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

• uglify-js supports JavaScript and most language features in ECMAScript.
• For more exotic parts of ECMAScript, process your source file with transpilerslikeBabel before passing ontouglify-js.
• uglify-js@3 has a simplifiedAPI andCLIthat is not backwards compatible withuglify-js@2.

First make sure you have installed the latest version ofnode.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 theinput files first, then pass the options. UglifyJS will parse input filesin sequence and apply any compression options. The files are parsed in thesame global scope, that is, a reference from a file to somevariable/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 witha 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.                                `globals`  Mangle variable access via global object.                                `keep_quoted`  Only mangle unquoted properties.                                `regex`  Only mangle matched property names.                                `reserved`  List of names that should not be mangled.    -b, --beautify [options]    Beautify output/specify output options:                                `beautify`  Enabled with `--beautify` by default.                                `preamble`  Preamble to prepend to the output. You                                            can use this to insert a comment, for                                            example for licensing information.                                            This will not be parsed, but the source                                            map will adjust for its presence.                                `quote_style`  Quote style:                                               0 - auto                                               1 - single                                               2 - double                                               3 - original                                `wrap_iife`  Wrap IIFEs in parentheses. Note: you may                                             want to disable `negate_iife` under                                             compressor options.    -O, --output-opts [options] Specify output options (`beautify` disabled by default).    -o, --output <file>         Output file path (default STDOUT). Specify `ast` or                                `spidermonkey` to write UglifyJS or SpiderMonkey AST                                as JSON to STDOUT respectively.    --annotations               Process and preserve comment annotations.                                (`/*@__PURE__*/` or `/*#__PURE__*/`)    --no-annotations            Ignore and discard comment annotations.    --comments [filter]         Preserve copyright comments in the output. By                                default this works like Google Closure, keeping                                JSDoc-style comments that contain "@license" or                                "@preserve". You can optionally pass one of the                                following arguments to this flag:                                - "all" to keep all comments                                - a valid JS RegExp like `/foo/` or `/^!/` to                                keep only matching comments.                                Note that currently not *all* comments can be                                kept when compression is on, because of dead                                code removal or cascading statements into                                sequences.    --config-file <file>        Read `minify()` options from JSON file.    -d, --define <expr>[=value] Global definitions.    -e, --enclose [arg[:value]] Embed everything in a big function, with configurable                                argument(s) & value(s).    --expression                Parse a single expression, rather than a program                                (for parsing JSON).    --ie                        Support non-standard Internet Explorer.                                Equivalent to setting `ie: true` in `minify()`                                for `compress`, `mangle` and `output` options.                                By default UglifyJS will not try to be IE-proof.    --keep-fargs                Do not mangle/drop function arguments.    --keep-fnames               Do not mangle/drop function names.  Useful for                                code relying on Function.prototype.name.    --module                    Process input as ES module (implies --toplevel)    --no-module                 Avoid optimizations which may alter runtime behavior                                under prior versions of JavaScript.    --name-cache <file>         File to hold mangled name mappings.    --self                      Build UglifyJS as a library (implies --wrap UglifyJS)    --source-map [options]      Enable source map/specify source map options:                                `base`  Path to compute relative paths from input files.                                `content`  Input source map, useful if you're compressing                                           JS that was generated from some other original                                           code. Specify "inline" if the source map is                                           included within the sources.                                `filename`  Filename and/or location of the output source                                            (sets `file` attribute in source map).                                `includeSources`  Pass this flag if you want to include                                                  the content of source files in the                                                  source map as sourcesContent property.                                `names` Include symbol names in the source map.                                `root`  Path to the original source to be included in                                        the source map.                                `url`  If specified, path to the source map to append in                                       `//# sourceMappingURL`.    --timings                   Display operations run time on STDERR.    --toplevel                  Compress and/or mangle variables in top level scope.    --v8                        Support non-standard Chrome & Node.js                                Equivalent to setting `v8: true` in `minify()`                                for `mangle` and `output` options.                                By default UglifyJS will not try to be v8-proof.    --verbose                   Print diagnostic messages.    --warn                      Print warning messages.    --webkit                    Support non-standard Safari/Webkit.                                Equivalent to setting `webkit: true` in `minify()`                                for `compress`, `mangle` and `output` options.                                By default UglifyJS will not try to be Safari-proof.    --wrap <name>               Embed everything in a big function, making the                                “exports” and “global” variables available. You                                need to pass an argument to this option to                                specify the name that your module will take                                when included in, say, a browser.

