Fork me on GitHub

UglifyJS 2

Build Status

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).


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://
cd UglifyJS2
npm link .


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

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
  --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.
  --source-map-inline           Write base64-encoded source map to the end of js output.
  --in-source-map               Input source map, useful if you're compressing
                                JS that was generated from some other original
  --screw-ie8                   Use this flag if you don't wish to support
                                Internet Explorer 6/7/8.
                                By default UglifyJS will not try to be IE-proof.
  --support-ie8                 Use this flag to support Internet Explorer 6/7/8.
                                Equivalent to setting `screw_ie8: false` in `minify()`
                                for `compress`, `mangle` and `output` options.
                                Note: `--support-ie8` may generate incorrect code
                                for `try`/`catch` in ES5 compliant browsers.
  --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
  -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 like `/foo/` or `/^!/` to
                                keep only matching comments.
                                Note that currently not *all* comments can be
                                kept when compression is on, because of dead
                                code removal or cascading statements into
  --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
  --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
  --reserved-file               File containing reserved names
  --reserve-domprops            Make (most?) DOM properties reserved for
  --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 (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

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 \
         --source-map-root \
         -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 The source
mapping will refer to and (in fact it will list
as the source map root, and the original files as js/file1.js and

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

To use this feature you need to pass --in-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:

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
}; = 2;
x["baz"] = 3;
x[condition ? "moo" : "boo"] = 4;

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
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.

Mangling unquoted names (--mangle-props=unquoted or --mangle-props=2)

Using quoted property name (o["foo"]) reserves the property name (foo)
so that it is not mangled throughout the entire script even when used in an
unquoted style ( Example:

$ echo 'var o={"foo":1, bar:3}; +=; console.log(;' | uglifyjs --mangle-props=2 -mc
var o={"foo":1,a:3};,console.log(;

Debugging property name mangling

You can also pass --mangle-props-debug in order to mangle property names
without completely obscuring them. For example the property
would mangle to o._$foo$_ with this option. This allows property mangling
of a large codebase while still being able to debug the code and identify
where mangling is breaking things.

You can also pass a custom suffix using --mangle-props-debug=XYZ. This would then
mangle to o._$foo$XYZ_. You can change this each time you compile a
script to identify how a property got mangled. One technique is to pass a
random number on every compile to simulate mangling changing with different
inputs (e.g. as you update the input script with new properties), and to help
identify mistakes like writing mangled keys to storage.

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 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:

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:

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
will keep only comments that contain "foo" or "bar".

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

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.


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

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

  // Mangling (optional)
  if (mangle) {

  // 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.

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" ]);

To generate a source map:

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

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: "",
  outFileName: "out.js",
  fromString: true

Note that the source map is not saved in a file, it's just returned in The value passed for outSourceMap is only used to set
//# in result.code. The value of
outFileName is only used to set file attribute in source map file.

The file attribute in the source map (see the spec) will
use outFileName firstly, if it's falsy, then will be deduced from
outSourceMap (by removing '.map').

You can set option sourceMapInline to be true and source map will
be appended to code.

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

var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ], {
    outSourceMap: "",
    sourceRoot: ""

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: "",
    outSourceMap: ""
// 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: ""

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: "",
  sourceMapUrl: "localhost/"

Other options:

mangleProperties options

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:

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;
    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:



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

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


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:


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);
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 (as string starting and closing with /
or pass a RegExp object), a boolean or a function. Stringified options
all and some can be passed too, where some behaves like it's cli
equivalent --comments without passing a value. 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:

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

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).


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

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: