How to Identify Memory Leaks in Next.js

Memory leaks in Next.js applications can lead to degraded performance, increased server costs, and even application crashes. A memory leak occurs when allocated memory is not properly released, causing the application to consume more and more memory over time. In a Next.js environment, leaks can occur both in client-side and server-side rendering (SSR) scenarios. Identifying and resolving these leaks is crucial to maintaining a performant and scalable application.

A memory leak can be particularly troublesome in long-running applications, such as those hosted on Vercel, AWS Lambda, or dedicated Node.js servers. If an application continuously consumes more memory without freeing up unused allocations, it may lead to frequent restarts, slow responses, and increased infrastructure costs.

Understanding the root causes of memory leaks requires a combination of tools, performance monitoring, and code analysis. This article will explore different approaches to identifying memory leaks in both development and production environments.

Common Causes of Memory Leaks in Next.js

Memory leaks in Next.js applications typically originate from various sources, including:

• Unclosed database connections: Many ORM libraries, such as Prisma and Sequelize, maintain persistent database connections. Failing to close these connections properly can result in memory consumption increasing over time.
• Retaining references to large objects: If global variables or closures hold references to objects that are no longer needed, they prevent garbage collection from freeing up memory.
• Unnecessary event listeners: Components that add event listeners (such as window.addEventListener) but do not remove them when unmounted can cause memory leaks.
• Improper use of React hooks: The useEffect hook is a common source of memory leaks when dependencies are not properly managed, leading to stale references and redundant re-renders.
• Server-side memory retention: API handlers that store objects in memory beyond their intended lifespan can lead to increasing heap usage.
• Cache mismanagement: Storing too much data in in-memory caches (such as Redis or LRU caches) without eviction policies can cause excessive memory consumption.

By understanding these common pitfalls, developers can adopt strategies to detect and fix memory leaks effectively.

Using Chrome DevTools for Client-Side Memory Profiling

For client-side Next.js applications, Chrome DevTools provides a robust set of tools to analyze memory usage and detect leaks.

Steps to Identify Memory Leaks:

• Open Chrome DevTools (F12 or Ctrl+Shift+I).
• Navigate to the Memory tab.
• Select Heap Snapshot and take an initial snapshot.
• Perform interactions within the app that may cause memory issues (e.g., navigating between pages, opening modals).
• Take another snapshot and compare memory usage trends.
• Identify objects that persist unexpectedly between snapshots.

Key Metrics to Analyze:

• Detached DOM nodes: If elements persist in memory after being removed from the DOM, they may have lingering references.
• Objects growing over time: If the retained size of certain objects continuously increases, it indicates a memory leak.
• Event listeners: Unclosed listeners can prevent garbage collection.

Example of Fixing Event Listener Leaks:

A common mistake in React applications is forgetting to remove event listeners:

useEffect(() => {
  const handleResize = () => console.log("Resized");
  window.addEventListener("resize", handleResize);
​
  return () => {
    window.removeEventListener("resize", handleResize);
  };
}, []);

The cleanup function ensures that the event listener is removed when the component unmounts.

Using Autocannon to Simulate Load Testing for Server-Side Memory Leaks

To identify memory leaks in SSR or API routes, Autocannon can be used for load testing.

Installing Autocannon:

npm install -g autocannon

Running a Load Test:

autocannon -c 100 -d 30 -p 10 http://localhost:3000/api/data

• -c 100 sets 100 concurrent connections.
• -d 30 runs the test for 30 seconds.
• -p 10 sets a pipelining factor of 10.

If memory usage keeps increasing throughout the test without stabilizing, it’s a sign of a memory leak.

Analyzing Memory Usage with Node.js Heap Snapshots

For server-side memory analysis, Node.js provides built-in profiling tools.

Capturing a Heap Snapshot:

Add the following code to your Next.js server:

const v8 = require('v8');
const fs = require('fs');
​
function takeHeapSnapshot() {
    const snapshot = v8.getHeapSnapshot();
    snapshot.pipe(fs.createWriteStream(`heapdump-${Date.now()}.heapsnapshot`));
}
​
setInterval(takeHeapSnapshot, 60000); // Capture every 60 seconds

Analyze the generated snapshot using Chrome DevTools (Profiles tab → Load).

By comparing snapshots over time, you can pinpoint objects that continue growing.

Tracking Memory Leaks in Production with OpenTelemetry

For applications in production, integrating a monitoring tool like OpenTelemetry can provide real-time insights.

Setting Up OpenTelemetry:

Install OpenTelemetry:

⁠npm install @opentelemetry/sdk-node @opentelemetry/api

Initialize OpenTelemetry in your Next.js app:

const { NodeSDK } = require('@opentelemetry/sdk-node');
⁠const sdk = new NodeSDK();
⁠
⁠sdk.start();

By sending memory usage metrics to an observability platform like Grafana or Datadog, you can detect memory issues before they escalate.

Using Built-in V8 Profiling Tools

Node.js provides V8’s built-in profiling tools to analyze memory allocation and garbage collection.

Running Node.js with Memory Profiling:

node --inspect --trace-gc server.js

• --inspect enables debugging.
• --trace-gc logs garbage collection activity.

These logs help developers identify functions or processes that are retaining memory longer than necessary.

Preventing Memory Leaks in Next.js

To prevent memory leaks from occurring, follow these best practices:

• Always close database connections using finally or useEffect cleanup
• Remove event listeners when components unmount
• Use WeakMaps for caching to allow garbage collection
• Monitor heap usage using built-in Node.js tools
• Profile memory regularly with Chrome DevTools and heap snapshots
• Use structured logging to track memory consumption in production

Example: Cleaning Up API Request Subscriptions

Using AbortController to clean up fetch requests in React components:

useEffect(() => {
  const controller = new AbortController();
  fetch("/api/data", { signal: controller.signal });
​
  return () => controller.abort();
}, []);

This ensures that pending API calls are canceled when the component unmounts.

Conclusion

Memory leaks can significantly impact Next.js applications, whether on the client or server side. Using Chrome DevTools, Autocannon, Node.js heap snapshots, and OpenTelemetry, developers can diagnose and prevent leaks before they affect performance. By following best practices, you can ensure a scalable and efficient Next.js application.

Our experts in Next.js performance can help you improve the speed and performance of your app.Get in touchto learn how we can help.