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    UglifyJS 3

    Build Status

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


    • uglify-js@3.x has a new API and CLI and is not backwards compatible with uglify-js@2.x.
    • Documentation for UglifyJS 2.x releases can be found here.
    • Release versions of uglify-js only support ECMAScript 5 (ES5). If you wish to minify ES2015+ (ES6+) code then please use the harmony development branch.
    • Node 7 has a known performance regression and runs uglify-js twice as slow.


    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]

    UglifyJS 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:

        -h, --help                  Print usage information.
        -V, --version               Print version number.
        -p, --parse <options>       Specify parser options:
                                    `acorn`  Use Acorn for parsing.
                                    `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`.
                                    `expression`  Parse a single expression, rather than
                                                  a program (for parsing JSON).
                                    `spidermonkey`  Assume input files are SpiderMonkey
                                                    AST format (as JSON).
        -c, --compress [options]    Enable compressor/specify compressor options:
                                    `pure_funcs`  List of functions that can be safely
                                                  removed when their return values are
                                                  not used.
        -m, --mangle [options]      Mangle names/specify mangler options:
                                    `reserved`  List of names that should not be mangled.
        --mangle-props [options]    Mangle properties/specify mangler options:
                                    `builtins`  Mangle property names that overlaps
                                                with standard JavaScript globals.
                                    `debug`  Add debug prefix and suffix.
                                    `domprops`  Mangle property names that overlaps
                                                with DOM properties.
                                    `keep_quoted`  Only mangle unquoted properies.
                                    `regex`  Only mangle matched property names.
                                    `reserved`  List of names that should not be mangled.
        -b, --beautify [options]    Beautify output/specify output options:
                                    `beautify`  Enabled with `--beautify` by default.
                                    `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.
                                    `quote_style`  Quote style:
                                                   0 - auto
                                                   1 - single
                                                   2 - double
                                                   3 - original
                                    `wrap_iife`  Wrap IIFEs in parenthesis. Note: you may
                                                 want to disable `negate_iife` under
                                                 compressor options.
        -o, --output <file>         Output file (default STDOUT). Specify "spidermonkey"
                                    to dump SpiderMonkey AST format (as JSON) to STDOUT.
        --comments [filter]         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
        --config-file <file>        Read `minify()` options from JSON file.
        -d, --define <expr>[=value] Global definitions.
        --ie8                       Support non-standard Internet Explorer 8.
                                    Equivalent to setting `ie8: true` in `minify()`
                                    for `compress`, `mangle` and `output` options.
                                    By default UglifyJS will not try to be IE-proof.
        --keep-fnames               Do not mangle/drop function names.  Useful for
                                    code relying on
        --name-cache                File to hold mangled name mappings.
        --self                      Build UglifyJS as a library (implies --wrap UglifyJS)
        --source-map [options]      Enable source map/specify source map options:
                                    `base`  Path to compute relative paths from input files.
                                    `content`  Input source map, useful if you're compressing
                                               JS that was generated from some other original
                                               code. Specify "inline" if the source map is
                                               included within the sources.
                                    `filename`  Name and/or location of the output source.
                                    `includeSources`  Pass this flag if you want to include
                                                      the content of source files in the
                                                      source map as sourcesContent property.
                                    `root`  Path to the original source to be included in
                                            the source map.
                                    `url`  If specified, path to the source map to append in
                                           `//# sourceMappingURL`.
        --stats                     Display operations run time on STDERR.
        --toplevel                  Compress and/or mangle variables in toplevel scope.
        --verbose                   Print diagnostic messages.
        --warn                      Print warning messages.
        --wrap <name>               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.

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

    Source map options

    UglifyJS can generate a source map file, which is highly useful for debugging your compressed JavaScript. To get a source map, pass --source-map --output output.js (source map will be written out to

    Additionally you might need --source-map root=<URL> to pass the URL where the original files can be found. Use --source-map url=<URL> to specify the URL where the source map can be found.

    For example:

    uglifyjs /home/doe/work/foo/src/js/file1.js \
             /home/doe/work/foo/src/js/file2.js \
             -o foo.min.js -c -m \
             --source-map base="/home/doe/work/foo/src",root=""

    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/ or --in-source-map inline if the source map is included inline with the sources. 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 --mangle reserved — pass a comma-separated list of names. For example:

    uglifyjs ... -m reserved=[$,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 avoid mangling standard JS names by default (--mangle-props builtins to override).

