Your .map() is Slower Than You Think — Here’s How to Speed It Up

Boost your JavaScript performance by ditching common .map() mistakes and learning faster alternatives

JavaScript’s .map() is like that reliable old friend who always shows up — but takes forever to get ready.

It’s convenient, readable, and works most of the time — but what if I told you it’s secretly slowing down your app?

Let’s dive into why .map() isn’t as fast as you think, what’s really happening under the hood, and how you can speed things up — without writing unreadable code.

Photo by cottonbro studio

🧠 Why .map() Isn’t Always Your Friend

Let’s start with a classic example:

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(num => num * 2);

console.log(doubled); // [2, 4, 6, 8, 10]

Looks clean, right? But .map() has a hidden cost — it creates a new array every time. That’s great for immutability, but terrible for performance when working with large datasets or chaining multiple transformations.

Let’s see what’s really going on under the hood:

1. Loop through each element.
2. Run the callback function.
3. Push the result into a new array.

That third step — the constant pushing — is where things start to drag. The more data you process, the more memory you chew up.

Benchmark Time: .map() vs. for Loops

Let’s put it to the test with a large array:

const bigArray = Array.from({ length: 1_000_000 }, (_, i) => i);

// Method 1: .map()
console.time("map");
const mapResult = bigArray.map(num => num * 2);
console.timeEnd("map");

// Method 2: for loop
console.time("for-loop");
const forLoopResult = [];
for (let i = 0; i < bigArray.length; i++) {
  forLoopResult.push(bigArray[i] * 2);
}
console.timeEnd("for-loop");

For me, the results looked like this:

map: 120ms  
for-loop: 45ms

👀 The for loop won — and by a lot.