10 Ways to Optimize JavaScript Code Performance

JavaScript is a key language for web development, but with growing applications, its performance is of great significance. Optimize JavaScript code performance improves the speed and efficiency of your applications. According to Google, 53% of mobile users leave a site that takes longer than 3 seconds to load. Moreover, Akamai’s survey found that conversion rates can decrease by 7% just because website load time was delayed by only 100 milliseconds. This means that JavaScript performance should be focused on so as to improve speed and enhance user experience, as well as possibly increase conversions.

1. Understand JavaScript Performance

JavaScript engines such as V8 (used in Chrome) and SpiderMonkey (used in Firefox) are highly optimized, but having some knowledge about how they work will help you write faster code.

JIT Compilation – At runtime, JavaScript engines use Just-In-Time (JIT) compilation to convert JavaScript into machine code. Optimizing the code becomes better if it has more predictability.
Garbage Collection – Memory that is no longer used is automatically freed up through this process. Time taken during garbage collection can be reduced by having effective memory management.

2. Profiling and Benchmarking

it is very important to identify where the bottlenecks are before optimizing Javascript code. Profiling and benchmarking tools are useful and play an important role.

Chrome DevTools: Examine the runtime performance of your app using the Performance panel. Here is how to get started:

Open Chrome DevTools (right click on your page and choose “Inspect”).
Switch to “Performance” tab.
Press “Record” and enact the actions you want to profile.
Click “Stop” for the recording to finish and see performance data.

function exampleFunction() {  
    for (let i = 0; i < 1000000; i++) {  
        // Simulate a heavy task  
    }  
}  
exampleFunction();

While exampleFunction is running, record its performance and calculate the time taken.

Lighthouse: An inbuilt tool within Chrome DevTools for auditing web pages.

Start Chrome DevTools.
Select the “Lighthouse” tab.
Click on “Generate Report” to execute an audit.

Benchmark.js: A JavaScript library that measures the execution speed of your code.

const Benchmark = require('benchmark');  
const suite = new Benchmark.Suite;  

suite.add('Example', function() {  
    for (let i = 0; i < 1000000; i++) {  
        // Simulate a heavy task  
    }  
})  
.on('cycle', function(event) {  
    console.log(String(event.target));  
})  
.on('complete', function() {  
    console.log('Fastest is ' + this.filter('fastest').map('name'));  
})  
.run({ 'async': true });

3. Optimizing Loops and Iterations

Loops are often a major cause of inefficiency. Below are a few tips on how you can optimize them:

Avoid Expensive Operations Inside Loops: Moving costly operations out of loops can help avoid repeated calculations.

// Inefficient way to write loops  
for (let i = 0; i < items.length; i++) {  
    process(items[i]);  
}  

// Efficient way write loops 
const length = items.length;  
for (let i = 0; i < length; i++) {  
    process(items[i]);  
}

Use Array Methods: forEach, map, reduce etc. These methods are often more optimized than traditional loops, especially when working with arrays or array-like objects as well as collections like sets or maps among others.

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

4. Efficient DOM Manipulation

Manipulating the DOM entails some of the most expensive operations in JavaScript. For better performance, minimize these operations as much as possible.

Batch DOM Updates: DocumentFragment is used to batch updates together so as to reduce reflows and repaints while updating the browser’s rendering tree.

const fragment = document.createDocumentFragment();  
items.forEach(item => {  
    const element = createElement(item);  
    fragment.appendChild(element);  
});  
document.body.appendChild(fragment);

Virtual DOM: React makes use of virtual DOM which allows for faster update process through making comparisons between newly generated VDOM trees after changes have been made by comparing it with old ones in order to determine whether changes are needed or not before committing actual frequent updates.

5. Event Handling Best Practices

For a responsible application, it is necessary to have effective event handling.

Event Delegation: Add one listener to a parent element instead of adding multiple listeners for an event on many elements at once.

document.body.addEventListener('click', event => {  
    if (event.target.matches('.item')) {  
        handleItemClick(event);  
    }  
});

Throttling and Debouncing: Restrict the frequency of calls made by event handlers that are associated with events, such as scroll and resize.

function debounce(func, wait) {  
    let timeout;  
    return function(...args) {  
        clearTimeout(timeout);  
        timeout = setTimeout(() => func.apply(this, args), wait);  
    };  
}  

const handleResize = debounce(() => {  
    console.log('Window resized');  
}, 200);  

window.addEventListener('resize', handleResize);

6. Memory Management Techniques

Efficient memory management helps avoid leaks and ensures smooth performance.

