Mastering Tree Shaking in JavaScript Libraries
Mastering Tree Shaking in JavaScript Libraries
Tree shaking, a powerful optimization technique, eliminates unused code from JavaScript bundles, resulting in smaller, faster applications. However, the way you structure your JavaScript library's exports significantly impacts the effectiveness of tree shaking. This post delves into the nuances of default versus named exports, highlighting best practices to ensure your library users reap the full benefits of this optimization.
Understanding Tree Shaking Limitations with Default Exports
While seemingly convenient, default exports can often hinder tree shaking, leading to larger-than-necessary bundles for your library consumers. Let's explore some common pitfalls.
The Object Pattern Problem
A frequent anti-pattern involves exporting multiple functions as properties of a single default-exported object:
// utils.js
function formatDate(date) { /* implementation */ }
function formatMoney(amount) { /* implementation */ }
function formatPhoneNumber(number) { /* implementation */ }
export default {
formatDate,
formatMoney,
formatPhoneNumber
};
When a consumer imports this module, even if they only use one function, the entire object is included:
// consumer.js
import utils from './utils';
utils.formatDate(new Date()); // Only using formatDate
The bundler can't identify and remove the unused formatMoney and formatPhoneNumber functions because the entire object is imported as a single unit. This defeats tree shaking.
Namespace Re-exports
Re-exporting namespace imports presents a similar challenge:
// constants.js
export const foo = 'foo';
export const bar = 'bar';
// namespaced-constants.js
import * as constants from './constants';
export { constants };
// consumer.js
import { constants } from './namespaced-constants';
console.log(constants.foo); // Only using foo
The bundler can't determine that only foo is used, so both foo and bar are included in the final bundle.
React Component Libraries
This problem is especially common in React component libraries:
// components.js
const Button = () => { </* implementation */> };
const Input = () => { </* implementation */> };
const Select = () => { </* implementation */> };
export default {
Button,
Input,
Select
};
If a consumer only needs Button, they still receive the entire library:
// App.js
import Components from './components';
function App() {
return <Components.Button />; // Only using Button
}
Enabling Tree Shaking with Named Exports
Named exports provide the solution for enabling effective tree shaking. Let's see how.
Individual Named Exports
The simplest approach is to use individual named exports:
// utils.js
export function formatDate(date) { /* implementation */ }
export function formatMoney(amount) { /* implementation */ }
export function formatPhoneNumber(number) { /* implementation */ }
Consumers can then import only what they need:
// consumer.js
import { formatDate } from './utils';
formatDate(new Date());
Bundlers can now precisely identify and remove unused functions.
Effective Re-exports
When aggregating exports from multiple files, use export * for optimal tree shaking:
// constants/index.js
export * from './dateConstants';
export * from './currencyConstants';
export * from './validationConstants';
// consumer.js
import { isoDateFormatter } from './constants';
Bundlers can efficiently trace imports through modules, including only the necessary code.
Component Library Example
For React, named exports are crucial:
// components.js
export const Button = () => { </* implementation */> };
export const Input = () => { </* implementation */> };
export const Select = () => { </* implementation */> };
// App.js
import { Button } from './components';
function App() {
return <Button />;
}
Or, for combining multiple exports:
// index.js
export * from "./buttons";
export * from "./inputs";
export * from "./select";
Modern bundlers will only include the Button component if that's the only one used.
Practical Impact on Bundle Size
The impact of this difference can be significant. A UI library with 50 components might be 500KB. With proper tree shaking, an app using only 5 components might only include 50KB, whereas without tree shaking, the entire 500KB would be included.
Key Takeaways
- Use named exports for all public API components.
- Avoid exporting objects with multiple properties as default exports.
- When aggregating exports, use
export *or individual named exports. - Consider ESLint rules to enforce these patterns.
- When using
export default, export from different modules; avoid re-exporting from a single module unless usingexport { default as Name }.
Conclusion
The choice between default and named exports significantly impacts performance. Named exports enable precise code inclusion, resulting in smaller, faster applications. Prioritize named exports for optimal tree shaking in your JavaScript libraries.
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