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UglifyJS 3

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


  • uglify-js@3 has a simplified API and CLI that is not backwards compatible with uglify-js@2.
  • Documentation for UglifyJS 2.x releases can be found here.
  • uglify-js only supports JavaScript (ECMAScript 5).
  • To minify ECMAScript 2015 or above, transpile using tools like Babel.


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`.
                                `expression`  Parse a single expression, rather than
                                              a program (for parsing JSON).
                                `spidermonkey`  Assume input files are SpiderMonkey
                                                AST format (as JSON).
    -c, --compress [options]    Enable compressor/specify compressor options:
                                `pure_funcs`  List of functions that can be safely
                                              removed when their return values are
                                              not used.
    -m, --mangle [options]      Mangle names/specify mangler options:
                                `reserved`  List of names that should not be mangled.
    --mangle-props [options]    Mangle properties/specify mangler options:
                                `builtins`  Mangle property names that overlaps
                                            with standard JavaScript globals.
                                `debug`  Add debug prefix and suffix.
                                `domprops`  Mangle property names that overlaps
                                            with DOM properties.
                                `keep_quoted`  Only mangle unquoted properties.
                                `regex`  Only mangle matched property names.
                                `reserved`  List of names that should not be mangled.
    -b, --beautify [options]    Beautify output/specify output options:
                                `beautify`  Enabled with `--beautify` by default.
                                `preamble`  Preamble to prepend to the output. You
                                            can use this to insert a comment, for
                                            example for licensing information.
                                            This will not be parsed, but the source
                                            map will adjust for its presence.
                                `quote_style`  Quote style:
                                               0 - auto
                                               1 - single
                                               2 - double
                                               3 - original
                                `wrap_iife`  Wrap IIFEs in parenthesis. Note: you may
                                             want to disable `negate_iife` under
                                             compressor options.
    -o, --output <file>         Output file path (default STDOUT). Specify `ast` or
                                `spidermonkey` to write UglifyJS or SpiderMonkey AST
                                as JSON to STDOUT respectively.
    --comments [filter]         Preserve copyright comments in the output. By
                                default this works like Google Closure, keeping
                                JSDoc-style comments that contain "@license" or
                                "@preserve". You can optionally pass one of the
                                following arguments to this flag:
                                - "all" to keep all comments
                                - a valid JS RegExp like `/foo/` or `/^!/` to
                                keep only matching comments.
                                Note that currently not *all* comments can be
                                kept when compression is on, because of dead
                                code removal or cascading statements into
    --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).
    --ie8                       Support non-standard Internet Explorer 8.
                                Equivalent to setting `ie8: true` in `minify()`
                                for `compress`, `mangle` and `output` options.
                                By default UglifyJS will not try to be IE-proof.
    --keep-fnames               Do not mangle/drop function names.  Useful for
                                code relying on
    --name-cache <file>         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.
                                `root`  Path to the original source to be included in
                                        the source map.
                                `url`  If specified, path to the source map to append in
                                       `//# sourceMappingURL`.
    --timings                   Display operations run time on STDERR.
    --toplevel                  Compress and/or mangle variables in top level scope.
    --verbose                   Print diagnostic messages.
    --warn                      Print warning messages.
    --wrap <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

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 "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='',url=''"

The above will compress and mangle file1.js and file2.js, will drop the output in foo.min.js and the source map in The source mapping will refer to and (in fact it will list as the source map root, and the original files as js/file1.js and js/file2.js).

Composed source map

When you‘re compressing JS code that was output by a compiler such as CoffeeScript, mapping to the JS code won’t be too helpful. Instead, you'd like to map back to the original code (i.e. CoffeeScript). UglifyJS has an option to take an input source map. Assuming you have a mapping from CoffeeScript → compiled JS, UglifyJS can generate a map from CoffeeScript → compressed JS by mapping every token in the compiled JS to its original location.

To use this feature pass --source-map "content='/path/to/input/'" 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).


uglifyjs file.js -c toplevel,sequences=false

CLI mangle options

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

  • toplevel (default false) -- mangle names declared in the top level scope.

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

When mangling is enabled but you want to prevent certain names from being mangled, you can declare those names with --mangle reserved — pass a comma-separated list of names. For example:

uglifyjs ... -m reserved=['$','require','exports']

to prevent the require, exports and $ names from being changed.

