Build Tools & Bundlers Interview Questions and Answers

Tree shaking removes unused exports from the final bundle. It relies on ES modules being statically analysable: imports and exports are known at parse time, so the bundler can build a graph, mark reachable bindings, and drop the rest. It works best when modules are side-effect free, which is why package.json has "sideEffects": false or an array listing files that do have side effects. Without that hint the bundler keeps imports whose removal could change behaviour.

What breaks it: CommonJS require, because imports can be dynamic and conditional. Importing a namespace and accessing properties dynamically, such as import * as x and x[name]. Re-export barrels that pull in everything. Top-level code with side effects. Usage that cannot be statically traced.

Minifiers then remove dead code within a module, so production builds must be minified; unminified output often still contains unused code. Use named imports, avoid side effects at module scope, and inspect the bundle with rollup-plugin-visualizer or webpack-bundle-analyzer.

Library builds differ from app builds: you usually bundle to ESM and CommonJS, externalise peer dependencies such as React or Vue so consumers do not get duplicates, generate type declarations, and avoid bundling polyfills. Rollup is the classic choice because its output is clean and tree-shakeable, and tools like tsup, unbuild and Vite's library mode wrap it with sensible defaults. Configure exports with conditions for import, require and types, set sideEffects: false when true, and keep entry points minimal so consumers only pull what they use.

{
  "main": "./dist/index.cjs",
  "module": "./dist/index.js",
  "types": "./dist/index.d.ts",
  "sideEffects": false
}

For analysis, webpack-bundle-analyzer and rollup-plugin-visualizer show a treemap of module sizes. Common findings are a library imported wholesale, duplicated versions, all moment locales, or lodash pulling everything. Fix with per-function imports, aliasing to lighter alternatives, manualChunks for vendors, and checking duplicates with npm ls. Always measure gzipped or brotli sizes, not raw bytes.