Specify--output (-o) to declare the output file. Otherwise the outputgoes to STDOUT.

UglifyJS can generate a source map file, which is highly useful fordebugging your compressed JavaScript. To get a source map, pass--source-map --output output.js (source map will be written out tooutput.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 (seethe 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 sizeof 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 manglefile1.js andfile2.js, will drop theoutput infoo.min.js and the source map infoo.min.js.map. The sourcemapping will refer tohttp://foo.com/src/js/file1.js andhttp://foo.com/src/js/file2.js (in fact it will listhttp://foo.com/srcas the source map root, and the original files asjs/file1.js andjs/file2.js).

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

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 withthe sources.

You need to pass--compress (-c) to enable the compressor. Optionallyyou can pass a comma-separated list ofcompress options.

Options are in the formfoo=bar, or justfoo (the latter impliesa boolean option that you want to settrue; it's effectively ashortcut forfoo=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 whereeval orwith are used.

• reserved (default:[]) — when mangling is enabled but you want toprevent 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 therequire,exports and$ names from being changed.

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

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

Mangle all properties (except for JavaScriptbuiltins):

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

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

Mangle all properties except forreserved properties:

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

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

Mangle all properties matching aregex:

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

varx={o:0,_:1,calc:function(){returnthis._+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_]

varx={o:0,_:1,calc:function(){returnthis._+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 bydefault (--mangle-props builtins to override).

Specify--mangle-props globals to mangle property names of globalobject (e.g.self.foo) as global variables.

A default exclusion file is provided intools/domprops.json which shouldcover 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 bemangled. For example,--mangle-props regex=/^_/ will only mangle propertynames that start with an underscore.

When you compress multiple files using this option, in order for them towork together in the end we need to ensure somehow that one property getsmangled to the same name in all of them. For this, pass--name-cache filename.jsonand 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 andpart2.js will be consistent with each other in termsof mangled property names.

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

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 anunquoted style (o.foo). Example:

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

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

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

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

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

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

You can also pass--mangle-props debug in order to mangle property nameswithout completely obscuring them. For example the propertyo.foowould mangle too._$foo$_ with this option. This allows property manglingof a large codebase while still being able to debug the code and identifywhere mangling is breaking things.

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

varo={_$foo$_: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 thenmangleo.foo too._$foo$XYZ_. You can change this each time you compile ascript to identify how a property got mangled. One technique is to pass arandom number on every compile to simulate mangling changing with differentinputs (e.g. as you update the input script with new properties), and to helpidentify mistakes like writing mangled keys to storage.

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

varUglifyJS=require("uglify-js");

There is a single high level function,minify(code, options),which will perform all minificationphases in a configurablemanner. By defaultminify() will enable the optionscompressandmangle. Example:

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

You canminify more than one JavaScript file at a time by using an objectfor the first argument where the keys are file names and the values are sourcecode:

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

Thetoplevel option:

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

ThenameCache option:

varoptions={mangle:{toplevel:true,},nameCache:{}};varresult1=UglifyJS.minify({"file1.js":"function add(first, second) { return first + second; }"},options);varresult2=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:

varcacheFileName="/tmp/cache.json";varoptions={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 ofminify() options:

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

To produce warnings:

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

An error example:

varresult=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: unlikeuglify-js@2.x, the3.x API does not throw errors. Toachieve a similar effect one could do the following:

varresult=UglifyJS.minify(code,options);if(result.error)throwresult.error;

• annotations — passfalse to ignore all comment annotations and elide themfrom output. Useful when, for instance, external tools incorrectly applied/*@__PURE__*/ or/*#__PURE__*/. Passtrue to both compress and retaincomment annotations in output to allow for further processing downstream.

• compress (default:{}) — passfalse to skip compressing entirely.Pass an object to specify customcompress 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 manglingof function arguments.

