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UglifyJS2

by mishoo

mishoo /UglifyJS2

JavaScript parser / mangler / compressor / beautifier toolkit

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UglifyJS 3

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

Note:

2.x

 releases can be found [here](https://github.com/mishoo/UglifyJS/tree/v2.x)**.
- 

uglify-js

 only supports JavaScript (ECMAScript 5).
- To minify ECMAScript 2015 or above, transpile using tools like [Babel](https://babeljs.io/).

## Install

First make sure you have installed the latest version of [node.js](http://nodejs.org/)(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 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 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 parenthesis. Note: you may want to disable negate_iife under compressor options. -O, --output-opts [options] Specify output options (beautify disabled by default). -o, --output Output file path (default STDOUT). Specify ast or spidermonkey to write UglifyJS or SpiderMonkey AST as JSON to STDOUT respectively. --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 Read minify() options from JSON file. -d, --define [=value] Global definitions. -e, --enclose [arg[:value]] Embed everything in a big function, with configurable argument(s) & value(s). --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 Function.prototype.name. --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 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. --verbose Print diagnostic messages. --warn Print warning messages. --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.


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=''"

 to specify the name of the source map. The value of

filename

 is only used to set 

file

 attribute (see [the spec](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k)) in source map file.
- 

--source-map "root=''"

 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=''"

 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](https://github.com/mishoo/UglifyJS2/blob/master/#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:
- 

toplevel

 (default 

false

) -- mangle names declared in the top level scope.
- 

eval

 (default 

false

) -- mangle names visible in scopes where 

eval

 or 

with

 are used.

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

):

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

javascript 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:

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

javascript 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

:

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

javascript 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:

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

javascript 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);


