UglifyJS 3

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

Note:

  - uglify-js supports JavaScript and most language features in ECMAScript.
  - For more exotic parts of ECMAScript, process your source file with transpilers
    like Babel before passing onto uglify-js.
  - uglify-js@3 has a simplified API and CLI
    that is not backwards compatible with uglify-js@2.

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

Command line usage

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 no input file is specified, UglifyJS will read from STDIN.

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

Command line options

    -h, --help                  Print usage information.
                                `--help options` for details on available options.
    -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`.
                                `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 properties.
                                `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 parentheses. Note: you may
                                             want to disable `negate_iife` under
                                             compressor options.
    -O, --output-opts [options] Specify output options (`beautify` disabled by default).
    -o, --output <file>         Output file path (default STDOUT). Specify `ast` or
                                `spidermonkey` to write UglifyJS or SpiderMonkey AST
                                as JSON to STDOUT respectively.
    --annotations               Process and preserve comment annotations.
                                (`/*@__PURE__*/` or `/*#__PURE__*/`)
    --no-annotations            Ignore and discard comment annotations.
    --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
                                sequences.
    --config-file <file>        Read `minify()` options from JSON file.
    -d, --define <expr>[=value] Global definitions.
    -e, --enclose [arg[:value]] Embed everything in a big function, with configurable
                                argument(s) & value(s).
    --expression                Parse a single expression, rather than a program
                                (for parsing JSON).
    --ie                        Support non-standard Internet Explorer.
                                Equivalent to setting `ie: true` in `minify()`
                                for `compress`, `mangle` and `output` options.
                                By default UglifyJS will not try to be IE-proof.
    --keep-fargs                Do not mangle/drop function arguments.
    --keep-fnames               Do not mangle/drop function names.  Useful for
                                code relying on Function.prototype.name.
    --module                    Process input as ES module (implies --toplevel)
    --no-module                 Avoid optimizations which may alter runtime behavior
                                under prior versions of JavaScript.
    --name-cache <file>         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`  Filename and/or location of the output source
                                            (sets `file` attribute in source map).
                                `includeSources`  Pass this flag if you want to include
                                                  the content of source files in the
                                                  source map as sourcesContent property.
                                `names` Include symbol names in the source map.
                                `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`.
    --timings                   Display operations run time on STDERR.
    --toplevel                  Compress and/or mangle variables in top level scope.
    --v8                        Support non-standard Chrome & Node.js
                                Equivalent to setting `v8: true` in `minify()`
                                for `mangle` and `output` options.
                                By default UglifyJS will not try to be v8-proof.
    --verbose                   Print diagnostic messages.
    --warn                      Print warning messages.
    --webkit                    Support non-standard Safari/Webkit.
                                Equivalent to setting `webkit: true` in `minify()`
                                for `compress`, `mangle` and `output` options.
                                By default UglifyJS will not try to be Safari-proof.
    --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.

CLI 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
output.js.map).

Additional options:

  - --source-map "filename='<NAME>'" to specify the name of the source map. The value of
    filename is only used to set file attribute (see the spec)
    in source map file.

  - --source-map "root='<URL>'" to pass the URL where the original files can be found.

  - --source-map "names=false" to omit symbol names if you want to reduce size
    of the source map file.

  - --source-map "url='<URL>'" to specify the URL where the source map can be found.
    Otherwise UglifyJS assumes HTTP X-SourceMap is being used and will omit the
    //# sourceMappingURL= directive.

For example:

uglifyjs js/file1.js js/file2.js \
         -o foo.min.js -c -m \
         --source-map "root='http://foo.com/src',url='foo.min.js.map'"

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 pass --source-map "content='/path/to/input/source.map'"
or --source-map "content=inline" if the source map is included inline with
the sources.

CLI compress options

You need to pass --compress (-c) to enable the compressor.  Optionally
you can pass a comma-separated list of compress 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).

