Designing a tree-shakable library means structuring it so a consumer's bundler can confidently drop the parts they don't import.

The requirements

1. Ship ESM with named exports. Tree shaking relies on ES modules being statically analyzable. Ship an ESM build (alongside CJS) and use named exports — export function debounce() — not a single default-exported object bundling everything (which can't be partially dropped).

2. Mark sideEffects: false in package.json. This is the explicit signal: "importing any module in this package has no side effects, so if an export isn't used, it's safe to remove the whole module." Without it, bundlers are conservative and keep modules "just in case." If some files do have side effects (a CSS import, a polyfill), list them: "sideEffects": ["*.css", "./src/polyfill.js"].

3. Actually be side-effect-free. The flag is a promise — the code must keep it. No work at import time: don't mutate globals, don't run code at module top-level, don't register things on import. Side effects are exactly what blocks the bundler from removing a module.

4. Many small pure modules, careful barrels.

• Keep functionality in small, focused modules.
• A barrel file (index.js re-exporting everything) is fine if every module is side-effect-free and sideEffects: false is set — then unused re-exports get dropped. If not, the barrel drags everything in.
• Avoid re-exporting giant namespaces.

5. Don't force namespace imports. Design so consumers can do import { thing } from "lib" and get just thing. Avoid APIs that only work via import * as Lib.

6. Keep modules pure and decoupled. Pure functions, minimal cross-module coupling — so dropping one export doesn't accidentally require keeping ten others.

How to verify

• Build a test app that imports one thing, inspect the output bundle (or use a bundle analyzer) — confirm the rest didn't come along.
• This is exactly why lodash-es exists: ESM, per-function modules, side-effect-free — so import { debounce } ships only debounce.

The framing

"Tree-shakability means a consumer's bundler can safely drop what they don't import. So: ship ESM with named exports — not one default object — because tree shaking needs static analyzability. Mark sideEffects: false so the bundler isn't conservative, and actually be side-effect-free — no work at import time, no global mutation, since side effects are what block removal. Structure it as many small pure modules; barrels are fine only if everything's side-effect-free. And design the API so named imports work — don't force import * as. Then verify with a bundle analyzer that importing one thing doesn't drag the rest."