UglifyJS2 can take multiple input files. It's recommended that you pass the input files first, then pass the options. UglifyJS will parse input files in sequence and apply any compression options. The files are parsed in the same global scope, that is, a reference from a file to some variable/function declared in another file will be matched properly.

If you want to read from STDIN instead, pass a single dash instead of input files.

The available options are:

--source-map       Specify an output file where to generate source map.
--source-map-root  The path to the original source to be included in the
                   source map.
--in-source-map    Input source map, useful if you're compressing JS that was
                   generated from some other original code.
-p, --prefix       Skip prefix for original filenames that appear in source
                   maps. For example -p 3 will drop 3 directories from file
                   names and ensure they are relative paths.
-o, --output       Output file (default STDOUT).
-b, --beautify     Beautify output/specify output options.            [string]
-m, --mangle       Mangle names/pass mangler options.                 [string]
-r, --reserved     Reserved names to exclude from mangling.
-c, --compress     Enable compressor/pass compressor options. Pass options
                   like -c hoist_vars=false,if_return=false. Use -c with no
                   argument to use the default compression options.   [string]
-d, --define       Global definitions                                 [string]
--comments         Preserve copyright comments in the output. By default this
                   works like Google Closure, keeping JSDoc-style comments
                   that contain "@license" or "@preserve". You can optionally
                   pass one of the following arguments to this flag:
                   - "all" to keep all comments
                   - a valid JS regexp (needs to start with a slash) to keep
                   only comments that match.
                   Note that currently not *all* comments can be kept when
                   compression is on, because of dead code removal or
                   cascading statements into sequences.               [string]
--stats            Display operations run time on STDERR.            [boolean]
-v, --verbose      Verbose                                           [boolean]

Specify --output (-o) to declare the output file. Otherwise the output goes to STDOUT.

Source map options

UglifyJS2 can generate a source map file, which is highly useful for debugging your compressed JavaScript. To get a source map, pass --source-map output.js.map (full path to the file where you want the source map dumped).

Additionally you might need --source-map-root to pass the URL where the original files can be found. In case you are passing full paths to input files to UglifyJS, you can use --prefix (-p) to specify the number of directories to drop from the path prefix when declaring files in the source map.

For example:

uglifyjs2 /home/doe/work/foo/src/js/file1.js \
          /home/doe/work/foo/src/js/file2.js \
          -o foo.min.js \
          --source-map foo.min.js.map \
          --source-map-root http://foo.com/src \
          -p 5 -c -m

The above will compress and mangle file1.js and file2.js, will drop the output in foo.min.js and the source map in foo.min.js.map. The source mapping will refer to http://foo.com/src/js/file1.js and http://foo.com/src/js/file2.js (in fact it will list http://foo.com/src as the source map root, and the original files as js/file1.js and js/file2.js).

Composed source map

When you're compressing JS code that was output by a compiler such as CoffeeScript, mapping to the JS code won't be too helpful. Instead, you'd like to map back to the original code (i.e. CoffeeScript). UglifyJS has an option to take an input source map. Assuming you have a mapping from CoffeeScript → compiled JS, UglifyJS can generate a map from CoffeeScript → compressed JS by mapping every token in the compiled JS to its original location.

To use this feature you need to pass --in-source-map /path/to/input/source.map. Normally the input source map should also point to the file containing the generated JS, so if that's correct you can omit input files from the command line.

Mangler options

To enable the mangler you need to pass --mangle (-m). Optionally you can pass -m sort (we'll possibly have other flags in the future) in order to assign shorter names to most frequently used variables. This saves a few hundred bytes on jQuery before gzip, but the output is bigger after gzip (and seems to happen for other libraries I tried it on) therefore it's not enabled by default.

When mangling is enabled but you want to prevent certain names from being mangled, you can declare those names with --reserved (-r) — pass a comma-separated list of names. For example:

uglifyjs2 ... -m -r '$,require,exports'

to prevent the require, exports and $ names from being changed.

Compressor options

You need to pass --compress (-c) to enable the compressor. Optionally you can pass a comma-separated list of options. Options are in the form foo=bar, or just foo (the latter implies a boolean option that you want to set true; it's effectively a shortcut for foo=true).