Example:

uglifyjs file.js -c toplevel,sequences=false

CLI mangle options

To enable the mangler you need to pass --mangle (-m).  The following
(comma-separated) options are supported:

  - eval (default: false) — mangle names visible in scopes where eval or
    with are used.

  - reserved (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.

CLI 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 to enable it.  It will mangle all properties in the
input code with the exception of built in DOM properties and properties
in core JavaScript classes.  For example:

// example.js
var x = {
    baz_: 0,
    foo_: 1,
    calc: function() {
        return this.foo_ + this.baz_;
    }
};
x.bar_ = 2;
x["baz_"] = 3;
console.log(x.calc());

Mangle all properties (except for JavaScript builtins):

$ uglifyjs example.js -c -m --mangle-props

var x={o:0,_:1,l:function(){return this._+this.o}};x.t=2,x.o=3,console.log(x.l());

Mangle all properties except for reserved properties:

$ uglifyjs example.js -c -m --mangle-props reserved=[foo_,bar_]

var x={o:0,foo_:1,_:function(){return this.foo_+this.o}};x.bar_=2,x.o=3,console.log(x._());

Mangle all properties matching a regex:

$ uglifyjs example.js -c -m --mangle-props regex=/_$/

var x={o:0,_:1,calc:function(){return this._+this.o}};x.l=2,x.o=3,console.log(x.calc());

Combining mangle properties options:

$ uglifyjs example.js -c -m --mangle-props regex=/_$/,reserved=[bar_]

var x={o:0,_:1,calc:function(){return this._+this.o}};x.bar_=2,x.o=3,console.log(x.calc());

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.

A regular expression can be used 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 (o.foo). Example:

// stuff.js
var o = {
    "foo": 1,
    bar: 3,
};
o.foo += o.bar;
console.log(o.foo);

$ uglifyjs stuff.js --mangle-props keep_quoted -c -m

var o={foo:1,o:3};o.foo+=o.o,console.log(o.foo);

If the minified output will be processed again by UglifyJS, consider specifying
keep_quoted_props so the same property names are preserved:

$ uglifyjs stuff.js --mangle-props keep_quoted -c -m -O keep_quoted_props

var o={"foo":1,o:3};o.foo+=o.o,console.log(o.foo);

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 o.foo
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.

$ uglifyjs stuff.js --mangle-props debug -c -m

var o={_$foo$_:1,_$bar$_:3};o._$foo$_+=o._$bar$_,console.log(o._$foo$_);

You can also pass a custom suffix using --mangle-props debug=XYZ. This would then
mangle o.foo 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.

API Reference

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

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

There is a single high level function, minify(code, options),
which will perform all minification phases in a configurable
manner. By default minify() will enable the options compress
and mangle. Example:

var code = "function add(first, second) { return first + second; }";
var result = UglifyJS.minify(code);
console.log(result.error); // runtime error, or `undefined` if no error
console.log(result.code);  // minified output: function add(n,d){return n+d}

You can minify more than one JavaScript file at a time by using an object
for the first argument where the keys are file names and the values are source
code:

var code = {
    "file1.js": "function add(first, second) { return first + second; }",
    "file2.js": "console.log(add(1 + 2, 3 + 4));"
};
var result = UglifyJS.minify(code);
console.log(result.code);
// function add(d,n){return d+n}console.log(add(3,7));

The toplevel option:

var code = {
    "file1.js": "function add(first, second) { return first + second; }",
    "file2.js": "console.log(add(1 + 2, 3 + 4));"
};
var options = { toplevel: true };
var result = UglifyJS.minify(code, options);
console.log(result.code);
// console.log(3+7);

The nameCache option:

var options = {
    mangle: {
        toplevel: true,
    },
    nameCache: {}
};
var result1 = UglifyJS.minify({
    "file1.js": "function add(first, second) { return first + second; }"
}, options);
var result2 = UglifyJS.minify({
    "file2.js": "console.log(add(1 + 2, 3 + 4));"
}, options);
console.log(result1.code);
// function n(n,r){return n+r}
console.log(result2.code);
// console.log(n(3,7));

