JavaScript Tree Shaking for Production Build Optimization
Jul 20, 2025 am 03:37 AMTree shaking is an optimization technique based on ES Module static analysis, used to remove unused code to reduce bundle volume. It relies on the import/export syntax, improves deletion efficiency by tagging no side effects codes (such as setting sideEffects: false), and supports it by default in tools such as Webpack, Rollup, Vite, etc. Common misunderstandings include functions being referenced but not called, modules having side effects, or third-party libraries not correctly label sideEffects. The verification method can be done by observing the change in the build volume or using an analysis tool to confirm the effect.
JavaScript Tree Shaking is a very practical technology in modern front-end construction processes, especially when packaging and optimization of production environments. It helps us remove unused code in the project, thereby significantly reducing the volume of the final bundle. If you are using packaging tools like Webpack, Rollup, or Vite, tree shaking is actually serving you silently.

Let’s take a look at some common problems and operation suggestions in actual development.
What is Tree Shaking?
Tree shaking is not a specific configuration item, but an optimization strategy to remove unused exported modules by statically analyzing the ES Module structure . Its core premise is that your code must use import
and export
(rather than CommonJS require
) so that the packaging tool can accurately determine which code is "dead code" during the build stage and can be safely removed.

To give a simple example:
// math.js export function add(a, b) { return ab; } export function multiply(a, b) { return a * b; }
// main.js import { add } from './math.js'; console.log(add(2, 3));
In this case, the packaging tool will find that the multiply
function is not introduced, so it will not be packaged into the final bundle file.

How to ensure Tree Shaking takes effect?
Not all writing methods can trigger tree shaking, you have to pay attention to some details:
- ? Use ES Module writing method (
import
/export
) - ? Avoid non-static paths in
require()
or dynamic import (import()
) - ? Mark code that you do not want to be packaged as side-effect-free (such as setting
"sideEffects": false
inpackage.json
)
Here are a few tips:
If you add
package.json
{ "sideEffects": false }
This means that your module has no side effects and can make the packaging tool more secure to delete useless code.
If some files do have side effects (such as CSS introduction, polyfill, etc.), you can write this:
{ "sideEffects": ["./src/some-polyfill.js", "**/*.css"] }
Common misunderstandings and precautions
Sometimes you think the code will be shaken, but the result is not. It may be because of these reasons:
- Although the function is not called, it is referenced , such as being an object property or being passed to a higher-order function.
- The module itself has side effects , such as performing global assignments, modifying the prototype chain, etc.
- The third-party library does not correctly label sideEffects , resulting in the unused code inside it being unable to be cleared.
For example:
import { createApp } from 'vue'; createApp(App).mount('#app');
If Vue does not set sideEffects correctly, or you use some plug-in injection logic, you may not be able to completely shake other code even if you use only some of the functions.
Build tool configuration suggestions
Different tools have slightly different support for tree shaking, but mainstream tools have enabled this feature by default:
- Webpack : enabled by default, but be careful that mode is set to
production
- Rollup : naturally supports tree shaking, suitable for packaging library
- Vite : Based on Rollup, the code is not compressed in dev mode, and it is packaged using Rollup when building, and tree shaking is also enabled by default.
You can check whether the useless code has been successfully removed by viewing the build bundle file size changes, or using visualization tools such as Webpack Bundle Analyzer .
Basically that's it. Although Tree shaking seems to be a high-end concept, it has become a standard configuration in modern projects. As long as you write a standard ES Module and configure the packaging tool reasonably, you can easily enjoy the performance improvement it brings.
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