The defaults should be tuned for maximum compression on most code. Here are the available options (all are true by default, except hoist_vars):

sequences -- join consecutive simple statements using the comma operator
properties -- rewrite property access using the dot notation, for example foo["bar"] → foo.bar
dead-code -- remove unreachable code
drop-debugger -- remove debugger; statements
unsafe -- apply "unsafe" transformations (discussion below)
conditionals -- apply optimizations for if-s and conditional expressions
comparisons -- apply certain optimizations to binary nodes, for example: !(a <= b) → a > b (only when unsafe), attempts to negate binary nodes, e.g. a = !b && !c && !d && !e → a=!(b||c||d||e) etc.
evaluate -- attempt to evaluate constant expressions
booleans -- various optimizations for boolean context, for example !!a ? b : c → a ? b : c
loops -- optimizations for do, while and for loops when we can statically determine the condition
unused -- drop unreferenced functions and variables
hoist-funs -- hoist function declarations
hoist-vars -- hoist var declarations (this is false by default because it seems to increase the size of the output in general)
if-return -- optimizations for if/return and if/continue
join-vars -- join consecutive var statements
cascade -- small optimization for sequences, transform x, x into x and x = something(), x into x = something()
warnings -- display warnings when dropping unreachable code or unused declarations etc.

Conditional compilation

You can use the --define (-d) switch in order to declare global variables that UglifyJS will assume to be constants (unless defined in scope). For example if you pass --define DEBUG=false then, coupled with dead code removal UglifyJS will discard the following from the output:

if (DEBUG) {
    console.log("debug stuff");
}

UglifyJS will warn about the condition being always false and about dropping unreachable code; for now there is no option to turn off only this specific warning, you can pass warnings=false to turn off all warnings.

Another way of doing that is to declare your globals as constants in a separate file and include it into the build. For example you can have a build/defines.js file with the following:

const DEBUG = false;
const PRODUCTION = true;
// etc.

and build your code like this:

uglifyjs2 build/defines.js js/foo.js js/bar.js... -c

UglifyJS will notice the constants and, since they cannot be altered, it will evaluate references to them to the value itself and drop unreachable code as usual. The possible downside of this approach is that the build will contain the const declarations.

Beautifier options

The code generator tries to output shortest code possible by default. In case you want beautified output, pass --beautify (-b). Optionally you can pass additional arguments that control the code output:

beautify (default true) -- whether to actually beautify the output. Passing -b will set this to true, but you might need to pass -b even when you want to generate minified code, in order to specify additional arguments, so you can use -b beautify=false to override it.
indent-level (default 4)
indent-start (default 0) -- prefix all lines by that many spaces
quote-keys (default false) -- pass true to quote all keys in literal objects
space-colon (default true) -- insert a space after the colon signs
ascii-only (default false) -- escape Unicode characters in strings and regexps
inline-script (default false) -- escape the slash in occurrences of </script in strings
width (default 80) -- only takes effect when beautification is on, this specifies an (orientative) line width that the beautifier will try to obey. It refers to the width of the line text (excluding indentation). It doesn't work very well currently, but it does make the code generated by UglifyJS more readable.
max-line-len (default 32000) -- maximum line length (for uglified code)
ie-proof (default true) -- generate “IE-proof” code (for now this means add brackets around the do/while in code like this: if (foo) do something(); while (bar); else ....
bracketize (default false) -- always insert brackets in if, for, do, while or with statements, even if their body is a single statement.

Keeping copyright notices or other comments

You can pass --comments to retain certain comments in the output. By default it will keep JSDoc-style comments that contain "@preserve" or "@license". You can pass --comments all to keep all the comments, or a valid JavaScript regexp to keep only comments that match this regexp. For example --comments '/foo|bar/' will keep only comments that contain "foo" or "bar".

Note, however, that there might be situations where comments are lost. For example:

function f() {
  /** @preserve Foo Bar */
  function g() {
    // this function is never called
  }
  return something();
}

Even though it has "@preserve", the comment will be lost because the inner function g (which is the AST node to which the comment is attached to) is discarded by the compressor as not referenced.

The safest comments where to place copyright information (or other info that needs to me kept in the output) are comments attached to toplevel nodes.