You may persist the name cache to the file system in the following way:

var cacheFileName = "/tmp/cache.json";
var options = {
    mangle: {
        properties: true,
    },
    nameCache: JSON.parse(fs.readFileSync(cacheFileName, "utf8"))
};
fs.writeFileSync("part1.js", UglifyJS.minify({
    "file1.js": fs.readFileSync("file1.js", "utf8"),
    "file2.js": fs.readFileSync("file2.js", "utf8")
}, options).code, "utf8");
fs.writeFileSync("part2.js", UglifyJS.minify({
    "file3.js": fs.readFileSync("file3.js", "utf8"),
    "file4.js": fs.readFileSync("file4.js", "utf8")
}, options).code, "utf8");
fs.writeFileSync(cacheFileName, JSON.stringify(options.nameCache), "utf8");

An example of a combination of minify() options:

var code = {
    "file1.js": "function add(first, second) { return first + second; }",
    "file2.js": "console.log(add(1 + 2, 3 + 4));"
};
var options = {
    toplevel: true,
    compress: {
        global_defs: {
            "@console.log": "alert"
        },
        passes: 2
    },
    output: {
        beautify: false,
        preamble: "/* uglified */"
    }
};
var result = UglifyJS.minify(code, options);
console.log(result.code);
// /* uglified */
// alert(10);"

To produce warnings:

var code = "function f(){ var u; return 2 + 3; }";
var options = { warnings: true };
var result = UglifyJS.minify(code, options);
console.log(result.error);    // runtime error, `undefined` in this case
console.log(result.warnings); // [ 'Dropping unused variable u [0:1,18]' ]
console.log(result.code);     // function f(){return 5}

An error example:

var result = UglifyJS.minify({"foo.js" : "if (0) else console.log(1);"});
console.log(JSON.stringify(result.error));
// {"message":"Unexpected token: keyword (else)","filename":"foo.js","line":1,"col":7,"pos":7}

Note: unlike uglify-js@2.x, the 3.x API does not throw errors. To
achieve a similar effect one could do the following:

var result = UglifyJS.minify(code, options);
if (result.error) throw result.error;

Minify options

  - annotations — pass false to ignore all comment annotations and elide them
    from output. Useful when, for instance, external tools incorrectly applied
    /*@__PURE__*/ or /*#__PURE__*/. Pass true to both compress and retain
    comment annotations in output to allow for further processing downstream.

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

  - expression (default: false) — parse as a single expression, e.g. JSON.

  - ie (default: false) — enable workarounds for Internet Explorer bugs.

  - keep_fargs (default: false) — pass true to prevent discarding or mangling
    of function arguments.

  - keep_fnames (default: false) — pass true to prevent discarding or mangling
    of function names.  Useful for code relying on Function.prototype.name.

  - mangle (default: true) — pass false to skip mangling names, or pass
    an object to specify mangle options (see below).
    
      - mangle.properties (default: false) — a subcategory of the mangle option.
        Pass an object to specify custom mangle property options.

  - module (default: true) — process input as ES module, i.e. implicit
    "use strict"; and support for top-level await. When explicitly specified,
    also enables toplevel.

  - nameCache (default: null) — pass an empty object {} or a previously
    used nameCache object if you wish to cache mangled variable and
    property names across multiple invocations of minify(). Note: this is
    a read/write property. minify() will read the name cache state of this
    object and update it during minification so that it may be
    reused or externally persisted by the user.

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

  - parse (default: {}) — pass an object if you wish to specify some
    additional parse options.

  - sourceMap (default: false) — pass an object if you wish to specify
    source map options.

  - toplevel (default: false) — set to true if you wish to enable top level
    variable and function name mangling and to drop unused variables and functions.

  - v8 (default: false) — enable workarounds for Chrome & Node.js bugs.

  - warnings (default: false) — pass true to return compressor warnings
    in result.warnings. Use the value "verbose" for more detailed warnings.