CLI mangling property names (--mangle-props)

Note: THIS WILL PROBABLY BREAK YOUR CODE. Mangling property names is a separate step, different from variable name mangling. Pass --mangle-props to enable it. It will mangle all properties in the input code with the exception of built in DOM properties and properties in core JavaScript classes. For example:

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

Mangle all properties (except for JavaScript builtins):

$ uglifyjs example.js -c -m --mangle-props
var x={o:0,_:1,l:function(){return this._+this.o}};x.t=2,x.o=3,console.log(x.l());

Mangle all properties except for reserved properties:

$ uglifyjs example.js -c -m --mangle-props reserved=[foo_,bar_]
var x={o:0,foo_:1,_:function(){return this.foo_+this.o}};x.bar_=2,x.o=3,console.log(x._());

Mangle all properties matching a regex:

$ uglifyjs example.js -c -m --mangle-props regex=/_$/
var x={o:0,_:1,calc:function(){return this._+this.o}};x.l=2,x.o=3,console.log(x.calc());

Combining mangle properties options:

$ uglifyjs example.js -c -m --mangle-props regex=/_$/,reserved=[bar_]
var x={o:0,_:1,calc:function(){return this._+this.o}};x.bar_=2,x.o=3,console.log(x.calc());

In order for this to be of any use, we avoid mangling standard JS names by default (--mangle-props builtins to override).

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

A regular expression can be used to define which property names should be mangled. For example, --mangle-props regex=/^_/ will only mangle property names that start with an underscore.

When you compress multiple files using this option, in order for them to work together in the end we need to ensure somehow that one property gets mangled to the same name in all of them. For this, pass --name-cache filename.json and UglifyJS will maintain these mappings in a file which can then be reused. It should be initially empty. Example:

$ rm -f /tmp/cache.json  # start fresh
$ uglifyjs file1.js file2.js --mangle-props --name-cache /tmp/cache.json -o part1.js
$ uglifyjs file3.js file4.js --mangle-props --name-cache /tmp/cache.json -o part2.js

Now, part1.js and part2.js will be consistent with each other in terms of mangled property names.

Using the name cache is not necessary if you compress all your files in a single call to UglifyJS.

Mangling unquoted names (--mangle-props keep_quoted)

Using quoted property name (o["foo"]) reserves the property name (foo) so that it is not mangled throughout the entire script even when used in an unquoted style ( Example:

// stuff.js
var o = {
    "foo": 1,
    bar: 3
}; +=;
$ uglifyjs stuff.js --mangle-props keep_quoted -c -m
var o={foo:1,o:3};,console.log(;

Debugging property name mangling

You can also pass --mangle-props debug in order to mangle property names without completely obscuring them. For example the property would mangle to o._$foo$_ with this option. This allows property mangling of a large codebase while still being able to debug the code and identify where mangling is breaking things.

$ 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 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);
// 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(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);
// function n(n,r){return n+r}
// 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);
// /* 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);"});
// {"message":"Unexpected token: keyword (else)","filename":"foo.js","line":1,"col":7,"pos":7}

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

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

Minify options

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

  • ie8 (default false) -- set to true to support IE8.

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

  • mangle (default true) — pass false to skip mangling names, or pass an object to specify mangle options (see below).

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

  • output (default null) — pass an object if you wish to specify additional output options. 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.

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

Minify options structure

    parse: {
        // parse options
    compress: {
        // compress options
    mangle: {
        // mangle options

        properties: {
            // mangle property options
    output: {
        // output options
    sourceMap: {
        // source map options
    nameCache: null, // or specify a name cache object
    toplevel: false,
    ie8: false,
    warnings: false,

Source map options

To generate a source map:

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

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

You can set option sourceMap.url to be "inline" and source map will be appended to code.

You can also specify sourceRoot property to be included in source map:

var result = UglifyJS.minify({"file1.js": "var a = function() {};"}, {
    sourceMap: {
        root: "",
        url: ""

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

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

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

Parse options

  • bare_returns (default false) -- support top level return statements

  • html5_comments (default true)

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

Compress options

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

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

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

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

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

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

  • dead_code (default: true) -- remove unreachable code

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

  • drop_console (default: false) -- Pass true to discard calls to console.* functions. If you wish to drop a specific function call such as 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 varto function whenever possible.