### 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:

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


There is a single high level function, 
**```
minify(code, options)
```**, which will perform all minification [phases](https://github.com/mishoo/UglifyJS2/blob/master/#minify-options) in a configurable manner. By default

minify()

 will enable the options [

compress

](https://github.com/mishoo/UglifyJS2/blob/master/#compress-options)and [

mangle

](https://github.com/mishoo/UglifyJS2/blob/master/#mangle-options). Example:

javascript 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:

javascript 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:

javascript 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:

javascript 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:

javascript 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:

javascript 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:

javascript 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:

javascript 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 

[email protected]

, the 

3.x

 API does not throw errors. To achieve a similar effect one could do the following:

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


## Minify options

- 

compress

 (default 

{}

) — pass 

false

 to skip compressing entirely. Pass an object to specify custom [compress options](https://github.com/mishoo/UglifyJS2/blob/master/#compress-options).
- 

ie8

 (default 

false

) -- set to 

true

 to support IE8.
- 

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](https://github.com/mishoo/UglifyJS2/blob/master/#mangle-options) (see below).
  - 

mangle.properties

 (default 

false

) — a subcategory of the mangle option. Pass an object to specify custom [mangle property options](https://github.com/mishoo/UglifyJS2/blob/master/#mangle-properties-options).
- 

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](https://github.com/mishoo/UglifyJS2/blob/master/#output-options). The defaults are optimized for best compression.
- 

parse

 (default 

{}

) — pass an object if you wish to specify some additional [parse options](https://github.com/mishoo/UglifyJS2/blob/master/#parse-options).
- 

sourceMap

 (default 

false

) -- pass an object if you wish to specify[source map options](https://github.com/mishoo/UglifyJS2/blob/master/#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.
- 

warnings

 (default 

false

) — pass 

true

 to return compressor warnings in 

result.warnings

. Use the value 

"verbose"

 for more detailed warnings.

## 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, ie8: false, warnings: false, }


### Source map options

To generate a source map:

javascript 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](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k)) 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:

javascript 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

:

javascript 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

)
- 

shebang

 (default 

true

) -- support 

#!command

 as the first line

## Compress options

- 

arguments

 (default: 

true

) -- replace 

arguments[index]

 with function parameter name whenever possible.
- 

assignments

 (default: 

true

) -- apply optimizations to assignment 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
- 

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](https://github.com/mishoo/UglifyJS2/blob/master/#conditional-compilation)
- 

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 

mangle

 enabled, the 

compress

 option 

passes

 set to 

2

 or higher, and the 

compress

 option 

toplevel

 enabled.
- 

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
- 

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
  - 

true

 -- same as 

3

- 

join_vars

 (default: 

true

) -- join consecutive 

var

 statements
- 

keep_fargs

 (default: 

strict

) -- Discard unused function arguments. Code which relies on 

Function.length

 will break if this is done indiscriminately, i.e. when passing 

true

. Pass 

false

 to always retain function arguments.
- 

keep_fnames

 (default: 

false

) -- Pass 

true

 to prevent the compressor from discarding function names. Useful for code relying on

Function.prototype.name

. See also: the 

keep_fnames

 [mangle option](https://github.com/mishoo/UglifyJS2/blob/master/#mangle-options).
- 

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.
- 

negate_iife

 (default: 

true

) -- negate "Immediately-Called Function Expressions" where the return value is discarded, to avoid the parens 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"

) -- If you pass 

true

 for this, UglifyJS will assume that object property access (e.g. 

foo.bar

 or 

foo["bar"]

) doesn't have any side effects. Specify 

"strict"

 to treat 

foo.bar

 as side-effect-free only when

foo

 is certain to not throw, i.e. not 

null

 or 

undefined

.
- 

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.
- 

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

) -- 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();

- 

strings

 (default: 

true

) -- compact string concatenations.
- 

switches

 (default: 

true

) -- de-duplicate and remove unreachable 

switch

 branches
- 

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)
- 

top_retain

 (default: 

null

) -- prevent specific toplevel functions and variables from 

unused

 removal (can be array, comma-separated, RegExp or function. Implies 

toplevel

)
- 

typeofs

 (default: 

true

) -- Transforms 

typeof foo == "undefined"

 into

foo === void 0

. Note: recommend to set this value to 

false

 for IE10 and earlier versions due to known issues.
- 

unsafe

 (default: 

false

) -- apply "unsafe" transformations (discussion below)
- 

unsafe_comps

 (default: 

false

) -- compress expressions like 

a <= b

 assuming none of the operands can be (coerced to) 

NaN

.
- 

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"

)

## Mangle options

- 

eval

 (default 

false

) -- Pass 

true

 to mangle names visible in scopes where 

eval

 or 

with

 are used.
- 

keep_fnames

 (default 

false

) -- Pass 

true

 to not mangle function names. Useful for code relying on 

Function.prototype.name

. See also: the 

keep_fnames

[compress option](https://github.com/mishoo/UglifyJS2/blob/master/#compress-options).
- 

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 builtin DOM properties. 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.
- 

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:
- 

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, 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.
- 

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.

js function(node, comment) { return comment.value.indexOf("@type " + node.TYPE) >= 0; }

- 

indent_level

 (default 

4

)
- 

indent_start

 (default 

0

) -- prefix all lines by that many spaces
- 

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)
- 

webkit

 (default 

false

) -- enable workarounds for WebKit bugs. PhantomJS users should set this option to 

true

.
- 

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](https://github.com/mishoo/UglifyJS/issues/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:

javascript 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:

javascript 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:

javascript 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:

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


Otherwise it would be replaced as string literal:

javascript 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

](https://github.com/mishoo/UglifyJS/blob/master/lib/ast.js) and[

TreeTransformer

](https://github.com/mishoo/UglifyJS/blob/master/lib/transform.js)respectively.

### ESTree / SpiderMonkey AST

UglifyJS has its own abstract syntax tree format; for[practical reasons](http://lisperator.net/blog/uglifyjs-why-not-switching-to-spidermonkey-ast/)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](https://github.com/ternjs/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 4 times. In this fast 

mangle

-only mode Uglify has comparable minify speeds and gzip sizes to[

butternut

](https://www.npmjs.com/package/butternut):

| d3.js | minify size | gzip size | minify time (seconds) | | --- | ---: | ---: | ---: | | original | 451,131 | 108,733 | - | | [email protected] mangle=false, compress=false | 316,600 | 85,245 | 0.70 | | [email protected] mangle=true, compress=false | 220,216 | 72,730 | 1.13 | | [email protected] | 217,568 | 72,738 | 1.41 | | [email protected] mangle=true, compress=true | 212,511 | 71,560 | 3.36 | | [email protected] | 210,713 | 72,140 | 12.64 |

To enable fast minify mode from the CLI use:

uglifyjs file.js -m

To enable fast minify mode with the API use:

js 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:

- 

.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()

``` ).

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