Avoid Global Variables: These can lead to memory leaks. Instead use local variables and closures.
Use WeakMap and WeakSet: They prevent memory leaks by allowing garbage collection of their entries.

const weakMap = new WeakMap();  
const obj = {};  
weakMap.set(obj, 'value');

7. Lazy Loading and Code Splitting

Increase performance by only loading code resources when required.

Lazy Loading: Load images, scripts, or other resources when they are needed such as through IntersectionObserver API.

<img src="placeholder.jpg" data-src="actual-image.jpg" class="lazy-load">

Code Splitting: Break your code into smaller chunks using tools such as Webpack that can be loaded on demand.

import(/* webpackChunkName: "myChunk" */ './myModule').then(module => {  
    module.doSomething();  
});

8. Using Web Workers for Parallel Processing

Web Workers give you the ability to run scripts in background threads, thus freeing up the main thread which results in better performance.

Creating a Web Worker: When dealing with heavy computations and data manipulations, web workers come handy as they carry out CPU-intensive tasks well.

const worker = new Worker('worker.js');  
worker.postMessage('start');  

worker.onmessage = function(event) {  
    console.log('Message from worker:', event.data);  
};

Web Workers are useful for CPU-intensive tasks like data processing and complex calculations.

9. Leveraging Modern JavaScript Features

Modern JavaScript (ES6+) has some improvements that can be beneficial to your code’s optimization.

Arrow Functions: More concise and potentially faster.

const add = (a, b) => a + b;

Destructuring: It makes your code cleaner and possibly faster.

const { name, age } = person;

Async/Await: Makes writing asynchronous code easier, resulting in better performance.

async function fetchData() {  
    const response = await fetch('https://api.example.com/data');  
    const data = await response.json();  
    console.log(data);  
}

10. Other Important Areas

Minimize HTTP Requests

Combine Files: Reduce the number of HTTP requests by combining CSS and JavaScript files.
Use HTTP/2: HTTP/2 allows multiplexing multiple requests over a single connection, reducing the overhead of multiple HTTP connections.

Minify and Compress Code

Minification: Use tools like UglifyJS or Terser to minify your JavaScript code, removing unnecessary characters.
Compression: Enable Gzip or Brotli compression on your web server to compress JavaScript files before they are sent to the client.

Efficient Use of Caching

Cache Static Assets: Use cache headers to cache static assets like JavaScript files, CSS, and images.
Service Workers: Utilize Service Workers to cache assets and manage offline capabilities efficiently.

Don’t Block the Main Thread

Asynchronous Loading: Make JavaScript files load asynchronously by using the async or defer attributes18.

<script src="script.js" async></script>  
<script src="script.js" defer></script>

Avoid Long-Running Scripts: Break down long-running scripts into smaller tasks using setTimeout, setImmediate, or requestIdleCallback.

11. Best Practices for Writing Efficient JavaScript Code

Keep It Simple: Your code should be both simple and easy to read. On the other hand, it is very hard to optimize and debug complex codes.
Use Modern JavaScript Syntax: For instance, you can utilize let, const, arrow functions and destructuring in ES6+, that will make your code more efficient and easier to understand.
Modularize Your Code: Break your code into small, reusable modules. This helps with management and optimization.
Avoid Global Scope Pollution: Minimize global variables to avoid conflicts and potential memory leaks.
Use Strict Mode: Switch on strict mode to capture frequent coding errors in advance without allowing them to happen.

Conclusion

Optimizing JavaScript performance is a continuous process involving multiple strategies. These are all areas mentioned in this guide, plus some other techniques which provide a comprehensive approach towards making JavaScript apps faster as well as more efficient.

If these best practices above are adhered to alongside utilization of available tools and techniques then not only will your javascript code be performant but also scalable.

References

Footnotes

Google. “Find Out How You Stack Up to New Industry Benchmarks for Mobile Page Speed.” https://www.thinkwithgoogle.com/marketing-strategies/app-and-mobile/mobile-page-speed-new-industry-benchmarks/ ↩
Google Developers. “Lighthouse.” https://developers.google.com/web/tools/lighthouse ↩
Benchmark.js. “Benchmark.js Documentation.” https://benchmarkjs.com/ ↩
React Documentation. “Virtual DOM.” https://reactjs.org/docs/faq-internals.html ↩