How To Minify Javascript
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What minification actually does
Minifying JavaScript shrinks bundle size, speeds up page loads, and reduces the bytes users download on slow connections. Done right, it’s invisible — same behavior, fewer kilobytes. Done wrong, it breaks your app in production. This guide covers what minification actually does (whitespace removal, variable renaming, dead code elimination), the difference between minification and compression, when to use Terser vs esbuild vs SWC, source maps for debugging minified code, the gotchas that break minified bundles, and how modern bundlers handle all of this automatically.
Minification vs compression
A minifier reads your JavaScript source and outputs semantically equivalent but smaller code. The transformations include:
Source maps — debug minified code
Minification and gzip/brotli are complementary, not redundant.
Terser, esbuild, SWC — picking a minifier
Typical results: 100KB raw JS → 40KB minified → 12KB brotli. Both matter. Minification matters more for parse time (engine does less work on smaller input); compression matters more for transfer time.
Common breakages from minification
Most tuning happens through bundler config. Common Terser options worth knowing:
What minifiers can’t help with
You rarely need to run a standalone minifier. The exception: you’re shipping a single file or inline script that isn’t part of a build pipeline.