Should You Minify Your JavaScript? Here's What It Actually Does
Minifying removes everything a browser does not need: comments, indentation, line breaks and in some cases long variable names. The behaviour is identical and the file is smaller. The JS Minifier does the whitespace and comment pass.
What the saving really is
Typical plain minification takes twenty to forty per cent off a hand-written file, depending on how heavily commented it was. Worth having.
But here is the thing people miss: your server is probably already compressing the file in transit, with gzip or brotli, and compression is extremely good at repeated whitespace. Minifying first and compressing after saves less than minifying alone suggests, because compression was handling much of that anyway. The honest summary is that minification plus compression beats compression alone, by a smaller margin than the raw file sizes imply - so check that compression is switched on before you worry about the minifier.
What it never changes
Minification does not alter what your code does. It does not remove unused functions, bundle your modules, modernise your syntax or make anything faster to execute. A slow function is exactly as slow afterwards.
It also does not protect your code. Removing comments and shortening names makes it unpleasant to read, not secret - anyone can format it back into something legible. If it is in the browser, it is public. Keep secrets on the server, and never put an API key in a script because minification will hide it.
Keep the readable version
Minify as a build step, never in place. The file you edit stays readable, and the minified file is generated output you can regenerate at any time. Overwriting your source with minified code is how a codebase becomes unmaintainable in one afternoon.
The same applies to stylesheets, where the CSS Minifier does the equivalent job and the whitespace saving is often proportionally larger, since CSS is mostly structure.
Common questions
Will minifying break my code?
Whitespace and comment removal is safe for valid JavaScript. The one historical trap is missing semicolons, where removing line breaks changes the meaning - and that is an argument for writing them rather than against minifying.
Does minification help SEO?
Only through page speed, which is a real but modest factor. Image sizes are almost always the bigger win on a slow page.
Can I get the original back?
Not the comments or the original names. Keep the source in version control - the minified file is output, not a copy.
What about minifying HTML?
Possible, and rarely worth it. The saving is small and a stray removal inside a pre block or a template changes the page.
Does it matter which minifier I use?
For comments and whitespace, no. Tools that also rename variables and rewrite expressions save more and have more ways to go wrong, so test the output if you use one.
Got a script to trim down?