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What is tree shaking?

Have you heard someone use the phrase “tree shaking” while talking about JavaScript, and wondered what the hell they were talking about? This article’s for you!

Browser-native ES modules and performance issues

ES modules provide you with a native way to break your code into smaller, modular parts, and keep variables and functions scoped to just where they’re needed.

Note: this article will make a lot more sense if you’re familiar with ES modules already.

In my pocket guide on ES modules, I note:

When you import a function or variable, the entire file for that module has to be downloaded. If you’re importing just a single function from a file that contains hundreds of them, you end up downloading far more JavaScript than you actually need.

For example, let’s imagine we have a helper library that exports three utility functions.

// All of the library code...

export {shuffle, debounce, dedupe};

In another file, we want to use the shuffle() function, so we import it.

import {shuffle} from './path/to/my-library.js';

// Shuffle an array of wizards
let wizards = ['Gandalf', 'Radagast', 'Merlin'];

If you’re using browser-native ES modules, loading your script with [type="module"], the entire my-library.js file is downloaded, compiled, and parsed by the browser.

You only needed shuffle(), but the browser has to grab the entire file to get that function for you. As you can imagine, this is a performance issue, especially with larger libraries.

This is where tree shaking comes in.

What is tree shaking, and how does it work?

Module bundlers are tools that take JavaScript files and combine or concatenate all of the import functions and variables into a single file.

To make files as performant as possible, modern module bundlers like rollup.js and esbuild use a process called tree shaking.

When they come across an import operator in a file, they only import the functions and variables you specified (and any internal variables or functions they use). Using our example above, the bundled file would contain only the shuffle() function and the wizards array.

function shuffle (arr) {
	// The code for this function...

// Shuffle an array of wizards
let wizards = ['Gandalf', 'Radagast', 'Merlin'];

Browser-native ES modules have other performance challenges as well, so I strongly recommend using a module bundler if you’re working with them.