UglifyJS 2

UglifyJS is a JavaScript parser, minifier, compressor or beautifier toolkit.

This page documents the command line utility. For
API and internals documentation see my website.
There's also an
in-browser online demo (for Firefox,
Chrome and probably Safari).

Install

First make sure you have installed the latest version of node.js
(You may need to restart your computer after this step).

From NPM for use as a command line app:

npm install uglify-js -g

From NPM for programmatic use:

npm install uglify-js

From Git:

git clone git://github.com/mishoo/UglifyJS2.git
cd UglifyJS2
npm link .

Usage

uglifyjs [input files] [options]

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.

If you wish to pass your options before the input files, separate the two with
a double dash to prevent input files being used as option arguments:

uglifyjs --compress --mangle -- input.js

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.
  --source-map-url              The path to the source map to be added in //#
                                sourceMappingURL.  Defaults to the value passed
                                with --source-map.
  --source-map-include-sources  Pass this flag if you want to include the
                                content of source files in the source map as
                                sourcesContent property.
  --in-source-map               Input source map, useful if you're compressing
                                JS that was generated from some other original
                                code.
  --screw-ie8                   Use this flag if you don't wish to support
                                Internet Explorer 6-8 quirks.
                                By default UglifyJS will not try to be IE-proof.
  --support-ie8                 Use this flag to support Internet Explorer 6-8 quirks.
                                Note: may break standards compliant `catch` identifiers.
  --expr                        Parse a single expression, rather than a
                                program (for parsing JSON)
  -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. You can also specify -p
                                relative, which will make UglifyJS figure out
                                itself the relative paths between original
                                sources, the source map and the output file.
  -o, --output                  Output file (default STDOUT).
  -b, --beautify                Beautify output/specify output options.
  -m, --mangle                  Mangle names/pass mangler options.
  -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.
  -d, --define                  Global definitions
  -e, --enclose                 Embed everything in a big function, with a
                                configurable parameter/argument list.
  --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.
  --preamble                    Preamble to prepend to the output.  You can use
                                this to insert a comment, for example for
                                licensing information.  This will not be
                                parsed, but the source map will adjust for its
                                presence.
  --stats                       Display operations run time on STDERR.
  --acorn                       Use Acorn for parsing.
  --spidermonkey                Assume input files are SpiderMonkey AST format
                                (as JSON).
  --self                        Build itself (UglifyJS2) as a library (implies
                                --wrap=UglifyJS --export-all)
  --wrap                        Embed everything in a big function, making the
                                “exports” and “global” variables available. You
                                need to pass an argument to this option to
                                specify the name that your module will take
                                when included in, say, a browser.
  --export-all                  Only used when --wrap, this tells UglifyJS to
                                add code to automatically export all globals.
  --lint                        Display some scope warnings
  -v, --verbose                 Verbose
  -V, --version                 Print version number and exit.
  --noerr                       Don't throw an error for unknown options in -c,
                                -b or -m.
  --bare-returns                Allow return outside of functions.  Useful when
                                minifying CommonJS modules and Userscripts that
                                may be anonymous function wrapped (IIFE) by the
                                .user.js engine `caller`.
  --keep-fnames                 Do not mangle/drop function names.  Useful for
                                code relying on Function.prototype.name.
  --reserved-file               File containing reserved names
  --reserve-domprops            Make (most?) DOM properties reserved for
                                --mangle-props
  --mangle-props                Mangle property names (default `0`). Set to
                                `true` or `1` to mangle all property names. Set
                                to `unquoted` or `2` to only mangle unquoted
                                property names. Mode `2` also enables the
                                `keep_quoted_props` beautifier option to
                                preserve the quotes around property names and
                                disables the `properties` compressor option to
                                prevent rewriting quoted properties with dot
                                notation. You can override these by setting
                                them explicitly on the command line.
  --mangle-regex                Only mangle property names matching the regex
  --name-cache                  File to hold mangled names mappings
  --pure-funcs                  List of functions that can be safely removed if
                                their return value is not used           [array]

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:

uglifyjs /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). The following
(comma-separated) options are supported:

• toplevel — mangle names declared in the toplevel scope (disabled by
  default).

