uglify-js
Advanced tools
The uglify-js npm package is a JavaScript parser, minifier, compressor, and beautifier toolkit. It is commonly used to reduce the size of JavaScript files by removing unnecessary characters (like whitespace, comments, etc.) without changing their functionality. This helps in decreasing the load time and bandwidth usage of web applications.
Minification
Minification is the process of removing unnecessary characters and whitespace from source code without changing its functionality. The code sample demonstrates how to read a JavaScript file, minify it using uglify-js, and then write the minified code to a new file.
const uglifyJS = require('uglify-js');
const fs = require('fs');
const code = fs.readFileSync('example.js', 'utf8');
const minifiedCode = uglifyJS.minify(code).code;
fs.writeFileSync('example.min.js', minifiedCode);Compression
Compression is a form of optimization that reduces code size by performing various transformations, such as removing dead code or simplifying expressions. The code sample shows how to use uglify-js to compress a JavaScript file by removing console statements and then writing the compressed code to a new file.
const uglifyJS = require('uglify-js');
const fs = require('fs');
const options = {
compress: {
drop_console: true
}
};
const code = fs.readFileSync('example.js', 'utf8');
const compressedCode = uglifyJS.minify(code, options).code;
fs.writeFileSync('example.compressed.js', compressedCode);Mangling
Mangling is the process of shortening variable and function names to reduce file size and potentially obfuscate code. The code sample illustrates how to use uglify-js to mangle a JavaScript file and then save the mangled code to a new file.
const uglifyJS = require('uglify-js');
const fs = require('fs');
const options = {
mangle: true
};
const code = fs.readFileSync('example.js', 'utf8');
const mangledCode = uglifyJS.minify(code, options).code;
fs.writeFileSync('example.mangled.js', mangledCode);Beautification
Beautification is the process of reformatting minified or obfuscated code into a more readable and maintainable format. The code sample demonstrates how to use uglify-js to beautify a JavaScript file and then write the beautified code to a new file.
const uglifyJS = require('uglify-js');
const fs = require('fs');
const options = {
output: {
beautify: true
}
};
const code = fs.readFileSync('example.js', 'utf8');
const beautifiedCode = uglifyJS.minify(code, options).code;
fs.writeFileSync('example.beautified.js', beautifiedCode);Terser is a modern fork of uglify-js that supports the ES6+ syntax. It offers similar functionalities such as minification, compression, and mangling. Terser is often preferred for projects using modern JavaScript syntax.
Google Closure Compiler is a JavaScript optimizing compiler that provides advanced optimizations including dead code removal, function inlining, and variable renaming. It is known for its aggressive optimization techniques and can produce smaller output files compared to uglify-js, but it may require more configuration.
Babel-minify is a minifier based on the Babel toolchain. It leverages Babel's understanding of the latest JavaScript syntax to provide minification and compression. It is suitable for projects that are already using Babel and need to support newer JavaScript features.
Esbuild is an extremely fast JavaScript bundler and minifier. It is written in Go and focuses on speed, boasting compile times significantly faster than other bundlers and minifiers. Esbuild also supports modern JavaScript and TypeScript out of the box.
UglifyJS is a JavaScript parser, minifier, compressor and beautifier toolkit.
uglify-js supports JavaScript and most language features in ECMAScript.uglify-js.uglify-js@3 has a simplified API and CLI
that is not backwards compatible with uglify-js@2.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
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
-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.
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).
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.
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
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.
--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.
--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);
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.
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;
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: false) — set to true if you wish to process input as
ES module, i.e. implicit "use strict"; and support for top-level await,
alongside with toplevel enabled.
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.
{
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,
}
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.
bare_returns (default: false) — support top level return statements
html5_comments (default: true) — process HTML comment as workaround for
browsers which do not recognise <script> 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
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 00 — disabled inlining1 — inline simple functions2 — inline functions with arguments3 — inline functions with arguments and variables4 — inline functions with arguments, variables and statementstrue — same as 4join_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"; alongside with toplevel enabled.
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.
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 declaractions into var
whenever safe to do so
yields (default: true) — apply optimizations to yield expressions
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;'
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_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.
