JavaScript Heap Out of Memory Error: Causes, Fixes, and Prevention Guide

In this guide, we will explore the root causes of these error messages and the fixes you can implement to resolve them. First, we will walk you through a quick summary of the key takeaways included in this guide.

Key Takeaways

• What the JavaScript heap out of memory error means: The “JavaScript Heap Out of Memory” error is a protective crash that occurs when the V8 engine exhausts its pre-allocated memory for dynamic data like objects and closures.
• Identify the Context: Use Node.js logs to find fatal error messages. You can also use the Chrome DevTools Memory tab to capture heap snapshots that reveal objects taking up memory.
• Scale the Hardware: For an immediate fix, manually increase the memory limit using the –max-old-space-size flag in your terminal.
• Refactor for Efficiency: Avoid batch processing large datasets in a single variable. Instead, use streams and pagination to process data in small, manageable chunks that the Garbage Collector can easily clear.
• Practice Proactive Prevention: Minimize memory leaks by avoiding global variables, clearing event listeners, and using WeakMaps to ensure the V8 engine can effectively remove unused references, minimizing chances of JavaScript heap out of memory errors. 

What Does the “JavaScript Heap Out of Memory” Error Mean?

The JavaScript Heap Out of Memory error is a frustrating crash that occurs when the V8 engine uses all its pre-allocated memory pool. This usually happens when the application in question is creating more objects than the Garbage Collector can clear. The common error message you will see is “call_and_retry_last allocation failed”.

Memory limits in JS affect both frontend and backend developers and can slow down the development process if not handled promptly. The memory limit will typically vary based on the resources of the systems the application is running on. Applications running on systems with more memory have more room to access more memory. 

Common Causes of the “Heap Out of Memory” Error

In this section, we will discuss some of the common causes of JavaScript heap out of memory error: 

Large or Unbounded Data Processing

For example, attempting to load massive datasets, such as multi-gigabyte JSON files or CSVs also causes Javascript heap errors. This is because loading these massive datasets directly into memory overwhelms the heap since JS must store the entire structure at once.

Memory Leaks in Code

Memory leaks occur when your code maintains references to objects that are no longer needed. Such objects can include unremoved event listeners and circular dependencies. Having these objects in memory prevents the Garbage Collector from freeing that space, which ultimately causes the application to exceed its memory allocations limit.

Inefficient Loops, Recursion, or Data Structures

Your program’s efficiency also significantly affects how much memory is utilized. For instance, poorly optimized logic, such as infinite recursion or loops that push data into an array indefinitely, can cause memory usage to spike exponentially until the system reaches its limit.

Below is a code example of an infinite loop:

constleakyArray = [];
// This loop simulates a process that never stops adding data
while(true) {
    constdataChunk = newArray(100000).fill('memory_sink');
    leakyArray.push(dataChunk); 
    // The heap cannot be cleared because 'leakyArray' holds a reference to everything
}

In the above code, the loop continuously pushes data into an array without ever clearing it, causing the heap to grow until it crashes.

Build Tools & Bundlers Using Excess Memory

Modern build tools such as Webpack, TypeScript, and Babel perform complex dependency mapping and transpilation in memory. These can often exceed default memory limits during the compilation of large projects, leading to Javascript heap errors. For instance, most of these tools tend to save previous work in memory to make the next build faster, which eventually fills up the available space and causes the JavaScript heap out of memory crashes. 

How to Identify the Error in Your Environment

Before fixing the JavaScript heap out of memory error, you first all need to identify it in your environment. Identifying these issues allows you to quickly find the most relevant fix for it. Let’s discuss the different ways to identify these issues. 

Reading Node.js Error Logs

The number one step you need to take when debugging in any language is reading logs.

One of the logs that usually appears in terminal is the  FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed – JavaScript heap out of memory. This allocation failed log is often followed by a stack trace pointing to the last failed allocation.

Using Chrome DevTools & Memory Snapshots

In the Google Chrome browser, use the Memory tab to take “Heap Snapshots” or record “Allocation Instrumentation.” You should take one snapshot, perform an action in your app such as opening a modal, and then take a second snapshot. This allows you to visually identify which objects are growing in size and which are failing to be garbage collected. You can then pay attention to that object that consumes memory in a way that wasn’t intended. 

Monitoring Build Tool Output (Webpack, TS, etc.)

During builds, the JavaScript heap of memory error usually appears as a sudden termination of the JS processes with an Exit Code 134 or a generic Command failed message right after the “Building” or “Emitting” progress step. Your built tools usually crash during the most “brain-heavy” parts of the build. Always watch your progress bar and if it dies right after building/compiling, emitting or minifying, it’s usually a memory issue.

Fix #1: Increase the Node.js Memory Limit

The most effective way to fix memory-related crashes is by increasing the default limits for memory allocation. But as discussed earlier, this will largely depend on the amount of memory available on the host system. 

Using –max-old-space-size

This flag tells the V8 engine to expand its “old space” (the largest part of the heap) to a specific number of megabytes, overriding the default environment limits. Depending on the memory available on the host system, you can increase the defaults using the commands below: 

For 4GB: node –max-old-space-size=4096 app.js

For 8GB: node –max-old-space-size=8192 app.js

The above commands will adjust the memory limit to 4G or 8GB respectively. You can even allocate more memory beyond this if the system has way more memory to spare. 

Please note: Allocating memory than what’s available on the host will crash the app immediately. Before issuing more memory to the application, first confirm the available memory on your system if you’re unsure. 