• eval — mangle names visible in scopes where eval or with are used
  (disabled 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:

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

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

Mangling property names (--mangle-props)

Note: this will probably break your code. Mangling property names is a
separate step, different from variable name mangling. Pass
--mangle-props. It will mangle all properties that are seen in some
object literal, or that are assigned to. For example:

 var x = {
  foo: 1
};

x.bar = 2;
x["baz"] = 3;
x[condition ? "moo" : "boo"] = 4;
console.log(x.something());

In the above code, foo, bar, baz, moo and boo will be replaced
with single characters, while something() will be left as is.

In order for this to be of any use, we should avoid mangling standard JS
names. For instance, if your code would contain x.length = 10, then
length becomes a candidate for mangling and it will be mangled throughout
the code, regardless if it's being used as part of your own objects or
accessing an array's length. To avoid that, you can use --reserved-file
to pass a filename that should contain the names to be excluded from
mangling. This file can be used both for excluding variable names and
property names. It could look like this, for example:

 {
  "vars": [ "define", "require", ... ],
  "props": [ "length", "prototype", ... ]
}

--reserved-file can be an array of file names (either a single
comma-separated argument, or you can pass multiple --reserved-file
arguments) — in this case it will exclude names from all those files.

A default exclusion file is provided in tools/domprops.json which should
cover most standard JS and DOM properties defined in various browsers. Pass
--reserve-domprops to read that in.

You can also use a regular expression to define which property names should be
mangled. For example, --mangle-regex="/^_/" will only mangle property names
that start with an underscore.

When you compress multiple files using this option, in order for them to
work together in the end we need to ensure somehow that one property gets
mangled to the same name in all of them. For this, pass --name-cache filename.json and UglifyJS will maintain these mappings in a file which can
then be reused. It should be initially empty. Example:

rm -f /tmp/cache.json  # start fresh
uglifyjs file1.js file2.js --mangle-props --name-cache /tmp/cache.json -o part1.js
uglifyjs file3.js file4.js --mangle-props --name-cache /tmp/cache.json -o part2.js

Now, part1.js and part2.js will be consistent with each other in terms
of mangled property names.

Using the name cache is not necessary if you compress all your files in a
single call to UglifyJS.

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).

• sequences (default: true) -- join consecutive simple statements using the
  comma operator. May be set to a positive integer to specify the maximum number
  of consecutive comma sequences that will be generated. If this option is set to
  true then the default sequences limit is 200. Set option to false or 0
  to disable. The smallest sequences length is 2. A sequences value of 1
  is grandfathered to be equivalent to true and as such means 200. On rare
  occasions the default sequences limit leads to very slow compress times in which
  case a value of 20 or less is recommended.

• 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 (default: false) -- apply "unsafe" transformations (discussion below)

• unsafe_comps (default: false) -- Reverse < and <= to > and >= to
  allow improved compression. This might be unsafe when an at least one of two
  operands is an object with computed values due the use of methods like get,
  or valueOf. This could cause change in execution order after operands in the
  comparison are switching. Compression only works if both comparisons and
  unsafe_comps are both set to true.

• 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_comps), 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 (default: false) -- 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()

• collapse_vars -- default false. Collapse single-use var and const
  definitions when possible.

• warnings -- display warnings when dropping unreachable code or unused
  declarations etc.

• negate_iife -- negate "Immediately-Called Function Expressions"
  where the return value is discarded, to avoid the parens that the
  code generator would insert.

• pure_getters -- the default is false. If you pass true for
  this, UglifyJS will assume that object property access
  (e.g. foo.bar or foo["bar"]) doesn't have any side effects.

• pure_funcs -- default null. You can pass an array of names and
  UglifyJS will assume that those functions do not produce side
  effects. DANGER: will not check if the name is redefined in scope.
  An example case here, for instance var q = Math.floor(a/b). If
  variable q is not used elsewhere, UglifyJS will drop it, but will
  still keep the Math.floor(a/b), not knowing what it does. You can
  pass pure_funcs: [ 'Math.floor' ] to let it know that this
  function won't produce any side effect, in which case the whole
  statement would get discarded. The current implementation adds some
  overhead (compression will be slower).

• drop_console -- default false. Pass true to discard calls to
  console.* functions.

