Fast JavaScript Isn’t Magic. It’s Just Knowing These Patterns.
You’re probably writing slow JavaScript.
Photo by Seyed Amir Mohammad Tabatabaee on Unsplash
You ever write something in JavaScript, swear it should be fast, and then watch Chrome’s performance tab light up like a Christmas tree?
Yeah. Same.
A few weeks ago, I wrote about how JavaScript — despite all the slander — can actually be fast. The trick isn’t in the language itself. It’s in how you use it.
But a few folks reached out asking how to actually write fast JavaScript.
So here it is. No magic. No deep V8 internals. Just a bunch of boring, reliable patterns that make your JS code fly, without sacrificing your sanity.
1. Loops beat everything else (when used right)
Array methods are cute. Until you’re calling .map() inside a .filter() inside a .reduce() and wondering why your page feels like it’s melting.
If you care about speed, a good old for loop still slaps. Why? Because it’s predictable. V8 knows exactly what you’re doing. No callback overhead, no abstractions, just raw iteration.
Want performance? Get primitive.
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Not sexy. Just fast.
2. Avoid object shape-shifting
JavaScript objects are flexible. Too flexible.
Change their shape mid-flight — add properties dynamically, delete stuff — and the engine throws away its optimized assumptions. What was once a slick hidden class becomes a clunky dictionary.
Stick to consistent object structures. Define all your fields upfront. If a key might exist later, initialize it as null or undefined early on.
The less your objects surprise the engine, the better.
3. Don’t re-declare your functions like a maniac
If you’re defining functions inside other functions inside loops, please stop.
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No. Just no.
Move that process function out. Let the engine optimize it once and be done.
4. Don’t box primitives just because you can
You can do this:
But you shouldn’t.
That one word — new—just cost you a bunch of performance and mental clarity. Boxed primitives behave weirdly. They’re objects pretending to be values.
Stick to primitives. They’re faster and saner.
5. Memoization isn’t cheating
If your function’s returning the same thing every time for the same input, just cache it. Especially if it involves DOM queries, expensive calculations, or deeply nested object traversals.
You don’t get points for recalculating something pointless 300 times a second.
6. Asynchronous doesn’t mean fast
Some folks think slapping async and await everywhere makes things faster. What it actually does is introduce microtask queues and a bunch of scheduling overhead.
Use it when you need it. Not because VS Code told you to.
If your code can run synchronously without blocking anything important, let it.
7. Watch your dependencies
Modern JavaScript apps love importing entire libraries to do three things. That 100KB utility library? Probably doing something you could’ve written in 10 lines.
Every dependency you add is another potential performance black hole.
Measure twice. npm install once.
Final Thoughts
JavaScript isn’t a slow language. But it does punish carelessness. The good news is, you don’t need to become a compiler engineer to write fast code.
You just need to stop doing the stuff that slows it down.
Most performance wins don’t come from clever tricks, they come from knowing the boring stuff and doing it well.
Just like everything else in software.
