JS Minifier

How it actually works

The pass is region-aware before it is anything else. The source is split into code, string literals, template literals, regular-expression literals and comments, and whitespace is only removed between regions - never inside them. Two consequences that a naive regex minifier gets wrong are handled: a slash that opens a regular expression is distinguished from a division sign, and a newline is kept wherever it can change parsing, because an automatic semicolon insertion boundary is decided by the line break.

Comments are removed on request, whitespace is collapsed to the minimum that still parses, and a single space is retained only where two tokens genuinely need it, decided by looking at the characters on either side of the gap. Identifier shortening is a separate, optional switch and is off by default.

When mangling is enabled, only names declared inside the snippet are candidates: var, let, const, function names and parameter lists are collected, keywords, known globals, object keys and anything after a dot are left alone, and a generated short name is skipped if the code already uses that spelling somewhere.

What that means for your results

Because strings and regular expressions are stashed and restored verbatim, the output matches the same text the input did. A minifier that squeezed spaces inside a pattern or a template literal would produce a smaller file that no longer works, which is worse than no minification at all.

The savings figure is measured on the real output, and the tool will not show a compression ratio for a file it could not process.

Where it falls short — honestly

This is whitespace and local-name compression, not a compiler. There is no dead-code elimination, no constant folding, no property renaming and no scope analysis, so the result is smaller but not optimised the way a bundler pipeline would produce. Renaming without scope analysis is inherently conservative: a name used through eval or a string lookup is not visible to it, which is why the option is off by default and why you should test after enabling it.

Source maps are not generated, so a minified file is a poor thing to debug; keep the formatted original alongside it. Code that is already minified cannot be improved much further here.

A real end-to-end workflow

Use it before pasting a script into a page with a character limit, or to shrink an inline block in a template. Test the output once with the console open, especially if you enabled mangling, and keep the beautified source for future edits using JS Beautifier.

For the rest of the page, CSS Minifier and HTML Minifier apply the same region-preservation idea to stylesheets and markup, which is where most of the remaining bytes usually are.