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JavaScript parser, mangler/compressor and beautifier toolkit

UglifyJS 2

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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 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.                                        [string]
  --source-map-url   The path to the source map to be added in //#
                     sourceMappingURL.  Defaults to the value passed with
                     --source-map.                                      [string]
                     Pass this flag if you want to include the content of
                     source files in the source map as sourcesContent
                     property.                                         [boolean]
  --in-source-map    Input source map, useful if you're compressing JS that was
                     generated from some other original code.
  --screw-ie8        Pass this flag if you don't care about full compliance
                     with Internet Explorer 6-8 quirks (by default UglifyJS
                     will try to be IE-proof).                         [boolean]
  --expr             Parse a single expression, rather than a program (for
                     parsing JSON)                                     [boolean]
  -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.                    [string]
  -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]
  -e, --enclose      Embed everything in a big function, with a configurable
                     parameter/argument list.                           [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]
  --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.            [boolean]
  --acorn            Use Acorn for parsing.                            [boolean]
  --spidermonkey     Assume input files are SpiderMonkey AST format (as JSON).
  --self             Build itself (UglifyJS2) as a library (implies
                     --wrap=UglifyJS --export-all)                     [boolean]
  --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.                 [boolean]
  --lint             Display some scope warnings                       [boolean]
  -v, --verbose      Verbose                                           [boolean]
  -V, --version      Print version number and exit.                    [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 (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 \
         --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 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/ 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:

  • sort — 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.

  • 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.

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 -- join consecutive simple statements using the comma operator

  • properties -- rewrite property access using the dot notation, for example foo["bar"] →

  • dead_code -- remove unreachable code

  • drop_debugger -- remove debugger; statements

  • unsafe (default: false) -- 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 (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()

  • 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. 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.

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 0undefined (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;
// 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 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)
  • 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.

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.

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" ]);

To generate a source map:

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

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 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: "",
    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` 

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

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 — pass false to skip mangling names.

  • 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.

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
  • 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;
    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 scripts.

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 otions, 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).


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:

  • 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.