• keep_fargs -- default true. Prevents the
  compressor from discarding unused function arguments. You need this
  for code which relies on Function.length.

• keep_fnames -- default false. Pass true to prevent the
  compressor from discarding function names. Useful for code relying on
  Function.prototype.name. See also: the keep_fnames mangle option.

• passes -- default 1. Number of times to run compress. Use an
  integer argument larger than 1 to further reduce code size in some cases.
  Note: raising the number of passes will increase uglify compress time.

The unsafe option

It enables some transformations that might break code logic in certain
contrived cases, but should be fine for most code. You might want to try it
on your own code, it should reduce the minified size. Here's what happens
when this flag is on:

• new Array(1, 2, 3) or Array(1, 2, 3) → [ 1, 2, 3 ]
• new Object() → {}
• String(exp) or exp.toString() → "" + exp
• new Object/RegExp/Function/Error/Array (...) → we discard the new
• typeof foo == "undefined" → foo === void 0
• void 0 → undefined (if there is a variable named "undefined" in
  scope; we do it because the variable name will be mangled, typically
  reduced to a single character)

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;
// Alternative for environments that don't support `const`
/** @const */ var STAGING = false;
// etc.

and build your code like this:

uglifyjs 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 build will contain the const declarations if you use
them. If you are targeting < ES6 environments, use /** @const */ var.

Conditional compilation, API

You can also use conditional compilation via the programmatic API. With the difference that the
property name is global_defs and is a compressor property:

 uglifyJS.minify([ "input.js"], {
    compress: {
        dead_code: true,
        global_defs: {
            DEBUG: false
        }
    }
});

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 (affects directives with non-ascii characters becoming invalid)
• 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)
• bracketize (default false) -- always insert brackets in if, for,
  do, while or with statements, even if their body is a single
  statement.
• semicolons (default true) -- separate statements with semicolons. If
  you pass false then whenever possible we will use a newline instead of a
  semicolon, leading to more readable output of uglified code (size before
  gzip could be smaller; size after gzip insignificantly larger).
• preamble (default null) -- when passed it must be a string and
  it will be prepended to the output literally. The source map will
  adjust for this text. Can be used to insert a comment containing
  licensing information, for example.
• quote_style (default 0) -- preferred quote style for strings (affects
  quoted property names and directives as well):
  • 0 -- prefers double quotes, switches to single quotes when there are
    more double quotes in the string itself.
  • 1 -- always use single quotes
  • 2 -- always use double quotes
  • 3 -- always use the original quotes
• keep_quoted_props (default false) -- when turned on, prevents stripping
  quotes from property names in object literals.

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",
"@license" or "@cc_on" (conditional compilation for IE). 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 be kept in the output) are comments attached to toplevel nodes.

Support for the SpiderMonkey AST

UglifyJS2 has its own abstract syntax tree format; for
practical reasons
we can't easily change to using the SpiderMonkey AST internally. However,
UglifyJS now has a converter which can import a SpiderMonkey AST.

For example Acorn is a super-fast parser that produces a
SpiderMonkey AST. It has a small CLI utility that parses one file and dumps
the AST in JSON on the standard output. To use UglifyJS to mangle and
compress that:

acorn file.js | uglifyjs --spidermonkey -m -c

The --spidermonkey option tells UglifyJS that all input files are not
JavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore we
don't use our own parser in this case, but just transform that AST into our
internal AST.

Use Acorn for parsing

More for fun, I added the --acorn option which will use Acorn to do all
the parsing. If you pass this option, UglifyJS will require("acorn").

Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), but
converting the SpiderMonkey tree that Acorn produces takes another 150ms so
in total it's a bit more than just using UglifyJS's own parser.

Using UglifyJS to transform SpiderMonkey AST

Now you can use UglifyJS as any other intermediate tool for transforming
JavaScript ASTs in SpiderMonkey format.

Example:

 function uglify(ast, options, mangle) {
  // Conversion from SpiderMonkey AST to internal format
  var uAST = UglifyJS.AST_Node.from_mozilla_ast(ast);

  // Compression
  uAST.figure_out_scope();
  uAST = uAST.transform(UglifyJS.Compressor(options));

  // Mangling (optional)
  if (mangle) {
    uAST.figure_out_scope();
    uAST.compute_char_frequency();
    uAST.mangle_names();
  }

  // Back-conversion to SpiderMonkey AST
  return uAST.to_mozilla_ast();
}

Check out
original blog post
for details.

