What is tree shaking and what conditions must be met for it to work?

Tree shaking is the process of removing unused exports from a bundle by statically analyzing which imports are referenced.

Conditions required:

• ES Modules (ESM) — import/export are statically analyzable. CommonJS require() is dynamic so bundlers can't determine what's used at build time.
• sideEffects: false in package.json — tells the bundler it's safe to drop any module that isn't imported. Without this, bundlers assume every module might have side effects (e.g., polyfills modifying globals).
• No dynamic import patterns — import(variable) defeats static analysis.
• Bundler support — Webpack, Rollup, esbuild, Vite all support tree shaking by default for ESM.

// utils.ts — only 'add' is imported, 'multiply' will be shaken out
export function add(a: number, b: number) { return a + b; }
export function multiply(a: number, b: number) { return a * b; }

// index.ts
import { add } from './utils'; // multiply never bundled