Updating NPM/Yarn Scripts

To make the fix permanent for your project, add the flag directly to your package.json scripts:

“start”: “node –max-old-space-size=4096 server.js” or for build tools: “build”: “NODE_OPTIONS=’–max-old-space-size=4096′ next build”.

In the above script, npm start command to explicitly launch the Node.js runtime with 4GB of memory allocated to the heap.

Fix #2: Optimize Your Code to Reduce Memory Usage

Even when you have enough hardware resources, writing efficient code is crucial to reducing JavaScript heap out of memory errors. Let’s discuss some of the best practices for optimizing your code: 

Freeing Unused Variables and References

Start by manually clearing references by setting larger objects, arrays, or event listeners to null or undefined signals to the Garbage Collector that the memory is safe to reclaim.

Avoiding Heavy In-Memory Operations

You can redesign your app’s logic to avoid creating deep copies of massive arrays or storing entire database result sets in a single variable. Instead, you can use iterative methods that process only what is necessary for the current operation to avoid overloading memory. Here are examples showing the bad way and the good way to do it. 

The bad way it’s done:

consthugeData = newArray(1000000).fill({ name: "User"});
constmodifiedData = hugeData.map(item => ({ ...item, active: true}));

In the code above, a full copy of a huge array was created just to change one property, which doubles your memory usage instantly.

The good way:

for (let i = 0; i < hugeData.length; i++) {
    hugeData[i].active = true;
}

In the above code items were modified without creating a second 1-million-item array, minimizing memory usage. 

Fix #3: Use Streams for Large Data Handling

Reading/Writing Large Files with Streams

Instead of fs.readFile(), which loads a file entirely into the heap, use fs.createReadStream() to process the file in small, manageable chunks that are cleared from memory as soon as they are handled.

const fs = require('fs');
const readStream = fs.createReadStream('massive-log.txt');
readStream.on('data', (chunk) => {
    console.log(`Received ${chunk.length} bytes of data.`);
});
readStream.on('end', () => {
    console.log('Finished reading file.');
});

In the code above, using streams reads the file in small chunks (usually 64KB each). So, as soon as one chunk is processed, it is removed out of memory to make room for the next one.

Using Streams with APIs or Databases

When fetching thousands of rows from a database or sending large API responses, use cursor-based streaming to pipe data directly from the source to the destination. This method ensures the heap usage remains flat regardless of total data sizes.

Fix #4: Profile Your Memory Usage

Built-In Node.js Inspector

Start your application with the –inspect flag to connect the Node.js process to Chrome DevTools. This step allows you to monitor real-time memory consumption and trigger garbage collection manually. Include the flag when running the script in terminal as shown here: node –inspect app.js.

Heap Snapshots & Allocation Timelines

Capture “Heap Snapshots” at different intervals to compare object growth. You can then use ”Allocation Timelines” to identify exactly which function or operation is causing a sudden spike in memory usage.

Understanding How the JavaScript (V8) Engine Manages Memory

To have a clear understanding of the causes and fixes of  the “Heap Out of Memory” error, you need to know how the V8 engine works. This knowledge helps manage JavaScript heap out of memory errors from an informed perspective. 

The V8 engine divides memory into the Stack. The Stack is used for static memory allocation to store primitive variables such as numbers, strings, and booleans. The Heap is for storing dynamic data such as objects. 

To ensure efficient operations, the V8 engine uses a Garbage Collector to automatically find objects in the heap that the application no longer needs and delete them to free up space.

Key Concepts That Impact Memory Usage

• The Call Stack: It is like a to-do list that tracks all the functions that are currently running. Deep recursion occurs when a function calls itself too many times, which can lead to stack overflows or retained memory.
• Closures: A closure is a function that “remembers” the variables from the environment where it was created, even after that environment is gone. If you create a closure inside a large function that handles a massive array, that entire array stays in memory as long as the closure exists, which can become a source of “hidden” memory leaks.
• Object References: JS uses a “mark-and-sweep” algorithm. This means any object still referenced by a global variable or active function cannot be deleted.

How to Prevent Memory Issues Before They Occur

Here are steps you can take to potentially prevent memory issues from ever happening again: 

• Implement Pagination: Never fetch an entire database table. You should instead always use LIMIT and OFFSET (or cursor-based pagination).
• Use WeakMaps and WeakSets: Use these for object caching as they do not prevent the Garbage Collector from removing objects when they are otherwise unreferenced. This allows effective memory clean up. 
• Clean Up Listeners: Always remove event listeners and clear setInterval timers when a component or service is destroyed.
• Set Memory Limits in CI/CD: You should always configure your build pipelines to fail if memory usage exceeds a specific threshold to catch leaks before deployment. Such fails minimize chances of unintended JavaScript heap out of memory errors.  

FAQs

Is the “heap out of memory” issue specific to Node.js?

No, the JavaScript heap out of memory error can happen with several other runtime environments besides Node.js. For instance, it can also occur in web browsers when a script exceeds the memory limits set by the browser for a single tab.

What should I do if this error appears during development?

First, identify if the crash happened during a build or at runtime. If this issue happens due to a build tools crash, consider increasing the memory limit via NODE_OPTIONS. However, it is the code that caused the error, use a profiler to find memory leaks.

Are there best practices for managing memory in JavaScript?

Some of the best practices you can follow include:

• Avoid global variables.
• Use local scoping.
• Clean up event listeners/intervals.
• Always use streams or pagination over loading large datasets into memory at once.

Can bundlers like Webpack or TypeScript cause this issue?

Yes, it is common for these tools to exhaust the heap because they store entire dependency trees and source maps in memory during the compilation of large-scale projects.