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, 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.
function(node, comment) {
return comment.value.indexOf("@type " + node.TYPE) >= 0;
}
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 </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) — 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 quotes2 — always use double quotes3 — always use the original quotessemicolons (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.
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 optionIt 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() → "" + expnew Object/RegExp/Function/Error/Array (...) → we discard the newYou 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.
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()// 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.
Transversal and transformation of the native AST can be performed through
TreeWalker and
TreeTransformer
respectively.
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.
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.
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 });
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.
To allow for better optimizations, the compiler makes various assumptions:
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.Function.prototype.toString() or
Error.prototype.stack to be anything in particular..watch() or Proxy).Object.defineProperty(), Object.defineProperties(), Object.freeze(),
Object.preventExtensions() or Object.seal()).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);
SyntaxError with the following:
({
p: 42,
get p() {},
});
// SyntaxError: Object literal may not have data and accessor property with
// the same name
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);
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.ReferenceError with the
following:
var a;
try {
throw 42;
} catch ({
[a]: b,
// ReferenceError: a is not defined
}) {
let a;
}
SyntaxError with the following:
a => {
let a;
};
// SyntaxError: Identifier 'a' has already been declared
SyntaxError with the following:
try {
// ...
} catch ({ message: a }) {
var a;
}
// SyntaxError: Identifier 'a' has already been declared
ReferenceError with the
following:
console.log(((a, b = function() {
return a;
// ReferenceError: a is not defined
}()) => b)());
BigInt may throw TypeError:
1n + 1;
// TypeError: can't convert BigInt to number
SyntaxError with the
following:
console.log(String.raw`\uFo`);
// SyntaxError: Invalid Unicode escape sequence
SyntaxError with the
following:
try {} catch (e) {
for (var e of []);
}
// SyntaxError: Identifier 'e' has already been declared
console.log({
...{
set 42(v) {},
42: "PASS",
},
});
// Expected: { '42': 'PASS' }
// Actual: { '42': undefined }
SyntaxError with the following:
var await;
class A {
static p = await;
}
// SyntaxError: Unexpected reserved word
SyntaxError with the following:
var async;
for (async of []);
// SyntaxError: The left-hand side of a for-of loop may not be 'async'.
console.log({
...console,
get 42() {
return "FAIL";
},
[42]: "PASS",
}[42], {
...console,
get 42() {
return "FAIL";
},
42: "PASS",
}[42]);
// Expected: "PASS PASS"
// Actual: "PASS FAIL"
TypeError with the following:
(function() {
{
const a = "foo";
}
{
const a = "bar";
}
})();
// TypeError: const 'a' has already been declared
try {
class A {
static 42;
static get 42() {}
}
console.log("PASS");
} catch (e) {
console.log("FAIL");
}
// Expected: "PASS"
// Actual: "FAIL"
(async function(a) {
(function() {
var b = await => console.log("PASS");
b();
})();
})().catch(console.error);
// Expected: "PASS"
// Actual: SyntaxError: Unexpected reserved word
try {
f();
function f() {
throw 42;
}
} catch (e) {
console.log(typeof f, e);
}
// Expected: "function 42"
// Actual: "undefined 42"
SyntaxError with the following:
"use strict";
console.log(function f() {
return f = "PASS";
}());
// Expected: "PASS"
// Actual: TypeError: invalid assignment to const 'f'
FAQs
JavaScript parser, mangler/compressor and beautifier toolkit
We found that uglify-js demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 3 open source maintainers collaborating on the project.
Did you know?
Socket for GitHub automatically highlights issues in each pull request and monitors the health of all your open source dependencies. Discover the contents of your packages and block harmful activity before you install or update your dependencies.
Security News
Research
An advanced npm supply chain attack is leveraging Ethereum smart contracts for decentralized, persistent malware control, evading traditional defenses.
Security News
Research
Attackers are impersonating Sindre Sorhus on npm with a fake 'chalk-node' package containing a malicious backdoor to compromise developers' projects.
Security News
The npm package for the LottieFiles Player web component was hit with a supply chain attack after a software engineer's npmjs credentials were compromised.