⚡ 5 JavaScript Performance Traps That Make Your Code Slow

Intro

JavaScript engines (V8, SpiderMonkey, etc.) are incredibly fast. But bad code patterns can cripple performance. The worst part? These traps are often invisible until your app scales.

Let’s dive deep into 5 common performance killers in JS, with simple examples and fixes.
Read for free here✨

🐌 Trap 1: Repeated DOM Manipulations

const list = document.getElementById("list");
for (let i = 0; i < 1000; i++) {
  const item = document.createElement("li");
  item.textContent = i;
  list.appendChild(item);
}

Why slow: Each appendChild triggers layout recalculations.

✅ Fix: Use a fragment.

const frag = document.createDocumentFragment();
for (let i = 0; i < 1000; i++) {
  const item = document.createElement("li");
  item.textContent = i;
  frag.appendChild(item);
}
list.appendChild(frag);

🐌 Trap 2: Blocking the Event Loop

while (true) {
  // infinite loop blocks UI
}

Why slow: Long-running sync tasks block rendering & input.

✅ Fix: Split work into chunks.

function heavyTask(items) {
  if (items.length === 0) return;
  doWork(items.shift());
  setTimeout(() => heavyTask(items), 0);
}

🐌 Trap 3: Overusing forEach on Huge Arrays

largeArray.forEach(item => process(item));

Why slow: forEach has function call overhead.

✅ Fix: For massive arrays, use a for loop.

for (let i = 0; i < largeArray.length; i++) {
  process(largeArray[i]);
}

🐌 Trap 4: Memory Leaks

setInterval(() => console.log("tick"), 1000);
// never cleared → memory keeps growing

Why slow: Growing memory → GC overhead → sluggish app.

✅ Fix: Always clean up.

const id = setInterval(() => console.log("tick"), 1000);
clearInterval(id);

🐌 Trap 5: Using JSON.stringify for Deep Clones

const clone = JSON.parse(JSON.stringify(obj));

Why slow: Inefficient, loses methods/dates.

✅ Fix: Use structuredClone (modern JS).

const clone = structuredClone(obj);

Wrap-Up

Performance traps are subtle but fixable. Watch out for DOM thrashing, event loop blocking, huge loops, leaks, and cloning hacks. Small changes = massive speed gains.

Happy coding 💻✨

✍️ By Vishal Solanki

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⚡ 5 JavaScript Performance Traps That Make Your Code Slow was originally published in JavaScript in Plain English on Medium, where people are continuing the conversation by highlighting and responding to this story.