    A default exclusion file is provided in tools/domprops.json which should cover most standard JS and DOM properties defined in various browsers. Pass --mangle-props domprops to disable this feature.

    You can also use a regular expression to define which property names should be mangled. For example, --mangle-props 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.

    Mangling unquoted names (--mangle-props keep_quoted)

    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 keep_quoted -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).

    • 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"] →

    • 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.

    • unsafe_math (default: false) -- optimize numerical expressions like 2 * x * 3 into 6 * x, which may give imprecise floating point results.

    • unsafe_proto (default: false) -- optimize expressions like into []

    • 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 (simple direct variable assignments do not count as references unless set to "keep_assign")

    • toplevel -- drop unreferenced functions ("funcs") and/or variables ("vars") in the toplevel scope (false by default, true to drop both unreferenced functions and variables)

    • top_retain -- prevent specific toplevel functions and variables from unused removal (can be array, comma-separated, RegExp or function. Implies toplevel)

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

    • reduce_vars -- Improve optimization on variables assigned with and used as constant values.

    • 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. Specify "strict" to treat as side-effect-free only when foo is certain to not throw, i.e. not null or undefined.

    • 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. If you wish to drop a specific function call such as and/or retain side effects from function arguments after dropping the function call then use pure_funcs instead.

    • expression -- default false. Pass true to preserve completion values from terminal statements without return, e.g. in bookmarklets.

    • 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 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.

    • keep_infinity -- default false. Pass true to prevent Infinity from being compressed into 1/0, which may cause performance issues on Chrome.

    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");

    You can specify nested constants in the form of --define env.DEBUG=false.

    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 build will contain the const declarations if you use them. If you are targeting < ES6 environments which does not support const, using var with reduce_vars (enabled by default) should suffice.

    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 /^!/ will keep comments like /*! Copyright Notice */.

    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

    UglifyJS 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 -p spidermonkey -m -c

    The -p 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 -p 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");

    There is a single toplevel function, minify(files, options), which will performs all the steps in a configurable manner. Example:

    var result = UglifyJS.minify("var b = function() {};");
    console.log(result.code); // minified output

    You can also compress multiple files:

    var result = UglifyJS.minify({
      "file1.js": "var a = function() {};",
      "file2.js": "var b = function() {};"

    To generate a source map:

    var result = UglifyJS.minify({"file1.js": "var a = function() {};"}, {
      sourceMap: {
        filename: "out.js",
        url: ""
    console.log(result.code); // minified output
    console.log(;  // source map

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

    You can set option sourceMap.url to be "inline" 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": "var a = function() {};"}, {
      sourceMap: {
        root: "",
        url: ""

    If you're compressing compiled JavaScript and have a source map for it, you can use sourceMap.content:

    var result = UglifyJS.minify({"compiled.js": "compiled code"}, {
      sourceMap: {
        content: "content from",
        url: ""
    // same as before, it returns `code` and `map`

    If you're using the X-SourceMap header instead, you can just omit sourceMap.url.

    Other options:

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

    • 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.

    • reserved - 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 See also: the keep_fnames compress option.


    var globalVar;
    function funcName(firstLongName, anotherLongName)
      var myVariable = firstLongName +  anotherLongName;
    // 'function funcName(a,n){}var globalVar;'
    UglifyJS.minify("tst.js", { mangle: { reserved: ['firstLongName'] } }).code;
    // 'function funcName(firstLongName,a){}var globalVar;'
    UglifyJS.minify("tst.js", { mangle: { toplevel: true } }).code;
    // 'function n(n,a){}var a;' options
    • regex — Pass a RegExp to only mangle certain names
    • keep_quoted — Only mangle unquoted property names
    • debug — Mangle names with the original name still present. Defaults to false. Pass an empty string to enable, or a non-empty string to set the suffix.


    If you wish to use the experimental harmony branch to minify ES2015+ (ES6+) code please use the following in your package.json file:

    "uglify-js": "git+"

    or to directly install the experimental harmony version of uglify:

    npm install --save-dev uglify-js@github:mishoo/UglifyJS2#harmony

    See #448 for additional details.


    npm i uglify-js@3.0.0





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