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

  • hoist_funs (default: false) -- hoist function declarations

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

  • hoist_vars (default: false) -- hoist var declarations (this is false by default because it seems to increase the size of the output in general)

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

  • inline (default: true) -- inline calls to function with simple/return statement:

    • false -- same as 0
    • 0 -- disabled inlining
    • 1 -- inline simple functions
    • 2 -- inline functions with arguments
    • 3 -- inline functions with arguments and variables
    • true -- same as 3
  • join_vars (default: true) -- join consecutive var statements

  • keep_fargs (default: strict) -- Discard unused function arguments. Code which relies on Function.length will break if this is done indiscriminately, i.e. when passing true. Pass false to always retain function arguments.

  • keep_fnames (default: false) -- Pass true to prevent the compressor from discarding function names. Useful for code relying on See also: the keep_fnames mangle option.

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

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

  • negate_iife (default: true) -- negate “Immediately-Called Function Expressions” where the return value is discarded, to avoid the parens that the code generator would insert.

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

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

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

  • 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. or foo["bar"]) doesn't have any side effects. Specify "strict" to treat as side-effect-free only when foo is certain to not throw, i.e. not null or undefined.

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

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

  • sequences (default: true) -- join consecutive simple statements using the comma operator. May be set to a positive integer to specify the maximum number of consecutive comma sequences that will be generated. If this option is set to true then the default sequences limit is 200. Set option to false or 0 to disable. The smallest sequences length is 2. A sequences value of 1 is grandfathered to be equivalent to true and as such means 200. On rare occasions the default sequences limit leads to very slow compress times in which case a value of 20 or less is recommended.

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

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

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

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

  • typeofs (default: true) -- Transforms typeof foo == "undefined" into foo === void 0. Note: recommend to set this value to false for IE10 and earlier versions due to known issues.

  • unsafe (default: false) -- apply “unsafe” transformations (discussion below)

  • unsafe_comps (default: false) -- compress expressions like a <= b assuming none of the operands can be (coerced to) NaN.

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

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

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

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

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

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

Mangle options

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

  • keep_fnames (default false) -- Pass true to not mangle function names. Useful for code relying on See also: the keep_fnames compress option.

  • 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.


// test.js
var globalVar;
function funcName(firstLongName, anotherLongName) {
    var myVariable = firstLongName +  anotherLongName;
var code = fs.readFileSync("test.js", "utf8");

// 'function funcName(a,n){}var globalVar;'

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

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

Mangle properties options

  • builtins (default: false) -- Use true to allow the mangling of builtin DOM properties. Not recommended to override this setting.

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

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

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

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

Output options

The code generator tries to output shortest code possible by default. In case you want beautified output, pass --beautify (-b). Optionally you can pass additional arguments that control the code output:

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

  • beautify (default true) -- whether to actually beautify the output. Passing -b will set this to true, but you might need to pass -b even when you want to generate minified code, in order to specify additional arguments, so you can use -b beautify=false to override it.

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

  • comments (default false) -- pass true or "all" to preserve all comments, "some" to preserve some comments, a regular expression string (e.g. /^!/) or a function.

  • indent_level (default 4)

  • indent_start (default 0) -- prefix all lines by that many spaces

  • inline_script (default true) -- escape HTML comments and the slash in occurrences of </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 quotes
    • 2 -- always use double quotes
    • 3 -- always use the original quotes
  • semicolons (default true) -- separate statements with semicolons. If you pass false then whenever possible we will use a newline instead of a semicolon, leading to more readable output of uglified code (size before gzip could be smaller; size after gzip insignificantly larger).

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

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

  • width (default 80) -- only takes effect when beautification is on, this specifies an (orientative) line width that the beautifier will try to obey. It refers to the width of the line text (excluding indentation). It doesn't work very well currently, but it does make the code generated by UglifyJS more readable.

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


Keeping copyright notices or other comments

You can pass --comments to retain certain comments in the output. By default it will keep JSDoc-style comments that contain “@preserve”, “@license” or “@cc_on” (conditional compilation for IE). You can pass --comments all to keep all the comments, or a valid JavaScript regexp to keep only comments that match this regexp. For example --comments /^!/ will keep comments like /*! Copyright Notice */.

