Earlier quoted context omitted.
One massive thing minifying does is dead code elimination (slightly less applicable to CSS but it still applies using some build stacks) We can build a "prod" version of the app and the minifying process will drop all the debugging code as well as any unused or uncalled functions from the output.
What JS Minifier do you know that does dead code elimination? I would have thought that understanding what functions of a dynamic language that can be safely removed would require parsing/AST analysis beyond those found in the typical minifier.
We use blocks like this throughout our codebase:
if (process.env.NODE_ENV === 'development') {
fancyDebuggingFunction(stuff)
}
// ... some code later
fancyDebuggingFunction(var) {
/* do debugging things here */
}
Throw in Webpack 2's import/export bundling stuff, you can exclude whole modules which you don't use in production which can really reduce the size of your code.
And moving forward if the JS ecosystem can ever get a handle on a good way to move to import/export by default, you'll start seeing massive gains in this area by being able to strip out any parts of a library that you don't use.
Also, I believe most widely used minifiers use AST parsing to do their work.