  - webkit (default: false) — enable workarounds for Safari/WebKit bugs.
    PhantomJS users should set this option to true.

Minify options structure

{
    parse: {
        // parse options
    },
    compress: {
        // compress options
    },
    mangle: {
        // mangle options

        properties: {
            // mangle property options
        }
    },
    output: {
        // output options
    },
    sourceMap: {
        // source map options
    },
    nameCache: null, // or specify a name cache object
    toplevel: false,
    warnings: false,
}

Source map options

To generate a source map:

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

Note that the source map is not saved in a file, it's just returned in
result.map.  The value passed for sourceMap.url is only used to set
//# sourceMappingURL=out.js.map 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: "http://example.com/src",
        url: "out.js.map"
    }
});

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 compiled.js.map",
        url: "minified.js.map"
    }
});
// same as before, it returns `code` and `map`

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

If you wish to reduce file size of the source map, set option sourceMap.names
to be false and all symbol names will be omitted.

Parse options

  - bare_returns (default: false) — support top level return statements

  - html5_comments (default: true) — process HTML comment as workaround for
    browsers which do not recognize  tags

  - module (default: false) — set to true if you wish to process input as
    ES module, i.e. implicit "use strict"; and support for top-level await.

  - shebang (default: true) — support #!command as the first line

Compress options

  - annotations (default: true) — Pass false to disable potentially dropping
    functions marked as "pure".  A function call is marked as "pure" if a comment
    annotation /*@__PURE__*/ or /*#__PURE__*/ immediately precedes the call. For
    example: /*@__PURE__*/foo();

  - arguments (default: true) — replace arguments[index] with function
    parameter name whenever possible.

  - arrows (default: true) — apply optimizations to arrow functions

  - assignments (default: true) — apply optimizations to assignment expressions

  - awaits (default: true) — apply optimizations to await expressions

  - booleans (default: true) — various optimizations for boolean context,
    for example !!a ? b : c → a ? b : c

  - collapse_vars (default: true) — Collapse single-use non-constant variables,
    side effects permitting.

  - comparisons (default: true) — apply certain optimizations to binary nodes,
    e.g. !(a <= b) → a > b, attempts to negate binary nodes, e.g.
    a = !b && !c && !d && !e → a=!(b||c||d||e) etc.

  - conditionals (default: true) — apply optimizations for if-s and conditional
    expressions

  - dead_code (default: true) — remove unreachable code

  - default_values (default: true) — drop overshadowed default values

  - directives (default: true) — remove redundant or non-standard directives

  - drop_console (default: false) — Pass true to discard calls to
    console.* functions. If you wish to drop a specific function call
    such as console.info and/or retain side effects from function arguments
    after dropping the function call then use pure_funcs instead.

  - drop_debugger (default: true) — remove debugger; statements

  - evaluate (default: true) — Evaluate expression for shorter constant
    representation. Pass "eager" to always replace function calls whenever
    possible, or a positive integer to specify an upper bound for each individual
    evaluation in number of characters.

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

  - functions (default: true) — convert declarations from var to function
    whenever possible.

  - global_defs (default: {}) — see conditional compilation

  - hoist_exports (default: true) — hoist export statements to facilitate
    various compress and mangle optimizations.

  - hoist_funs (default: false) — hoist function declarations

  - hoist_props (default: true) — hoist properties from constant object and
    array literals into regular variables subject to a set of constraints. For example:
    var o={p:1, q:2}; f(o.p, o.q); is converted to f(1, 2);. Note: hoist_props
    works best with toplevel and mangle enabled, alongside with compress option
    passes set to 2 or higher.

  - 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 (default: true) — optimizations for if/return and if/continue

  - imports (default: true) — drop unreferenced import symbols when used with unused

  - inline (default: true) — inline calls to function with simple/return statement:
    
      - false — same as 0
      - 0 — disabled inlining
      - 1 — inline simple functions
      - 2 — inline functions with arguments
      - 3 — inline functions with arguments and variables
      - 4 — inline functions with arguments, variables and statements
      - true — same as 4

  - join_vars (default: true) — join consecutive var statements

  - keep_fargs (default: false) — discard unused function arguments except
    when unsafe to do so, e.g. code which relies on Function.prototype.length.
    Pass true to always retain function arguments.