• keep_fnames (default:false) — passtrue to prevent discarding or manglingof function names. Useful for code relying onFunction.prototype.name.

• mangle (default:true) — passfalse to skip mangling names, or passan object to specifymangle options (see below).

  • mangle.properties (default:false) — a subcategory of the mangle option.Pass an object to specify custommangle property options.

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

• nameCache (default:null) — pass an empty object{} or a previouslyusednameCache object if you wish to cache mangled variable andproperty names across multiple invocations ofminify(). Note: this isa read/write property.minify() will read the name cache state of thisobject and update it during minification so that it may bereused or externally persisted by the user.

• output (default:null) — pass an object if you wish to specifyadditionaloutput options. The defaults are optimizedfor best compression.

• parse (default:{}) — pass an object if you wish to specify someadditionalparse options.

• sourceMap (default:false) — pass an object if you wish to specifysource map options.

• toplevel (default:false) — set totrue if you wish to enable top levelvariable 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 warningsinresult.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.

{parse:{// parse options},compress:{// compress options},mangle:{// mangle optionsproperties:{// mangle property options}},output:{// output options},sourceMap:{// source map options},nameCache:null,// or specify a name cache objecttoplevel:false,warnings:false,}

To generate a source map:

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

Note that the source map is not saved in a file, it's just returned inresult.map. The value passed forsourceMap.url is only used to set//# sourceMappingURL=out.js.map inresult.code. The value offilename is only used to setfile attribute (seethe spec)in source map file.

You can set optionsourceMap.url to be"inline" and source map willbe appended to code.

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

varresult=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, youcan usesourceMap.content:

varresult=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 theX-SourceMap header instead, you can just omitsourceMap.url.

If you wish to reduce file size of the source map, set optionsourceMap.namesto befalse and all symbol names will be omitted.

• bare_returns (default:false) — support top levelreturn statements

• html5_comments (default:true) — process HTML comment as workaround forbrowsers which do not recognize<script> tags

• module (default:false) — set totrue if you wish to process input asES module, i.e. implicit"use strict"; and support for top-levelawait.

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

• annotations (default:true) — Passfalse to disable potentially droppingfunctions marked as "pure". A function call is marked as "pure" if a commentannotation/*@__PURE__*/ or/*#__PURE__*/ immediately precedes the call. Forexample:/*@__PURE__*/foo();

• arguments (default:true) — replacearguments[index] with functionparameter 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 conditionalexpressions

• 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 callsuch asconsole.info and/or retain side effects from function argumentsafter dropping the function call then usepure_funcs instead.

• drop_debugger (default:true) — removedebugger; statements

• evaluate (default:true) — Evaluate expression for shorter constantrepresentation. Pass"eager" to always replace function calls wheneverpossible, or a positive integer to specify an upper bound for each individualevaluation in number of characters.

• expression (default:false) — Passtrue to preserve completion valuesfrom terminal statements withoutreturn, e.g. in bookmarklets.

• functions (default:true) — convert declarations fromvar tofunctionwhenever possible.

• global_defs (default:{}) — seeconditional compilation

• hoist_exports (default:true) — hoistexport statements to facilitatevariouscompress andmangle optimizations.

• hoist_funs (default:false) — hoist function declarations

• hoist_props (default:true) — hoist properties from constant object andarray 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_propsworks best withtoplevel andmangle enabled, alongside withcompress optionpasses set to2 or higher.

• hoist_vars (default:false) — hoistvar declarations (this isfalseby 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 inlining
  • 1 — inline simple functions
  • 2 — inline functions with arguments
  • 3 — inline functions with arguments and variables
  • 4 — inline functions with arguments, variables and statements
  • true — same as4

• join_vars (default:true) — join consecutivevar statements

• keep_fargs (default:false) — discard unused function arguments exceptwhen 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 frombeing compressed into1/0, which may cause performance issues on Chrome.

• loops (default:true) — optimizations fordo,while andfor loopswhen 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 asES module, i.e. implicit"use strict";.

• negate_iife (default:true) — negate "Immediately-Called Function Expressions"where the return value is discarded, to avoid the parentheses that thecode 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 inmind more passes will take more time.