API Reference

Assuming installation via NPM, you can load UglifyJS in your application
like this:

 var UglifyJS = require("uglify-js");

It exports a lot of names, but I'll discuss here the basics that are needed
for parsing, mangling and compressing a piece of code. The sequence is (1)
parse, (2) compress, (3) mangle, (4) generate output code.

The simple way

There's a single toplevel function which combines all the steps. If you
don't need additional customization, you might want to go with minify.
Example:

 var result = UglifyJS.minify("/path/to/file.js");
console.log(result.code); // minified output
// if you need to pass code instead of file name
var result = UglifyJS.minify("var b = function () {};", {fromString: true});

You can also compress multiple files:

 var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ]);
console.log(result.code);

To generate a source map:

 var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ], {
	outSourceMap: "out.js.map"
});
console.log(result.code); // minified output
console.log(result.map);

To generate a source map with the fromString option, you can also use an object:

 var result = UglifyJS.minify({"file1.js": "var a = function () {};"}, {
  outSourceMap: "out.js.map",
  fromString: true
});

Note that the source map is not saved in a file, it's just returned in
result.map. The value passed for outSourceMap is only used to set the
file attribute in the source map (see the spec).

You can also specify sourceRoot property to be included in source map:

 var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ], {
	outSourceMap: "out.js.map",
	sourceRoot: "http://example.com/src"
});

If you're compressing compiled JavaScript and have a source map for it, you
can use the inSourceMap argument:

 var result = UglifyJS.minify("compiled.js", {
	inSourceMap: "compiled.js.map",
	outSourceMap: "minified.js.map"
});
// same as before, it returns `code` and `map`

If your input source map is not in a file, you can pass it in as an object
using the inSourceMap argument:

 var result = UglifyJS.minify("compiled.js", {
	inSourceMap: JSON.parse(my_source_map_string),
	outSourceMap: "minified.js.map"
});

The inSourceMap is only used if you also request outSourceMap (it makes
no sense otherwise).

To set the source map url, use the sourceMapUrl option.
If you're using the X-SourceMap header instead, you can just set the sourceMapUrl option to false.
Defaults to outSourceMap:

 var result = UglifyJS.minify([ "file1.js" ], {
  outSourceMap: "out.js.map",
  sourceMapUrl: "localhost/out.js.map"
});

Other options:

• warnings (default false) — pass true to display compressor warnings.

• fromString (default false) — if you pass true then you can pass
  JavaScript source code, rather than file names.

• mangle (default true) — pass false to skip mangling names, or pass
  an object to specify mangling options (see below).

• mangleProperties (default false) — pass an object to specify custom
  mangle property options.

• output (default null) — pass an object if you wish to specify
  additional output options. The defaults are optimized
  for best compression.

• compress (default {}) — pass false to skip compressing entirely.
  Pass an object to specify custom compressor options.

• parse (default {}) — pass an object if you wish to specify some
  additional parser options. (not all options available... see below)

mangle

• except - pass an array of identifiers that should be excluded from mangling

• toplevel — mangle names declared in the toplevel scope (disabled by
  default).

• eval — mangle names visible in scopes where eval or with are used
  (disabled by default).

• keep_fnames -- default false. Pass true to not mangle
  function names. Useful for code relying on Function.prototype.name.
  See also: the keep_fnames compress option.

Examples:

 //tst.js
var globalVar;
function funcName(firstLongName, anotherLongName)
{
  var myVariable = firstLongName +  anotherLongName;
}

UglifyJS.minify("tst.js").code;
// 'function funcName(a,n){}var globalVar;'

UglifyJS.minify("tst.js", { mangle: { except: ['firstLongName'] } }).code;
// 'function funcName(firstLongName,a){}var globalVar;'

UglifyJS.minify("tst.js", { mangle: { toplevel: true } }).code;
// 'function n(n,a){}var a;'

mangleProperties options

• regex — Pass a RegExp to only mangle certain names (maps to the --mangle-regex CLI arguments option)
• ignore_quoted – Only mangle unquoted property names (maps to the --mangle-props 2 CLI arguments option)

We could add more options to UglifyJS.minify — if you need additional
functionality please suggest!

