Dynamic Import for JavaScript Code-Splitting

This article explains how dynamic imports function as the primary mechanism for code-splitting in modern JavaScript applications. You will learn the difference between static and dynamic imports, how modern module bundlers process on-demand requests into separate bundles, and how implementing dynamic imports enhances application load times and performance.

Static Import vs. Dynamic Import

In standard ES6 modules, static imports use the import ... from '...' syntax. Static imports must be declared at the top level of a file and are evaluated at build time. This means all imported code is bundled into a single JavaScript file (or set of entry files), forcing the browser to download and execute the entire codebase before rendering the application.

Dynamic import uses the function-like syntax import('module-path'). It can be called conditionally anywhere in the codebase—inside functions, event listeners, or conditional branches. Instead of executing at build time, it returns a Promise that resolves to the requested module when called at runtime.

// Static import: Loaded immediately
import { heavyUtility } from './heavyUtility.js';

// Dynamic import: Loaded on demand
button.addEventListener('click', async () => {
  const { heavyUtility } = await import('./heavyUtility.js');
  heavyUtility();
});

How Bundlers Enable Code-Splitting

When modern bundlers such as Webpack, Vite, Rollup, or esbuild encounter a dynamic import(), they recognize it as a split point. Rather than packaging the imported module into the main bundle, the bundler isolates that code into an independent chunk (e.g., chunk-xyz.js).

The process works as follows:

  1. Compilation Analysis: The bundler scans the Abstract Syntax Tree (AST) of the project and flags any dynamic import expressions.
  2. Chunk Generation: The target module and its isolated dependencies are compiled into a separate file.
  3. Runtime Fetching: In the browser, executing the import() statement automatically triggers an HTTP request (via a script tag or fetch) to download the required chunk from the server.
  4. Resolution: Once the chunk finishes downloading and executing, the Promise resolves, making the exported functions, objects, or classes available to the runtime environment.

Common Implementation Patterns

Route-Based Splitting

Single Page Applications (SPAs) commonly split code by route. When a user visits /dashboard, only the dashboard code is requested, while the code for /settings remains on the server until navigated to.

const routes = {
  '/home': () => import('./views/Home.js'),
  '/dashboard': () => import('./views/Dashboard.js'),
};

Interaction-Based Loading

Non-critical features, such as modals, date pickers, or file export utilities, can be loaded only when the user interacts with the relevant UI component.

async function exportToPDF() {
  const { generatePDF } = await import('./pdfGenerator.js');
  generatePDF();
}

Performance Benefits

Using dynamic imports for code-splitting directly improves key web performance metrics: