Tree Shaking: A Deep Dive into Dead Code Elimination and Shipping Less JavaScript

Tree shaking, an essential technique in modern frontend performance optimization, promises to eliminate unused code from your JavaScript applications. It's akin to pruning a tree, trimming off unnecessary branches to keep it healthy and efficient.

This comprehensive guide will delve into:

• The concept and mechanics of tree shaking
• The role of bundlers like Webpack and Rollup in tree shaking
• Practices that undermine tree shaking
• Strategies for writing tree shakeable modules
• Real-world case studies of tree shaking in libraries like Lodash and Material UI

Understanding Tree Shaking

Tree shaking is a dead code elimination technique implemented during the bundling process. It statically analyzes your code to identify unused imports and dependencies, which are then removed from the final bundle.

This concept, originating from compiler theory, became viable in JavaScript with the advent of ES6 modules (ESM). Prior to ESM, JavaScript primarily used CommonJS (require()), a dynamic module system that isn't statically analyzable. However, with ESM (import { x } from "lib"), bundlers can determine which exports are utilized and discard the rest. Rollup initially popularized this technique in frontend development, with Webpack subsequently adding support from version 2 onwards.

The Mechanics of Tree Shaking

Tree shaking hinges on three primary elements:

1. Static imports: These are facilitated by ESM syntax, allowing a bundler to statically analyze your code.
2. Pure functions: Code that doesn't produce side effects is essential for tree shaking.
3. Bundlers that comprehend module graphs: These bundlers can understand and manipulate the dependency relationships between different modules.

Let's illustrate this with a simple example:

// math.js
export function add(x, y) { return x + y; }
export function subtract(x, y) { return x - y; }

// index.js
import { add } from './math';

In the above code, the subtract() function is never invoked. Therefore, a bundler can discard it during the tree shaking process.

Bundler Support for Tree Shaking

Rollup

Rollup was explicitly designed to support ESM. It automatically performs tree shaking, effectively discarding unused classes, functions, and imports.

export default {
  input: 'index.js',
  output: { file: 'bundle.js', format: 'esm' },
  treeshake: true
}

Webpack

Webpack facilitates tree shaking when it meets the following conditions:

• The code employs ESM syntax
• The mode is set to "production"
• sideEffects are correctly labeled in package.json

{
  "sideEffects": false
}

This configuration informs Webpack that it's safe to eliminate unused files/modules.

How to Break Tree Shaking

Tree shaking will fail under these situations:

• Using CommonJS (require())
• Importing entire libraries (e.g., import _ from "lodash")
• Modules producing top-level side effects
• Dynamically mutating exports

For instance, consider the following code:

export const result = someFunction();

If someFunction() has side effects, bundlers will be compelled to retain it, thereby breaking tree shaking.

Real-World Applications of Tree Shaking

Lodash

Previously, import _ from 'lodash' would incorporate the entire library, which is approximately 70KB when gzipped. Currently, you can import individual functions:

import debounce from 'lodash/debounce';

Alternatively, you can use the babel-plugin-lodash to automatically optimize imports.

Material UI

Earlier versions of Material UI (MUI) weren't tree-shakeable, necessitating the importation of complete component libraries. However, starting with version 5, MUI supports full tree shaking, thanks to ESM exports and emotion-styled components.

Designing Tree-Shakeable Code

To write tree-shakeable code:

• Use pure exports, such as stateless functions and constants
• Avoid export default unless necessary
• Mark sideEffects: false in package.json
• Refrain from using dynamic require() or eval
• Break your library into granular submodules

Tools for Visualizing Bundles

Tools like Webpack Bundle Analyzer, Rollup Plugin Visualizer, and Source Map Explorer can help you visually inspect your bundle, allowing you to see which parts are included and which are excluded.

Tree Shaking Anti-Patterns

Avoid these anti-patterns when aiming for tree shaking:

• Using libraries without considering their size
• Re-exporting everything in index.js
• Mixing CommonJS and ESM
• Depending on runtime configuration for modularity

Conclusion: Delivering Only What You Need

Tree shaking is more than a bundler feature — it's a design philosophy that rewards simplicity and penalizes complexity. Well-structured code results in a leaner bundle, leading to faster applications for your users. So, shake that tree, prune those branches, and let your bundle breathe.