How Can Developers Optimize JavaScript Execution to Reduce Script Evaluation Times and Improve Key Performance Metrics Like Time to Interactive (TTI)?


To optimize JavaScript execution and reduce script evaluation times, developers can follow several strategies such as code splitting, deferred loading, minimizing heavy computations, and more. Improving key performance metrics like Time to Interactive (TTI) involves not only optimizing JavaScript but also improving critical rendering paths and server response times. Below is a detailed guide on these practices.

Code Splitting

Understanding Code Splitting

Code splitting allows developers to break down their JavaScript bundle into smaller chunks that can be loaded on-demand or in parallel. This reduces the initial load time and improves performance.

For example, using Webpack for code splitting:

module.exports = {
optimization: {
splitChunks: {
chunks: 'all',


Deferred and Asynchronous Loading

Using defer and async Attributes

JavaScript files can block the rendering of the page, increasing the Time to Interactive (TTI). To mitigate this:

1. Use the <script defer src="script.js"></script> attribute to load JavaScript after the HTML has been parsed.

2. Use the <script async src="script.js"></script> attribute to load scripts asynchronously, which can run as soon as they've been downloaded.


Minimizing Heavy Computations

Offload Tasks to Web Workers

Web Workers provide a way to run scripts in background threads, freeing up the main thread from heavy computations:


// main.js
const worker = new Worker('worker.js');
worker.postMessage('Start computation');

worker.onmessage = (event) => {
console.log('Worker result:',;

// worker.js
onmessage = (event) => {
// Perform heavy computation
const result = heavyComputation(;


Lazy Loading

Loading JavaScript on Demand

Lazy loading allows the delay of loading non-critical JavaScript resources until they are needed:

Example using Intersection Observer API:

const lazyScriptObserver = new IntersectionObserver((entries) => {
entries.forEach((entry) => {
if (entry.isIntersecting) {
const script = document.createElement('script');
script.src ='data-src');

document.querySelectorAll('script[data-src]').forEach((script) => {


Optimizing Critical Rendering Path

Inlining Critical JavaScript

Inlining essential JavaScript reduces round-trip times by embedding critical scripts directly within the HTML:

// Inline critical JavaScript here



Optimizing JavaScript to reduce script evaluation times and improve Time to Interactive (TTI) involves a multifaceted approach. Adopting techniques like code splitting, deferred loading, minimizing computations, lazy loading, and optimizing the critical rendering path can significantly enhance performance. Carefully implementing these strategies ensures a faster, more responsive user experience.