As developers, we often rely on external hook libraries to save time, leverage well-tested solutions, and focus on the bigger picture of our projects. However, it’s crucial to consider the impact these libraries have on your bundle size—a key factor in your app’s performance and loading speed. Let’s explore how these libraries impact bundle size, how to check if tree-shaking is supported, and how to make informed decisions.

Why Bundle Size Matters

• User Experience: Larger bundles take longer to download, parse, and execute, especially on slower networks or devices.
• SEO and Performance Scores: Tools like Google Lighthouse penalize heavy bundles, impacting your search rankings.
• Long-Term Maintenance: Larger bundles can obscure performance bottlenecks as your project grows.

External Hook Libraries: Convenience vs. Cost

Hook libraries are a common solution for handling complex state or reusable patterns, but their bundle cost depends on their structure:

Granular (Modular)

• Install only the hooks you need, keeping dependencies minimal.
• Example:

  import { useDebounce } from "hook-lib/useDebounce";

Monolithic (Tree-Shakable)

• Install one library, but ensure your build tool removes unused exports.
• Example:

  import { useDebounce } from "hook-lib";

Each approach has trade-offs. Granular libraries offer precise control over what’s added, while monolithic libraries are easier to manage but require proper tree-shaking to avoid bloat.

How Much Weight Do Hook Libraries Add?

The weight depends on:

• Library Size: Some libraries are lightweight (a few KB), while others can balloon to dozens of KB if they rely on dependencies.
• Tree-Shaking Effectiveness: If the library doesn’t support tree-shaking, you might import unused code.
• Usage: Importing a single hook might pull in shared utilities or polyfills, inflating the size.

Example Scenario:

• A lightweight library (use-fetch-hook) adds 5KB.
• A large, monolithic library with poor tree-shaking might add 30KB+, even if you only use one hook.

How to Check if a Library Supports Tree-Shaking

To check if a library supports tree-shaking, you can follow several approaches based on understanding its code structure and how it's bundled. Tree-shaking is a feature supported by modern JavaScript bundlers like Webpack and Rollup, which removes unused code during the build process. Here’s how you can determine if a library supports it:

1. Check the Library’s Package Documentation

• Look for ES Module (ESM) Support: For tree-shaking to work, the library must use ES Modules (ESM). ESM allows the bundler to analyze the import/export structure and safely eliminate unused code.
  • Check if the library provides an ESM build (often specified in the module or exports field of its package.json).
  • Search the documentation or repository to see if ESM is mentioned as the preferred usage.

2. Check the package.json of the Library

• Exports Field: For more recent packages, check if the exports field is used. This can specify different entry points for different environments (like CommonJS or ESM), improving tree-shaking support.
• Module Field: Look at the package.json file of the library. If it includes a module field that points to an ESM build, it indicates the library is compatible with tree-shaking. Example:

{
  "module": "dist/library.esm.js",
  "main": "dist/library.cjs.js"
}

• module points to the ESM version, which is tree-shakable.
• main typically points to the CommonJS version, which isn’t ideal for tree-shaking.

3.Check the Library’s Source Code

• Use of import/export: Ensure that the library uses ES module syntax (e.g., import and export). Tree-shaking works best with this syntax.

  • If the library uses CommonJS (require, module.exports), tree-shaking won’t be as effective.

• No Side Effects: Libraries that support tree-shaking typically avoid side effects in their code. Check the library’s source code to ensure that functions or modules don’t perform actions when they are imported. For example, importing a module should not alter global state.

4. Use a Bundler to Test Tree-Shaking

• You can use a modern JavaScript bundler (like Webpack or Rollup) to test if tree-shaking works. Here's a simple test:
  • Create a minimal project with the library installed.
  • Import only a part of the library in your code (e.g., a single function).
  • Run the bundler and check the output:
  • a) If the unused code is excluded from the final bundle, the library supports tree-shaking.
  • b) If the unused code is still included, then the library either doesn’t support tree-shaking or requires further configuration (like marking certain code as side-effect-free).

5. Use a Bundle Analyzer

Use tools like Webpack Bundle Analyzer or Rollup's built-in analyzer to visualize the final bundle.

• Look for the size of the library in the output. If tree-shaking works, unused code should be excluded, and the final size should be smaller.

6. Check the Community and Issues

Look at issues or discussions in the library’s repository (e.g., GitHub) to see if there are any mentions of tree-shaking or issues related to it. The maintainers may also provide guidance on enabling tree-shaking.

7. Look for Specific Build Instructions

Some libraries might have specific instructions for enabling tree-shaking, especially when they are not entirely tree-shakable by default. Check for any guidance on how to configure the bundler for optimal tree-shaking.

Example:

If you are using a library like Lodash, it has specific "modular" imports:

import { debounce } from 'lodash';

This allows bundlers like Webpack to shake off unused methods when using Lodash's modular imports, as opposed to importing the entire library (import _ from 'lodash'), which would include the entire codebase and prevent tree-shaking.