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

• pure_funcs (default:null) — You can pass an array of names andUglifyJS will assume that those functions do not produce sideeffects. DANGER: will not check if the name is redefined in scope.An example case here, for instancevar q = Math.floor(a/b). Ifvariableq is not used elsewhere, UglifyJS will drop it, but willstill keep theMath.floor(a/b), not knowing what it does. You canpasspure_funcs: [ 'Math.floor' ] to let it know that thisfunction won't produce any side effect, in which case the wholestatement would get discarded. The current implementation adds someoverhead (compression will be slower). Make sure symbols underpure_funcsare also undermangle.reserved to avoid mangling.

• pure_getters (default:"strict") — Passtrue for UglifyJS to assume thatobject property access (e.g.foo.bar ora[42]) does not throw exception oralter program states via getter function. Pass"strict" to allow dropping orreorderingfoo.bar only iffoo is notnull orundefined and is safe toaccess as a variable. Passfalse to retain all property accesses.

• reduce_funcs (default:true) — Allows single-use functions to beinlined as function expressions when permissible allowing furtheroptimization. Enabled by default. Option depends onreduce_varsbeing enabled. Some code runs faster in the Chrome V8 engine if thisoption is disabled. Does not negatively impact other major browsers.

• reduce_vars (default:true) — Improve optimization on variables assigned with andused as constant values.

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

• sequences (default:true) — join consecutive simple statements using thecomma operator. May be set to a positive integer to specify the maximum numberof consecutive comma sequences that will be generated. If this option is set totrue then the defaultsequences limit is200. Set option tofalse or0to disable. The smallestsequences length is2. Asequences value of1is grandfathered to be equivalent totrue and as such means200. On rareoccasions the default sequences limit leads to very slow compress times in whichcase a value of20 or less is recommended.

• side_effects (default:true) — drop extraneous code which does not affectoutcome 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 expressionsand/or converting to string literals, e.g. `foo ${42}` → "foo 42"

• top_retain (default:null) — prevent specific toplevel functions andvariables fromunused removal (can be array, comma-separated, RegExp orfunction. Impliestoplevel)

• toplevel (default:false) — drop unreferenced functions ("funcs") and/orvariables ("vars") in the top level scope (false by default,true to dropboth 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)NaNin numeric comparisons, e.g.a <= b. In addition, expressions involvinginorinstanceof 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 avariable namedundefined in scope (variable name will be mangled, typicallyreduced to a single character)

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

• varify (default:true) — convert block-scoped declarations intovarwhenever safe to do so

• yields (default:true) — apply optimizations toyield expressions

• eval (default:false) — Passtrue to mangle names visible in scopeswhereeval orwith are used.

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

• toplevel (default:false) — Passtrue to mangle names declared in thetop level scope.

Examples:

// test.jsvarglobalVar;functionfuncName(firstLongName,anotherLongName){varmyVariable=firstLongName+anotherLongName;}

varcode=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) — Usetrue to allow the mangling of built-inproperties 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 propertiescommonly found in Document Object Model. Not recommended to override this setting.

• globals (default:false) — Usetrue to mangle properties of global objectalongside undeclared variables.

• keep_fargs (default:false) — Usetrue to prevent mangling of functionarguments.

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

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

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

The code generator tries to output shortest code possible by default. Incase you want beautified output, pass--beautify (-b). Optionally youcan 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 andregexps (affects directives with non-ascii characters becoming invalid)

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

• braces (default:false) — always insert braces inif,for,do,while orwith statements, even if their body is a singlestatement.

• comments (default:false) — passtrue or"all" to preserve allcomments,"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){returncomment.value.indexOf("@type "+node.TYPE)>=0;}

• extendscript (default:false) — enable workarounds for Adobe ExtendScriptbugs

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

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

• indent_start (default:0) — prefix all lines by whitespace sequencespecified in the same format asindent_level.

• inline_script (default:true) — escape HTML comments and the slash inoccurrences of</script> in strings

• keep_quoted_props (default:false) — when turned on, prevents strippingquotes 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 andit will be prepended to the output literally. The source map willadjust for this text. Can be used to insert a comment containinglicensing information, for example.

• preserve_line (default:false) — passtrue to retain line numbering ona best effort basis.

