then(console.log);

A polyfill introduces the global Promise object and implements:

  • internal state (pending, fulfilled, rejected)
  • microtask queues
  • static methods (resolve, reject, all, race)

Semantic adherence is fundamental to avoid inconsistent behaviors.


Polyfill Inclusion Strategies

Direct Inclusion

<script src="polyfill.js"></script>

Simple approach but not very scalable.

Selective Import via Bundler

import 'core-js/features/array/includes';

Allows granular control over bundle weight.

Automatic Polyfills Based on Targets

Tools like Babel allow you to:

  • define supported browsers
  • include only necessary polyfills
  • avoid duplication

This is the most common solution in production.


Most Commonly Used JavaScript Polyfills

core-js

The main reference for ECMAScript polyfills:

  • covers ES5, ES2015+, and standardized proposals
  • used internally by Babel
  • highly modular

regenerator-runtime

Necessary for:

  • generators
  • async / await

Works in synergy with transpiling.

Fetch API Polyfill

Implements fetch, Headers, Request, and Response in legacy environments.

Intersection Observer Polyfill

Used for:

  • lazy loading
  • visibility observation
  • rendering optimizations

classList Polyfill

Adds support for the Element.classList API in obsolete browsers.

polyfill.io (Historically known but to be avoided)

A service that provided dynamic polyfills based on the user agent. It was very popular for years and is important to mention as a historical reference, although today many solutions prefer local bundling for security and control reasons.


Modern Considerations

In the modern development context:

  • many projects limit support to evergreen browsers
  • excessive use of polyfills can impact performance and bundle size
  • it is essential to base decisions on real compatibility requirements

The goal is not to maximize compatibility, but to balance compatibility, performance, and maintainability.


Conclusion

Polyfills represent an essential mechanism to guarantee alignment between specifications and implementations. In JavaScript, their correct use requires a clear understanding of the distinction between syntax, runtime, and APIs.

Used selectively and consciously, polyfills allow for writing modern, standard-compliant, and robust code without compromising the user experience or the architectural quality of the project.

📝 Memo

Remember that if your project:

  • does not require backward compatibility with older browsers
  • uses widely supported APIs
then you don't need to worry about providing or implementing polyfills 😉



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