Alex Anderson

Tree Shaking

August 02, 2019 Alex Anderson Devops


This is a post in a series about code projects, open source, build tools, and deployment.

Modern JavaScript development is awesome! You can go to NPM (or another package repository) and get code that someone else has written and load it into your application without having to copy/paste or write it yourself. One great example is Lodash, a package that bills itself as

A modern JavaScript utility library delivering modularity, performance, and extras.

With Lodash, I have tools for arrays, objects, functions, and promises. I can throttle, debounce, map, sort, flatten, union, zip, and more! Think of the benefits of using this over writing my own utilities:

  • It is battle tested, since many other people are using it
  • It has unit and integration tests
  • It handles edge cases
  • It is largely faster than naïve implementations

There's only one problem.


Bundlephobia says the current version is 69.2kB minified. That might not seem like a lot (React-DOM weighs in at 103.7kB), but if I'm only using a single measly function I dont' want to be pulling in the whole bundle.

Enter Tree Shaking.

What is Tree Shaking?

The purpose of tree shaking is removing modules from packages that aren't being included in your app. The idea behind the term is that a package has its individual exports which have their own files they depend on, fanning out in tree structure. If I only need one of those exports, I can shake up the tree to remove the branches that aren't necessary1. This is also called dead code elimination.

So, instead of grabbing the whole lodash package, I only grab the debounce package, which only costs 1kB, saving me 22.7kB.

Let's see how this is done.

Tree Shaking in Practice

Suppose I have an app that throttles the number of times I click, so it only increments a counter once every 500 milliseconds:

import React from "react"
import ReactDOM from "react-dom"
import { throttle } from "lodash"
const App = () => {
const [count, setCount] = React.useState(0)
const throttleSetCount = React.useCallback(
throttle(() => {
setCount(count => count + 1)
}, 500),
return (
<h1>Count: {count}</h1>
<button onClick={throttleSetCount}>Add One</button>
ReactDOM.render(<App />, document.getElementById("root"))

For such simple functionality, I've added at least 21kB to my app! Lets fix this by selectively importing just the function that we need.

import React from "react"
import ReactDOM from "react-dom"
import throttle from "lodash/throttle"

Now, instead of pulling in the whole library, it only pulls in the parts that are necessary for it to function, nothing more.

You can also tree-shake your app's code, but it takes a bit of manual work. There's a [guide for doing this with Webpack.]


Tree shaking is great, but it doesn't automatically work. There are a couple of conditions that must be met:

  • You have to be using the new ES6 import/export syntax. Tree shaking doesn't work with CommonJS modules, like require()
  • This only works with packages that are configured to use it. It works great with lodash; your milage may vary with THREE.js
  • Modules with side-effects might accidentally be removed, which can cause errors for your app.

Regardless, this can be a really handy way of keeping your bundle size down, which in turn decreases the time to parse your JavaScript. That makes for a faster app and a happier user!

1 I suppose the term "prune" was already being used somewhere else.
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