Note, however, that there might be situations where comments are lost. For example:

function f() {
    /** @preserve Foo Bar */
    function g() {
        // this function is never called
    return something();

Even though it has “@preserve”, the comment will be lost because the inner function g (which is the AST node to which the comment is attached to) is discarded by the compressor as not referenced.

The safest comments where to place copyright information (or other info that needs to be kept in the output) are comments attached to toplevel nodes.

The unsafe compress option

It enables some transformations that might break code logic in certain contrived cases, but should be fine for most code. You might want to try it on your own code, it should reduce the minified size. Here's what happens when this flag is on:

  • new Array(1, 2, 3) or Array(1, 2, 3)[ 1, 2, 3 ]
  • new Object(){}
  • String(exp) or exp.toString()"" + exp
  • new Object/RegExp/Function/Error/Array (...) → we discard the new

Conditional compilation

You can use the --define (-d) switch in order to declare global variables that UglifyJS will assume to be constants (unless defined in scope). For example if you pass --define DEBUG=false then, coupled with dead code removal UglifyJS will discard the following from the output:

if (DEBUG) {
    console.log("debug stuff");

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

UglifyJS will warn about the condition being always false and about dropping unreachable code; for now there is no option to turn off only this specific warning, you can pass warnings=false to turn off all warnings.

Another way of doing that is to declare your globals as constants in a separate file and include it into the build. For example you can have a build/defines.js file with the following:

var DEBUG = false;
var PRODUCTION = true;
// etc.

and build your code like this:

uglifyjs build/defines.js js/foo.js js/bar.js... -c

UglifyJS will notice the constants and, since they cannot be altered, it will evaluate references to them to the value itself and drop unreachable code as usual. The build will contain the const declarations if you use them. If you are targeting < ES6 environments which does not support const, using var with reduce_vars (enabled by default) should suffice.

Conditional compilation API

You can also use conditional compilation via the programmatic API. With the difference that the property name is global_defs and is a compressor property:

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

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

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

Otherwise it would be replaced as string literal:

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

Using native Uglify AST with minify()

// example: parse only, produce native Uglify AST

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

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

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

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

Working with Uglify AST

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

ESTree / SpiderMonkey AST

UglifyJS has its own abstract syntax tree format; for practical reasons we can't easily change to using the SpiderMonkey AST internally. However, UglifyJS now has a converter which can import a SpiderMonkey AST.

For example Acorn is a super-fast parser that produces a SpiderMonkey AST. It has a small CLI utility that parses one file and dumps the AST in JSON on the standard output. To use UglifyJS to mangle and compress that:

acorn file.js | uglifyjs -p spidermonkey -m -c

The -p spidermonkey option tells UglifyJS that all input files are not JavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore we don't use our own parser in this case, but just transform that AST into our internal AST.

Use Acorn for parsing

More for fun, I added the -p acorn option which will use Acorn to do all the parsing. If you pass this option, UglifyJS will require("acorn").

Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), but converting the SpiderMonkey tree that Acorn produces takes another 150ms so in total it‘s a bit more than just using UglifyJS’s own parser.

Uglify Fast Minify Mode

It's not well known, but whitespace removal and symbol mangling accounts for 95% of the size reduction in minified code for most JavaScript - not elaborate code transforms. One can simply disable compress to speed up Uglify builds by 3 to 4 times. In this fast mangle-only mode Uglify has comparable minify speeds and gzip sizes to butternut:

d3.jsminify sizegzip sizeminify time (seconds)
uglify-js@3.0.24 mangle=false, compress=false316,60085,2450.70
uglify-js@3.0.24 mangle=true, compress=false220,21672,7301.13
uglify-js@3.0.24 mangle=true, compress=true212,51171,5603.36

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:

  • .toString() and .valueOf() don't have side effects, and for built-in objects they have not been overridden.
  • undefined, NaN and Infinity have not been externally redefined.
  • arguments.callee, arguments.caller and Function.prototype.caller are not used.
  • The code doesn't expect the contents of Function.prototype.toString() or Error.prototype.stack to be anything in particular.
  • Getting and setting properties on a plain object does not cause other side effects (using .watch() or Proxy).
  • Object properties can be added, removed and modified (not prevented with Object.defineProperty(), Object.defineProperties(), Object.freeze(), Object.preventExtensions() or Object.seal()).