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

  - loops (default: true) — optimizations for do, while and for loops
    when we can statically determine the condition.

  - merge_vars (default: true) — combine and reuse variables.

  - module (default: false) — set to true if you wish to process input as
    ES module, i.e. implicit "use strict";.

  - negate_iife (default: true) — negate "Immediately-Called Function Expressions"
    where the return value is discarded, to avoid the parentheses that the
    code generator would insert.

  - objects (default: true) — compact duplicate keys in object literals.

  - passes (default: 1) — The maximum number of times to run compress.
    In some cases more than one pass leads to further compressed code.  Keep in
    mind more passes will take more time.

  - properties (default: true) — rewrite property access using the dot notation, for
    example foo["bar"] → foo.bar

  - 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). Make sure symbols under pure_funcs
    are also under mangle.reserved to avoid mangling.

  - pure_getters (default: "strict") — Pass true for UglifyJS to assume that
    object property access (e.g. foo.bar or a[42]) does not throw exception or
    alter program states via getter function. Pass "strict" to allow dropping or
    reordering foo.bar only if foo is not null or undefined and is safe to
    access as a variable. Pass false to retain all property accesses.

  - reduce_funcs (default: true) — Allows single-use functions to be
    inlined as function expressions when permissible allowing further
    optimization.  Enabled by default.  Option depends on reduce_vars
    being enabled.  Some code runs faster in the Chrome V8 engine if this
    option is disabled.  Does not negatively impact other major browsers.

  - reduce_vars (default: true) — Improve optimization on variables assigned with and
    used as constant values.

  - rests (default: true) — apply optimizations to rest parameters

  - 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.

  - side_effects (default: true) — drop extraneous code which does not affect
    outcome of runtime execution.

  - spreads (default: true) — flatten spread expressions.

  - strings (default: true) — compact string concatenations.

  - switches (default: true) — de-duplicate and remove unreachable switch branches

  - templates (default: true) — compact template literals by embedding expressions
    and/or converting to string literals, e.g.  `foo ${42}` → "foo 42"

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

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

  - typeofs (default: true) — compress typeof expressions, e.g.
    typeof foo == "undefined" → void 0 === foo

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

  - unsafe_comps (default: false) — assume operands cannot be (coerced to) NaN
    in numeric comparisons, e.g. a <= b. In addition, expressions involving in
    or instanceof would never throw.

  - unsafe_Function (default: false) — compress and mangle Function(args, code)
    when both args and code are string literals.

  - 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
    Array.prototype.slice.call(a) into [].slice.call(a)

  - unsafe_regexp (default: false) — enable substitutions of variables with
    RegExp values the same way as if they are constants.

  - unsafe_undefined (default: false) — substitute void 0 if there is a
    variable named undefined in scope (variable name will be mangled, typically
    reduced to a single character)

  - unused (default: true) — drop unreferenced functions and variables (simple
    direct variable assignments do not count as references unless set to "keep_assign")

  - varify (default: true) — convert block-scoped declarations into var
    whenever safe to do so

  - yields (default: true) — apply optimizations to yield expressions

Mangle options

  - eval (default: false) — Pass true to mangle names visible in scopes
    where eval or with are used.

  - reserved (default: []) — Pass an array of identifiers that should be
    excluded from mangling. Example: ["foo", "bar"].

  - toplevel (default: false) — Pass true to mangle names declared in the
    top level scope.

Examples:

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

var code = fs.readFileSync("test.js", "utf8");

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

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

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

Mangle properties options

  - builtins (default: false) — Use true to allow the mangling of built-in
    properties of JavaScript API. Not recommended to override this setting.

  - debug (default: false) — Mangle names with the original name still present.
    Pass an empty string "" to enable, or a non-empty string to set the debug suffix.

