In modern web development, making your applications fast and lightweight is more important than ever. One way to achieve this is through a technique called Tree Shaking, which helps remove unnecessary code and shrink your JavaScript bundles.
If you’ve used tools like Webpack or Rollup, you’ve probably come across the term. But what does it really mean, and how does it work?
In this guide, we’ll break down tree shaking in simple terms—why it’s important, how it works, and how you can use it in your projects. Whether you’re just starting out or have experience building apps, this article will make the concept easy to understand and apply.
What is Tree Shaking?
Tree shaking is a technique used to remove code that isn’t being used (also known as dead code) from your JavaScript bundles during the build process. The name comes from the idea of “shaking a tree” to drop off the unused branches—leaving only what’s necessary.
This method works best with ES6 (ES2015) module syntax because it uses static imports and exports. Since the structure is predictable, bundlers can easily figure out which parts of the code are actually needed and safely eliminate the rest.
Each component in a program can create symbols (like variables or functions), use symbols from other parts of the code, and depend on different files to work correctly. In addition, the program keeps track of whether certain pieces of code cause changes, known as side effects.
For example, if you write:
let firstName = ‘Jane’;
this does not create any side effects because the program’s behavior stays exactly the same even if this line is removed, as long as firstName is never used.
But if you write:
let firstName = getName();
this does create a side effect. That is because calling the function getName() might perform some action—like reading data, changing something in memory, or interacting with an external system—and removing the line would change how the program works.
In simple terms, side effects happen when a piece of code does something beyond just storing a value, and removing it would change the overall behavior of the program.
Only JavaScript modules that use the ES2015 (also known as ES6) module syntax — specifically the import and export keywords — can be tree-shaken effectively.
The way you import a module plays an important role in whether or not the build system can identify unused parts and safely remove them. If a module is imported in a way that suggests it might be used for its side effects, then the build system may keep the entire module even if only a small part is needed. On the other hand, if the module is imported in a clean, direct way that clearly shows which parts are being used, then the unused portions can be safely excluded from the final bundle.
Tree shaking starts by looking at the application’s entry point file — the main file where the program begins. It scans this file for any code that has side effects or is directly referenced. Then, it begins to follow the “edges” or connections between files, exploring each part that is actually used by the application. This process continues until there are no more new sections of code to visit.
Once the process is complete, the bundler includes only the parts of the code that were visited. Everything else — functions, variables, or modules that were never reached during the traversal — is removed. As a result, the final JavaScript bundle is lighter and faster to load.
For instance, imagine you have a file named utilities.js that contains several helper functions. Tree shaking ensures that if your program only uses one or two of these functions, the rest will be excluded from the final bundle.
Best Practices for Effective Tree Shaking
1. Write Modular Code
Always use ES6 module syntax (import and export) instead of CommonJS (require and module.exports). ES6 modules allow bundlers to analyze your code statically and determine exactly which parts are being used. This makes it easier to remove unused functions or variables during the build process.
2. Mark Side Effects
If your files don’t produce any side effects when imported (e.g., they don’t modify global variables or execute code at the top level), indicate this in your package.json by setting “sideEffects”: false. This tells the bundler it’s safe to remove unused exports without worrying about breaking your application.
3. Choose the Right Tools
Not all bundlers handle tree shaking equally. Rollup is known for excellent tree-shaking capabilities, especially for libraries, because it creates smaller and cleaner bundles. Webpack is a strong choice for larger applications, and with proper configuration, it can also perform efficient tree shaking.
4. Keep Dependencies in Check
Third-party libraries can add unnecessary weight to your bundle, especially if you’re only using a small portion of their functionality. Choose lightweight libraries or only import the parts you need. Consider alternatives to bulky packages to reduce unused code.
5. Enable Production Mode
Most bundlers, including Webpack, only perform tree shaking when production mode is enabled. Development mode typically skips this step to speed up builds. Make sure your build command includes production settings so unused code is stripped out in the final output.
Conclusion
Tree shaking has become a key technique in modern JavaScript development, helping developers deliver faster, more efficient, and more maintainable applications. By eliminating unused or “dead” code from your bundles, it not only reduces file sizes but also improves loading speed, which is crucial for providing a smooth user experience—especially on slower networks or mobile devices.
Thanks to build tools like Webpack and Rollup, implementing tree shaking is simpler than ever. However, to get the best results, you need to follow best practices—such as writing modular code using ES6 imports and exports, marking side-effect-free modules in your package.json, and keeping a close eye on third-party dependencies to avoid unnecessary bloat.
Understanding how tree shaking works, what its limitations are, and how to configure your build environment effectively ensures you’re creating web applications that are optimized for performance from the ground up. As you continue to refine your development workflow, consider incorporating tree shaking as a standard practice. The result? Smaller bundles, quicker load times, and happier users.