• quote_keys (default:false) — passtrue to quote all keys in literalobjects

• quote_style (default:0) — preferred quote style for strings (affectsquoted property names and directives as well):

  • 0 — prefers double quotes, switches to single quotes when there aremore 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. Ifyou passfalse then whenever possible we will use a newline instead of asemicolon, leading to more readable output of uglified code (size beforegzip 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, thisspecifies an (orientative) line width that the beautifier will try toobey. It refers to the width of the line text (excluding indentation).It doesn't work very well currently, but it does make the code generatedby UglifyJS more readable.

• wrap_iife (default:false) — passtrue to wrap immediately invokedfunction expressions. See#640 for more details.

You can pass--comments to retain certain comments in the output. Bydefault 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 tokeep 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. Forexample:

functionf(){/**@preserve Foo Bar */functiong(){// this function is never called}returnsomething();}

Even though it has "@preserve", the comment will be lost because the innerfunctiong (which is the AST node to which the comment is attached to) isdiscarded by the compressor as not referenced.

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

It enables some transformations thatmight break code logic in certaincontrived cases, but should be fine for most code. You might want to try iton your own code, it should reduce the minified size. Here's what happenswhen 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

You can use the--define (-d) switch in order to declare globalvariables that UglifyJS will assume to be constants (unless defined inscope). For example if you pass--define DEBUG=false then, coupled withdead 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 droppingunreachable code; for now there is no option to turn off only this specificwarning, you can passwarnings=false to turn offall warnings.

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

varDEBUG=false;varPRODUCTION=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, itwill evaluate references to them to the value itself and drop unreachablecode as usual. The build will contain theconst declarations if you usethem. If you are targeting < ES6 environments which does not supportconst,usingvar withreduce_vars (enabled by default) should suffice.

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

varresult=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 isnecessary to prefix theglobal_defs key with"@" to instruct UglifyJSto 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");'

// example: parse only, produce native Uglify ASTvarresult=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.varresult=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 throughTreeWalker andTreeTransformerrespectively.

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

For exampleAcorn is a super-fast parser that produces aSpiderMonkey AST. It has a small CLI utility that parses one file and dumpsthe AST in JSON on the standard output. To use UglifyJS to mangle andcompress that:

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

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

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

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

It's not well known, but whitespace removal and symbol mangling accountsfor 95% of the size reduction in minified code for most JavaScript - notelaborate code transforms. One can simply disablecompress to speed upUglify builds by 3 to 5 times.

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});

Variouscompress transforms that simplify, rearrange, inline and remove codeare known to have an adverse effect on debugging with source maps. This isexpected as code is optimized and mappings are often simply not possible assome code no longer exists. For highest fidelity in source map debuggingdisable the Uglifycompress option and just usemangle.

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 easierinlining, but may be slower on rare occasions for a specific platform, e.g.seereduce_funcs.
• .toString() and.valueOf() don't have side effects, and for built-inobjects 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 ofFunction.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 withObject.defineProperty(),Object.defineProperties(),Object.freeze(),Object.preventExtensions() orObject.seal()).
• If array destructuring is present, index-like properties inArray.prototypehave not been overridden:

  Object.prototype[0]=42;var[a]=[];var{0:b}={};// 42 undefinedconsole.log([][0],a);// 42 42console.log({}[0],b);

• Earlier versions of JavaScript will throwSyntaxError with the following:

  ({p:42,getp(){},});// SyntaxError: Object literal may not have data and accessor property with//              the same name

  UglifyJS may modify the input which in turn may suppress those errors.
• Iteration order of keys over an object which contains spread syntax in laterversions of Chrome and Node.js may be altered.
• Whentoplevel is enabled, UglifyJS effectively assumes input code is wrappedwithinfunction(){ ... }, thus forbids aliasing of declared global variables:

  A="FAIL";varB="FAIL";// can be `global`, `self`, `window` etc.vartop=function(){returnthis;}();// "PASS"top.A="PASS";console.log(A);// "FAIL" after compress and/or mangletop.B="PASS";console.log(B);