  - domprops (default: false) — Use true to allow the mangling of properties
    commonly found in Document Object Model. Not recommended to override this setting.

  - keep_fargs (default: false) — Use true to prevent mangling of function
    arguments.

  - keep_quoted (default: false) — Only mangle unquoted property names.

  - regex (default: null) — Pass a RegExp literal to only mangle property
    names matching the regular expression.

  - reserved (default: []) — Do not mangle property names listed in the
    reserved array.

Output 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:

  - annotations (default: false) — pass true to retain comment annotations
    /*@__PURE__*/ or /*#__PURE__*/, otherwise they will be discarded even if
    comments is set.

  - ascii_only (default: false) — escape Unicode characters in strings and
    regexps (affects directives with non-ascii characters becoming invalid)

  - beautify (default: true) — whether to actually beautify the output.
    Passing -b will set this to true. Use -O if you want to generate minified
    code and specify additional arguments.

  - braces (default: false) — always insert braces in if, for,
    do, while or with statements, even if their body is a single
    statement.

  - comments (default: false) — pass true or "all" to preserve all
    comments, "some" to preserve multi-line comments that contain @cc_on,
    @license, or @preserve (case-insensitive), a regular expression string
    (e.g. /^!/), or a function which returns boolean, e.g.
    
    function(node, comment) {
        return comment.value.indexOf("@type " + node.TYPE) >= 0;
    }

  - extendscript (default: false) — enable workarounds for Adobe ExtendScript
    bugs

  - galio (default: false) — enable workarounds for ANT Galio bugs

  - indent_level (default: 4) — indent by specified number of spaces or the
    exact whitespace sequence supplied, e.g. "\t".

  - indent_start (default: 0) — prefix all lines by whitespace sequence
    specified in the same format as indent_level.

  - inline_script (default: true) — escape HTML comments and the slash in
    occurrences of  in strings

  - keep_quoted_props (default: false) — when turned on, prevents stripping
    quotes from property names in object literals.

  - max_line_len (default: false) — maximum line length (for uglified code)

  - 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.

  - preserve_line (default: false) — pass true to retain line numbering on
    a best effort basis.

  - quote_keys (default: false) — pass true to quote all keys in literal
    objects

  - 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. 0 is best for gzip size.
      - 1 — always use single quotes
      - 2 — always use double quotes
      - 3 — always use the original quotes

  - 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).

  - shebang (default: true) — preserve shebang #! in preamble (bash scripts)

  - 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.

  - wrap_iife (default: false) — pass true to wrap immediately invoked
    function expressions. See
    #640 for more details.

Miscellaneous

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.

The unsafe compress 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

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:

var DEBUG = false;
var 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:

var result = UglifyJS.minify(fs.readFileSync("input.js", "utf8"), {
    compress: {
        dead_code: true,
        global_defs: {
            DEBUG: false
        }
    }
});

To replace an identifier with an arbitrary non-constant expression it is
necessary to prefix the global_defs key with "@" to instruct UglifyJS
to parse the value as an expression:

UglifyJS.minify("alert('hello');", {
    compress: {
        global_defs: {
            "@alert": "console.log"
        }
    }
}).code;
// returns: 'console.log("hello");'

Otherwise it would be replaced as string literal:

UglifyJS.minify("alert('hello');", {
    compress: {
        global_defs: {
            "alert": "console.log"
        }
    }
}).code;
// returns: '"console.log"("hello");'

Using native Uglify AST with minify()

// example: parse only, produce native Uglify AST

var result = UglifyJS.minify(code, {
    parse: {},
    compress: false,
    mangle: false,
    output: {
        ast: true,
        code: false  // optional - faster if false
    }
});

// result.ast contains native Uglify AST

// example: accept native Uglify AST input and then compress and mangle
//          to produce both code and native AST.

var result = UglifyJS.minify(ast, {
    compress: {},
    mangle: {},
    output: {
        ast: true,
        code: true  // optional - faster if false
    }
});

// result.ast contains native Uglify AST
// result.code contains the minified code in string form.

