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 withuglify-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)
--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 offilename
is only used to setfile
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 HTTPX-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 whereeval
orwith
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.
--mangle-props
)
CLI mangling property names (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.
--mangle-props keep_quoted
)
Mangling unquoted names (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 [email protected]
, 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
— passfalse
to ignore all comment annotations and elide them from output. Useful when, for instance, external tools incorrectly applied/*@__PURE__*/
or/*#__PURE__*/
. Passtrue
to both compress and retain comment annotations in output to allow for further processing downstream. -
compress
(default:{}
) — passfalse
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
) — passtrue
to prevent discarding or mangling of function arguments. -
keep_fnames
(default:false
) — passtrue
to prevent discarding or mangling of function names. Useful for code relying onFunction.prototype.name
. -
mangle
(default:true
) — passfalse
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:false
) — set totrue
if you wish to process input as ES module, i.e. implicit"use strict";
and support for top-levelawait
, alongside withtoplevel
enabled. -
nameCache
(default:null
) — pass an empty object{}
or a previously usednameCache
object if you wish to cache mangled variable and property names across multiple invocations ofminify()
. 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 totrue
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
) — passtrue
to return compressor warnings inresult.warnings
. Use the value"verbose"
for more detailed warnings. -
webkit
(default:false
) — enable workarounds for Safari/WebKit bugs. PhantomJS users should set this option totrue
.
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 levelreturn
statements -
html5_comments
(default:true
) — process HTML comment as workaround for browsers which do not recognize<script>
tags -
module
(default:false
) — set totrue
if you wish to process input as ES module, i.e. implicit"use strict";
and support for top-levelawait
. -
shebang
(default:true
) — support#!command
as the first line
Compress options
-
annotations
(default:true
) — Passfalse
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
) — replacearguments[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 toawait
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 forif
-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
) — Passtrue
to discard calls toconsole.*
functions. If you wish to drop a specific function call such asconsole.info
and/or retain side effects from function arguments after dropping the function call then usepure_funcs
instead. -
drop_debugger
(default:true
) — removedebugger;
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
) — Passtrue
to preserve completion values from terminal statements withoutreturn
, e.g. in bookmarklets. -
functions
(default:true
) — convert declarations fromvar
tofunction
whenever possible. -
global_defs
(default:{}
) — see conditional compilation -
hoist_exports
(default:true
) — hoistexport
statements to facilitate variouscompress
andmangle
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 tof(1, 2);
. Note:hoist_props
works best withtoplevel
andmangle
enabled, alongside withcompress
optionpasses
set to2
or higher. -
hoist_vars
(default:false
) — hoistvar
declarations (this isfalse
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 withunused
-
inline
(default:true
) — inline calls to function with simple/return
statement:false
— same as0
0
— disabled inlining1
— inline simple functions2
— inline functions with arguments3
— inline functions with arguments and variables4
— inline functions with arguments, variables and statementstrue
— same as4
-
join_vars
(default:true
) — join consecutivevar
statements -
keep_fargs
(default:false
) — discard unused function arguments except when unsafe to do so, e.g. code which relies onFunction.prototype.length
. Passtrue
to always retain function arguments. -
keep_infinity
(default:false
) — Passtrue
to preventInfinity
from being compressed into1/0
, which may cause performance issues on Chrome. -
loops
(default:true
) — optimizations fordo
,while
andfor
loops when we can statically determine the condition. -
merge_vars
(default:true
) — combine and reuse variables. -
module
(default:false
) — set totrue
if you wish to process input as ES module, i.e. implicit"use strict";
alongside withtoplevel
enabled. -
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 examplefoo["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 instancevar q = Math.floor(a/b)
. If variableq
is not used elsewhere, UglifyJS will drop it, but will still keep theMath.floor(a/b)
, not knowing what it does. You can passpure_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 underpure_funcs
are also undermangle.reserved
to avoid mangling. -
pure_getters
(default:"strict"
) — If you passtrue
for this, UglifyJS will assume that object property access (e.g.foo.bar
orfoo["bar"]
) doesn't have any side effects. Specify"strict"
to treatfoo.bar
as side-effect-free only whenfoo
is certain to not throw, i.e. notnull
orundefined
. -
reduce_funcs
(default:true
) — Allows single-use functions to be inlined as function expressions when permissible allowing further optimization. Enabled by default. Option depends onreduce_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 totrue
then the defaultsequences
limit is200
. Set option tofalse
or0
to disable. The smallestsequences
length is2
. Asequences
value of1
is grandfathered to be equivalent totrue
and as such means200
. On rare occasions the default sequences limit leads to very slow compress times in which case a value of20
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 unreachableswitch
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 fromunused
removal (can be array, comma-separated, RegExp or function. Impliestoplevel
) -
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
) — compresstypeof
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 involvingin
orinstanceof
would never throw. -
unsafe_Function
(default:false
) — compress and mangleFunction(args, code)
when bothargs
andcode
are string literals. -
unsafe_math
(default:false
) — optimize numerical expressions like2 * x * 3
into6 * x
, which may give imprecise floating point results. -
unsafe_proto
(default:false
) — optimize expressions likeArray.prototype.slice.call(a)
into[].slice.call(a)
-
unsafe_regexp
(default:false
) — enable substitutions of variables withRegExp
values the same way as if they are constants. -
unsafe_undefined
(default:false
) — substitutevoid 0
if there is a variable namedundefined
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 intovar
whenever safe to do so -
yields
(default:true
) — apply optimizations toyield
expressions
Mangle options
-
eval
(default:false
) — Passtrue
to mangle names visible in scopes whereeval
orwith
are used. -
reserved
(default:[]
) — Pass an array of identifiers that should be excluded from mangling. Example:["foo", "bar"]
. -
toplevel
(default:false
) — Passtrue
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
) — Usetrue
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
) — Usetrue
to allow the mangling of properties commonly found in Document Object Model. Not recommended to override this setting. -
keep_fargs
(default:false
) — Usetrue
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 thereserved
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
) — passtrue
to retain comment annotations/*@__PURE__*/
or/*#__PURE__*/
, otherwise they will be discarded even ifcomments
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, 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 inif
,for
,do
,while
orwith
statements, even if their body is a single statement. -
comments
(default:false
) — passtrue
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 returnsboolean
, 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 asindent_level
. -
inline_script
(default:true
) — escape HTML comments and the slash in occurrences of</script>
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
) — passtrue
to retain line numbering on a best effort basis. -
quote_keys
(default:false
) — passtrue
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 quotes2
— always use double quotes3
— always use the original quotes
-
semicolons
(default:true
) — separate statements with semicolons. If you passfalse
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
) — passtrue
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.
