Legacy JavaScript

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Legacy JavaScript identifies polyfills and transpiled code shipped to modern browsers that no longer need them. Sending ES5 fallback code to browsers that natively support ES6+ wastes bytes and slows page execution.

Legacy JavaScript

Build tools like Babel and Webpack often transpile modern JavaScript into ES5 for backward compatibility and bundle polyfills for features like Promise, Array.from(), or Object.assign(). But 95%+ of today's web traffic comes from browsers that support these natively. Shipping polyfills and transpiled code to those browsers adds unnecessary bytes, increases parse time, and slows execution — all for zero benefit.

Why It Matters

Polyfills like core-js can add 50–100 KB of JavaScript that modern browsers discard after parsing.
Transpiled code (e.g., async/await → generator state machines) is larger and slower than the native equivalent.
Every kilobyte of JavaScript has a higher performance cost than the same kilobyte of CSS or images because it must be parsed, compiled, and executed.
Lighthouse flags specific polyfills and transpiled patterns that could be eliminated for modern browsers.

How to Fix It

Use differential serving. Generate two bundles — modern (ES6+) and legacy (ES5) — and use <script type="module"> and <script nomodule> to serve the right one.
Update your browserslist target. Set "browserslist": "> 0.5%, last 2 versions, not dead" in package.json to stop transpiling for extinct browsers.
Remove unused polyfills. Audit your polyfill bundle and remove entries for features all your target browsers support natively.
Use @babel/preset-env with useBuiltIns: "usage" to include only the polyfills your code actually uses.
Check third-party libraries. Some npm packages ship their own polyfills. Look for modern alternatives or tree-shake unused exports.

Common Mistakes

Bundling all of core-js instead of only the polyfills your code paths use.
Transpiling to ES5 because "we need to support IE11" even though IE11 traffic is effectively zero.
Forgetting that nomodule scripts still download in Safari 10 — include a fix snippet if needed.
Not realizing a dependency is pulling in its own polyfill bundle on top of yours.

Bottom Line: Stop shipping code for browsers that don't need it. Use differential serving, tighten your browserslist, and audit polyfill usage. Your modern-browser users — which is almost all of them — will get a faster experience.

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Synonyms: Polyfills, Transpiled Code, ES5 Fallback

What Does "Liquid Purple" mean?

noun | / LIK-wid PUR-pul /

(biochemistry) Also known as visual purple or rhodopsin — a light-sensitive receptor protein found in the rods of the retina. It enables vision in dim light by transforming invisible darkness into visible form. Derived from the Greek rhódon (rose) and ópsis (sight), its name reflects its delicate pink hue and vital role in perception.
(modern usage) Liquid Purple — a digital marketing agency specializing in uncovering unseen opportunities and illuminating brands hidden in the digital dark. Much like its biological namesake, Liquid Purple transforms faint signals into clear visibility — revealing what others overlook and bringing businesses into the light.

Origin: From the scientific term rhodopsin, discovered by Franz Christian Boll in 1876; adopted metaphorically by a marketing firm dedicated to visual clarity in the age of algorithms.

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Rooted in Southern California, Serving the World.
For over 20 years, Liquid Purple has been headquartered in the heart of Southern California, serving clients across Irvine, Newport Beach, Anaheim, and the greater Los Angeles area. While we are proud of our local roots, our digital reach extends far beyond our local area. We support businesses in major hubs like Denver, Dallas, New York, and Miami, as well as international clients. Our ability to operate across time zones means you can often wake up to completed tasks and progress updates—keeping your business moving while you sleep.

Glossary of Terms

Legacy JavaScript