Free JavaScript Minifier – Compress JS Online

What JavaScript Minification Does and How Terser Compresses Code

Unminified JavaScript makes your users download every convenience you wrote for yourself: the indentation, the explanatory comments, the variable named currentUserSubscriptionStatus. None of it reaches the runtime with any meaning attached. This minifier runs Terser, the compressor that sits inside Webpack, Rollup, and most modern build chains, directly in your browser and returns functionally identical code at a fraction of the size. It suits developers pasting a snippet into a CMS field, anyone self-hosting a small library, and people who want to see the real saving on a file before wiring minification into a build. Terser works in three distinct stages, and knowing which one is doing the work helps you read the result. First it parses your source into an abstract syntax tree, which is why invalid input produces a parse error instead of mangled output. Then the compress pass rewrites the tree: constant expressions are folded, unreachable code after a return is dropped, if-else chains collapse into ternaries, and repeated literals may be hoisted. Finally the mangle pass walks each scope and renames local bindings to single letters, starting with a, b, c. Mangling is scope-aware, so it only renames identifiers it can prove are local — your exported functions, global names, and object property keys stay untouched, which is why the compressed file still works when called from elsewhere. The drop-console option removes console calls entirely during compression rather than leaving empty statements behind. Here is the effect on real code. Take this function: function calculateTotalPrice with parameters itemPrice and taxRate, a const declaration for taxAmount equal to itemPrice times taxRate, a console.log of the tax amount, and a return of itemPrice plus taxAmount, with a two-line comment above it. That is 236 bytes formatted. Minify with mangle and drop-console enabled and you get roughly function calculateTotalPrice open-paren a comma b close-paren then a const of a times b and a return — around 74 bytes, a saving near 69 percent. Notice what survived: the function name calculateTotalPrice, because it is a top-level declaration something else might call. The parameters became a and b, the comment and the console line vanished, and the intermediate const may be inlined into the return expression entirely if the compress pass judges it single-use. Where this matters in practice. A marketer pasting a tracking snippet into a Shopify theme field has a character budget and no build step available. A developer self-hosting a 40 KB utility library instead of loading it from a CDN wants it down near 15 KB before Gzip. Someone chasing a Lighthouse warning about unused JavaScript uses the reported saving to decide whether minification alone is enough or the real problem is a dependency. A WordPress plugin author ships a single compressed file because users will not run npm install. A developer writing an inline script for the document head keeps it tiny since it blocks parsing. A few honest cautions. Minified code is undebuggable in a browser console — that is what source maps are for, and this tool does not produce them, so keep your readable source in version control and only minify the deployed copy. Do not confuse minification with obfuscation or with security: mangled names slow a curious reader down for about a minute, and anything shipped to a browser is readable, so never hide an API secret in there. The pitfall that actually bites people is code depending on a function's name or a variable name at runtime, for example reading Function.prototype.name or looking up a property by a string that a bundler cannot see; mangling breaks that silently. Test the minified output before you ship it. Compression runs entirely in your browser, so proprietary and client code is never uploaded.

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