• Use ofarguments alongside destructuring as function parameters, e.g.function({}, arguments) {} will result inSyntaxError in earlier versionsof Chrome and Node.js - UglifyJS may modify the input which in turn maysuppress those errors.
• Earlier versions of Chrome and Node.js will throwReferenceError with thefollowing:

  vara;try{throw42;}catch({[a]:b,// ReferenceError: a is not defined}){leta;}

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of JavaScript will throwSyntaxError with the following:

  a=>{leta;};// SyntaxError: Identifier 'a' has already been declared

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of JavaScript will throwSyntaxError with the following:

  try{// ...}catch({message:a}){vara;}// SyntaxError: Identifier 'a' has already been declared

  UglifyJS may modify the input which in turn may suppress those errors.
• Some versions of Chrome and Node.js will throwReferenceError with thefollowing:

  console.log(((a,b=function(){returna;// ReferenceError: a is not defined}())=>b)());

  UglifyJS may modify the input which in turn may suppress those errors.
• Some arithmetic operations withBigInt may throwTypeError:

  1n+1;// TypeError: can't convert BigInt to number

  UglifyJS may modify the input which in turn may suppress those errors.
• Some versions of JavaScript will throwSyntaxError with thefollowing:

  console.log(String.raw`\uFo`);// SyntaxError: Invalid Unicode escape sequence

  UglifyJS may modify the input which in turn may suppress those errors.
• Some versions of JavaScript will throwSyntaxError with thefollowing:

  try{}catch(e){for(vareof[]);}// SyntaxError: Identifier 'e' has already been declared

  UglifyJS may modify the input which in turn may suppress those errors.
• Some versions of Chrome and Node.js will give incorrect results with thefollowing:

  console.log({    ...{set42(v){},42:"PASS",},});// Expected: { '42': 'PASS' }// Actual:   { '42': undefined }

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of JavaScript will throwSyntaxError with the following:

  varawait;classA{staticp=await;}// SyntaxError: Unexpected reserved word

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of JavaScript will throwSyntaxError with the following:

  varasync;for(asyncof[]);// SyntaxError: The left-hand side of a for-of loop may not be 'async'.

  UglifyJS may modify the input which in turn may suppress those errors.
• Some versions of Chrome and Node.js will give incorrect results with thefollowing:

  console.log({    ...console,get42(){return"FAIL";},[42]:"PASS",}[42],{    ...console,get42(){return"FAIL";},42:"PASS",}[42]);// Expected: "PASS PASS"// Actual:   "PASS FAIL"

  UglifyJS may modify the input which in turn may suppress those errors.
• Earlier versions of JavaScript will throwTypeError with the following:

  (function(){{consta="foo";}{consta="bar";}})();// TypeError: const 'a' has already been declared

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of Chrome and Node.js will give incorrect results with thefollowing:

  try{classA{static42;staticget42(){}}console.log("PASS");}catch(e){console.log("FAIL");}// Expected: "PASS"// Actual:   "FAIL"

  UglifyJS may modify the input which in turn may suppress those errors.
• Some versions of Chrome and Node.js will give incorrect results with thefollowing:

  (asyncfunction(a){(function(){varb=await=>console.log("PASS");b();})();})().catch(console.error);// Expected: "PASS"// Actual:   SyntaxError: Unexpected reserved word

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of Chrome and Node.js will give incorrect results with thefollowing:

  try{f();functionf(){throw42;}}catch(e){console.log(typeoff,e);}// Expected: "function 42"// Actual:   "undefined 42"

  UglifyJS may modify the input which in turn may suppress those errors.
• Later versions of JavaScript will throwSyntaxError with the following:

  "use strict";console.log(functionf(){returnf="PASS";}());// Expected: "PASS"// Actual:   TypeError: invalid assignment to const 'f'

  UglifyJS may modify the input which in turn may suppress those errors.
• Adobe ExtendScript will give incorrect results with the following:

  alert(true ?"PASS" :false ?"FAIL" :null);// Expected: "PASS"// Actual:   "FAIL"

  UglifyJS may modify the input which in turn may suppress those errors.
• Adobe ExtendScript will give incorrect results with the following:

  alert(42 ?null ?"FAIL" :"PASS" :"FAIL");// Expected: "PASS"// Actual:   SyntaxError: Expected: :

  UglifyJS may modify the input which in turn may suppress those errors.