Working with Uglify AST

Transversal and transformation of the native AST can be performed through
TreeWalker and
TreeTransformer
respectively.

ESTree / 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.

Uglify Fast Minify Mode

It's not well known, but whitespace removal and symbol mangling accounts
for 95% of the size reduction in minified code for most JavaScript - not
elaborate code transforms. One can simply disable compress to speed up
Uglify builds by 3 to 5 times.

| d3.js | minify size | gzip size | minify time (seconds) |
| --- | ---: | ---: | ---: |
| original | 511,371 | 119,932 | - |
| uglify-js@3.13.0 mangle=false, compress=false | 363,988 | 95,695 | 0.56 |
| uglify-js@3.13.0 mangle=true, compress=false | 253,305 | 81,281 | 0.99 |
| uglify-js@3.13.0 mangle=true, compress=true | 244,436 | 79,854 | 5.30 |

To enable fast minify mode from the CLI use:

uglifyjs file.js -m

To enable fast minify mode with the API use:

UglifyJS.minify(code, { compress: false, mangle: true });

Source maps and debugging

Various compress transforms that simplify, rearrange, inline and remove code
are known to have an adverse effect on debugging with source maps. This is
expected as code is optimized and mappings are often simply not possible as
some code no longer exists. For highest fidelity in source map debugging
disable the Uglify compress option and just use mangle.

Compiler assumptions

To allow for better optimizations, the compiler makes various assumptions:

  - The code does not rely on preserving its runtime performance characteristics.
    Typically uglified code will run faster due to less instructions and easier
    inlining, but may be slower on rare occasions for a specific platform, e.g.
    see reduce_funcs.
  - .toString() and .valueOf() don't have side effects, and for built-in
    objects they have not been overridden.
  - undefined, NaN and Infinity have not been externally redefined.
  - arguments.callee, arguments.caller and Function.prototype.caller are not used.
  - The code doesn't expect the contents of Function.prototype.toString() or
    Error.prototype.stack to be anything in particular.
  - Getting and setting properties on a plain object does not cause other side effects
    (using .watch() or Proxy).
  - Object properties can be added, removed and modified (not prevented with
    Object.defineProperty(), Object.defineProperties(), Object.freeze(),
    Object.preventExtensions() or Object.seal()).
  - If array destructuring is present, index-like properties in Array.prototype
    have not been overridden:
    Object.prototype[0] = 42;
    var [ a ] = [];
    var { 0: b } = {};
    // 42 undefined
    console.log([][0], a);
    // 42 42
    console.log({}[0], b);
  - Earlier versions of JavaScript will throw SyntaxError with the following:
    ({
        p: 42,
        get p() {},
    });
    // SyntaxError: Object literal may not have data and accessor property with
    //              the same name
    UglifyJS may modify the input which in turn may suppress those errors.
  - Iteration order of keys over an object which contains spread syntax in later
    versions of Chrome and Node.js may be altered.
  - When toplevel is enabled, UglifyJS effectively assumes input code is wrapped
    within function(){ ... }, thus forbids aliasing of declared global variables:
    A = "FAIL";
    var B = "FAIL";
    // can be `global`, `self`, `window` etc.
    var top = function() {
        return this;
    }();
    // "PASS"
    top.A = "PASS";
    console.log(A);
    // "FAIL" after compress and/or mangle
    top.B = "PASS";
    console.log(B);
  - Use of arguments alongside destructuring as function parameters, e.g.
    function({}, arguments) {} will result in SyntaxError in earlier versions
    of Chrome and Node.js - UglifyJS may modify the input which in turn may
    suppress those errors.
  - Earlier versions of Chrome and Node.js will throw ReferenceError with the
    following:
    var a;
    try {
        throw 42;
    } catch ({
        [a]: b,
        // ReferenceError: a is not defined
    }) {
        let a;
    }
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of JavaScript will throw SyntaxError with the following:
    a => {
        let a;
    };
    // SyntaxError: Identifier 'a' has already been declared
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of JavaScript will throw SyntaxError with the following:
    try {
        // ...
    } catch ({ message: a }) {
        var a;
    }
    // SyntaxError: Identifier 'a' has already been declared
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some versions of Chrome and Node.js will throw ReferenceError with the
    following:
    console.log(((a, b = function() {
        return a;
        // ReferenceError: a is not defined
    }()) => b)());
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some arithmetic operations with BigInt may throw TypeError:
    1n + 1;
    // TypeError: can't convert BigInt to number
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some versions of JavaScript will throw SyntaxError with the
    following:
    console.log(String.raw`\uFo`);
    // SyntaxError: Invalid Unicode escape sequence
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some versions of JavaScript will throw SyntaxError with the
    following:
    try {} catch (e) {
        for (var e of []);
    }
    // SyntaxError: Identifier 'e' has already been declared
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some versions of Chrome and Node.js will give incorrect results with the
    following:
    console.log({
        ...{
            set 42(v) {},
            42: "PASS",
        },
    });
    // Expected: { '42': 'PASS' }
    // Actual:   { '42': undefined }
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of JavaScript will throw SyntaxError with the following:
    var await;
    class A {
        static p = await;
    }
    // SyntaxError: Unexpected reserved word
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of JavaScript will throw SyntaxError with the following:
    var async;
    for (async of []);
    // SyntaxError: The left-hand side of a for-of loop may not be 'async'.
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some versions of Chrome and Node.js will give incorrect results with the
    following:
    console.log({
        ...console,
        get 42() {
            return "FAIL";
        },
        [42]: "PASS",
    }[42], {
        ...console,
        get 42() {
            return "FAIL";
        },
        42: "PASS",
    }[42]);
    // Expected: "PASS PASS"
    // Actual:   "PASS FAIL"
    UglifyJS may modify the input which in turn may suppress those errors.
  - Earlier versions of JavaScript will throw TypeError with the following:
    (function() {
        {
            const a = "foo";
        }
        {
            const a = "bar";
        }
    })();
    // TypeError: const 'a' has already been declared
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of Chrome and Node.js will give incorrect results with the
    following:
    try {
        class A {
            static 42;
            static get 42() {}
        }
        console.log("PASS");
    } catch (e) {
        console.log("FAIL");
    }
    // Expected: "PASS"
    // Actual:   "FAIL"
    UglifyJS may modify the input which in turn may suppress those errors.
  - Some versions of Chrome and Node.js will give incorrect results with the
    following:
    (async function(a) {
        (function() {
            var b = await => console.log("PASS");
            b();
        })();
    })().catch(console.error);
    // Expected: "PASS"
    // Actual:   SyntaxError: Unexpected reserved word
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of Chrome and Node.js will give incorrect results with the
    following:
    try {
        f();
        function f() {
            throw 42;
        }
    } catch (e) {
        console.log(typeof f, e);
    }
    // Expected: "function 42"
    // Actual:   "undefined 42"
    UglifyJS may modify the input which in turn may suppress those errors.
  - Later versions of JavaScript will throw SyntaxError with the following:
    "use strict";
    console.log(function f() {
        return f = "PASS";
    }());
    // Expected: "PASS"
    // Actual:   TypeError: invalid assignment to const 'f'
    UglifyJS may modify the input which in turn may suppress those errors.
  - Adobe ExtendScript will give incorrect results with the following:
    alert(true ? "PASS" : false ? "FAIL" : null);
    // Expected: "PASS"
    // Actual:   "FAIL"
    UglifyJS may modify the input which in turn may suppress those errors.
  - Adobe ExtendScript will give incorrect results with the following:
    alert(42 ? null ? "FAIL" : "PASS" : "FAIL");
    // Expected: "PASS"
    // Actual:   SyntaxError: Expected: :
    UglifyJS may modify the input which in turn may suppress those errors.