The hard way

Following there's more detailed API info, in case the minify function is
too simple for your needs.

The parser

 var toplevel_ast = UglifyJS.parse(code, options);

options is optional and if present it must be an object. The following
properties are available:

• strict — disable automatic semicolon insertion and support for trailing
  comma in arrays and objects
• bare_returns — Allow return outside of functions. (maps to the
  --bare-returns CLI arguments option and available to minify parse
  other options object)
• filename — the name of the file where this code is coming from
• toplevel — a toplevel node (as returned by a previous invocation of
  parse)

The last two options are useful when you'd like to minify multiple files and
get a single file as the output and a proper source map. Our CLI tool does
something like this:

 var toplevel = null;
files.forEach(function(file){
	var code = fs.readFileSync(file, "utf8");
	toplevel = UglifyJS.parse(code, {
		filename: file,
		toplevel: toplevel
	});
});

After this, we have in toplevel a big AST containing all our files, with
each token having proper information about where it came from.

Scope information

UglifyJS contains a scope analyzer that you need to call manually before
compressing or mangling. Basically it augments various nodes in the AST
with information about where is a name defined, how many times is a name
referenced, if it is a global or not, if a function is using eval or the
with statement etc. I will discuss this some place else, for now what's
important to know is that you need to call the following before doing
anything with the tree:

 toplevel.figure_out_scope()

Compression

Like this:

 var compressor = UglifyJS.Compressor(options);
var compressed_ast = toplevel.transform(compressor);

The options can be missing. Available options are discussed above in
“Compressor options”. Defaults should lead to best compression in most
scripts.

The compressor is destructive, so don't rely that toplevel remains the
original tree.

Mangling

After compression it is a good idea to call again figure_out_scope (since
the compressor might drop unused variables / unreachable code and this might
change the number of identifiers or their position). Optionally, you can
call a trick that helps after Gzip (counting character frequency in
non-mangleable words). Example:

 compressed_ast.figure_out_scope();
compressed_ast.compute_char_frequency();
compressed_ast.mangle_names();

Generating output

AST nodes have a print method that takes an output stream. Essentially,
to generate code you do this:

 var stream = UglifyJS.OutputStream(options);
compressed_ast.print(stream);
var code = stream.toString(); // this is your minified code

or, for a shortcut you can do:

 var code = compressed_ast.print_to_string(options);

As usual, options is optional. The output stream accepts a lot of options,
most of them documented above in section “Beautifier options”. The two
which we care about here are source_map and comments.

Keeping comments in the output

In order to keep certain comments in the output you need to pass the
comments option. Pass a RegExp or a function. If you pass a RegExp, only
those comments whose body matches the regexp will be kept. Note that body
means without the initial // or /*. If you pass a function, it will be
called for every comment in the tree and will receive two arguments: the
node that the comment is attached to, and the comment token itself.

The comment token has these properties:

• type: "comment1" for single-line comments or "comment2" for multi-line
  comments
• value: the comment body
• pos and endpos: the start/end positions (zero-based indexes) in the
  original code where this comment appears
• line and col: the line and column where this comment appears in the
  original code
• file — the file name of the original file
• nlb — true if there was a newline before this comment in the original
  code, or if this comment contains a newline.

Your function should return true to keep the comment, or a falsy value
otherwise.

Generating a source mapping

You need to pass the source_map argument when calling print. It needs
to be a SourceMap object (which is a thin wrapper on top of the
source-map library).

Example:

 var source_map = UglifyJS.SourceMap(source_map_options);
var stream = UglifyJS.OutputStream({
	...
	source_map: source_map
});
compressed_ast.print(stream);

var code = stream.toString();
var map = source_map.toString(); // json output for your source map

The source_map_options (optional) can contain the following properties:

• file: the name of the JavaScript output file that this mapping refers to

• root: the sourceRoot property (see the spec)

• orig: the "original source map", handy when you compress generated JS
  and want to map the minified output back to the original code where it
  came from. It can be simply a string in JSON, or a JSON object containing
  the original source map.