In this lesson, you will learn why the JavaScript ecosystem transitioned from the legacy CommonJS module system to the modern ECMAScript Module standard.
The Era of CommonJS (CJS)
When Node.js was first created, JavaScript lacked a built-in way to organize code into separate files. The community introduced CommonJS, which uses require to import files and module.exports to share them. It was a brilliant "hack" that allowed Node.js to thrive by loading files synchronously—meaning when you called require, the code would pause until the file was fully read from the disk.
The Shift to ESM
As JavaScript moved beyond the server and into web browsers, it needed a standardized, "official" way to handle modules. This birthed ECMAScript Modules (ESM), which use import and export syntax. Unlike CommonJS, ESM is designed to be asynchronous.
Why the change? Because browsers cannot afford to freeze the entire page to wait for a file to download over the network. By shifting to ESM, the runtime can "parse" your file structure before actually running the code. This allows for features like static analysis, where tools can identify which parts of your code are actually used (a process called "tree-shaking") to make your applications much faster.
Comparing the Syntax
In CommonJS, you import everything at runtime:
const math = require('./math.js');
In ESM, you import as a structural dependency:
import { add } from './math.js';
The rationale for this evolution is clear: while CommonJS was essential for the early days of server-side Node.js, the shift to ESM creates a unified language standard that works identically in both browsers and servers. It allows for better tooling, faster startup times, and a safer, more predictable way to manage how your code relies on other files.
Key Takeaway: ESM provides a standardized, asynchronous module system that improves performance through static analysis, replacing the synchronous, runtime-based behavior of CommonJS.