unsafe
compress
option
The 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)
orArray(1, 2, 3)
→[ 1, 2, 3 ]
new Object()
→{}
String(exp)
orexp.toString()
→"" + exp
new Object/RegExp/Function/Error/Array (...)
→ we discard thenew
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");'
minify()
Using native Uglify AST with // 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 | - |
[email protected] mangle=false, compress=false | 363,988 | 95,695 | 0.56 |
[email protected] mangle=true, compress=false | 253,305 | 81,281 | 0.99 |
[email protected] 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
andInfinity
have not been externally redefined.arguments.callee
,arguments.caller
andFunction.prototype.caller
are not used.- The code doesn't expect the contents of
Function.prototype.toString()
orError.prototype.stack
to be anything in particular. - Getting and setting properties on a plain object does not cause other side effects
(using
.watch()
orProxy
). - Object properties can be added, removed and modified (not prevented with
Object.defineProperty()
,Object.defineProperties()
,Object.freeze()
,Object.preventExtensions()
orObject.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:UglifyJS may modify the input which in turn may suppress those errors.({ p: 42, get p() {}, }); // SyntaxError: Object literal may not have data and accessor property with // the same name
- 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 withinfunction(){ ... }
, 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 inSyntaxError
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:UglifyJS may modify the input which in turn may suppress those errors.var a; try { throw 42; } catch ({ [a]: b, // ReferenceError: a is not defined }) { let a; }
- Later versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors.a => { let a; }; // SyntaxError: Identifier 'a' has already been declared
- Later versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors.try { // ... } catch ({ message: a }) { var a; } // SyntaxError: Identifier 'a' has already been declared
- Some versions of Chrome and Node.js will throw
ReferenceError
with the following:UglifyJS may modify the input which in turn may suppress those errors.console.log(((a, b = function() { return a; // ReferenceError: a is not defined }()) => b)());
- Some arithmetic operations with
BigInt
may throwTypeError
:UglifyJS may modify the input which in turn may suppress those errors.1n + 1; // TypeError: can't convert BigInt to number
- Some versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors.console.log(String.raw`\uFo`); // SyntaxError: Invalid Unicode escape sequence
- Some versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors.try {} catch (e) { for (var e of []); } // SyntaxError: Identifier 'e' has already been declared
- Some versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.
console.log({ ...{ set 42(v) {}, 42: "PASS", }, }); // Expected: { '42': 'PASS' } // Actual: { '42': undefined }
- Later versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors.var await; class A { static p = await; } // SyntaxError: Unexpected reserved word
- Later versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors.var async; for (async of []); // SyntaxError: The left-hand side of a for-of loop may not be 'async'.
- Some versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.
console.log({ ...console, get 42() { return "FAIL"; }, [42]: "PASS", }[42], { ...console, get 42() { return "FAIL"; }, 42: "PASS", }[42]); // Expected: "PASS PASS" // Actual: "PASS FAIL"
- Earlier versions of JavaScript will throw
TypeError
with the following:UglifyJS may modify the input which in turn may suppress those errors.(function() { { const a = "foo"; } { const a = "bar"; } })(); // TypeError: const 'a' has already been declared
- Later versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.
try { class A { static 42; static get 42() {} } console.log("PASS"); } catch (e) { console.log("FAIL"); } // Expected: "PASS" // Actual: "FAIL"
- Some versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.
(async function(a) { (function() { var b = await => console.log("PASS"); b(); })(); })().catch(console.error); // Expected: "PASS" // Actual: SyntaxError: Unexpected reserved word
- Later versions of Chrome and Node.js will give incorrect results with the
following:
UglifyJS may modify the input which in turn may suppress those errors.
try { f(); function f() { throw 42; } } catch (e) { console.log(typeof f, e); } // Expected: "function 42" // Actual: "undefined 42"
- Later versions of JavaScript will throw
SyntaxError
with the following:UglifyJS may modify the input which in turn may suppress those errors."use strict"; console.log(function f() { return f = "PASS"; }()); // Expected: "PASS" // Actual: TypeError: invalid assignment to const 'f'
- Adobe ExtendScript will give incorrect results with the following:
UglifyJS may modify the input which in turn may suppress those errors.
alert(true ? "PASS" : false ? "FAIL" : null); // Expected: "PASS" // Actual: "FAIL"
- Adobe ExtendScript will give incorrect results with the following:
UglifyJS may modify the input which in turn may suppress those errors.
alert(42 ? null ? "FAIL" : "PASS" : "FAIL"); // Expected: "PASS" // Actual: SyntaxError: Expected: :