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UglifyJS 3

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


  • uglify-js@3has a simplified API and CLI that is not backwards compatible withuglify-js@2 .
  • Documentation for UglifyJS 2.xreleases can be found here .
  • uglify-jsonly 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 filesem, remos the s​​an s有al, spression options in files, sthr. 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

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-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 sequences. --config-file <file> Read `minify()` options from JSON file. -d, --define <expr>[=value] Global definitions. --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` Name and/or location of the output source. `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.

  • --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-SourceMapis being used and will omit the //# sourceMappingURL=directive.

For example:

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uglifyjs js/file1.js js/file2.js \ -o foo.min.js -c -m \ --source-map "root='',url=''"

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

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 evalor withare 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, exportsand $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-propsto enable it. It will mangle all properties in the input code with the exception of built in OM classes. For example:

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

Mangle all properties (except for JavaScript builtins):

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

Mangle all properties except for reservedproperties:

$ 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 builtinsto override).

A default exclusion file is provided in tools/domprops.jsonwhich should cover most standard JS and DOM properties defined in various browsers. Pass --mangle-props dompropsto 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--name-cache filename.json 現在file which can then be reused. It should be initially empty. Example:

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$ 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.jsand part2.jswill 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:

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// stuff.js var o = { "foo": 1, bar: 3 }; +=; console.log(;
$ 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 debugin 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 ablebreak to debugify the code and identthing code-is ident ident 電子郵件

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

You can also pass a custom suffix using --mangle-props debug=XYZ. This would then mangle o.footo 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 randk number on every mangled. One technique is to pass a random number number on every compiled mile; inputs (eg 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:

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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 minifymore 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:

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

The topleveloption:

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

The nameCacheoption:

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var options = { mangle: { toplevel: true, }, nameCache: {} }; var result1 = UglifyJS.minify({ "file1.js": "function add(first, second) { return first + second; }" }, options); var result2 = UglifyJS.minify({ "file2.js": "console.log(add(1 + 2, 3 + 4));" }, options); console.log(result1.code); // function n(n,r){return n+r} console.log(result2.code); // console.log(n(3,7));

You may persist the name cache to the file system in the following way:

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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:

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

To produce warnings:

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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:

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var result = UglifyJS.minify({"foo.js" : "if (0) else console.log(1);"}); console.log(JSON.stringify(result.error)); // {"message":"Unexpected token: keyword (else)","filename":"foo.js","line":1,"col":7,"pos":7}

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

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

Minify options

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

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

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

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

    • false) — a subcategory of the mangle option. Pass an object to specify custom mangle property options .
  • output(default null) — pass an object if you wish to specify additional output options . The defaults are optimized for best compression.

  • sourceMap(default false) - pass an object if you wish to specify source map options .

  • toplevel(default false) - set to trueif you wish to enable top level variable and function name mangling and to drop unused variables and functions.

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

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

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

Minify options structure

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{ 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:

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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.urlis only used to set //# sourceMappingURL=out.js.mapin result.code. The value of filenameis only used to set fileattribute (see the spec ) in source map file.

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

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

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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:

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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-SourceMapheader instead, you can just omit sourceMap.url.

Parse options

  • bare_returns(default false) -- support top level returnstatements

  • html5_comments(default true)

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

Compress options

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

  • 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, eg !(a <= b) → a > b, attempts to negate binary nodes, eg 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

  • drop_console(default: false) -- Pass trueto discard calls to console.*functions. If you wish to drop a specific function call such as console.infoand/or retain side effects from function arguments after dropping the function call then use pure_funcsinstead.

  • drop_debugger(default: true) -- remove debugger;statements

  • evaluate(default: true) -- attempt to evaluate constant expressions

  • expression(default: false) -- Pass trueto preserve completion values from terminal statements without return, eg in bookmarklets.

  • 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 mangleenabled, the compressoption passesset to 2or compressoption set and toplevelthed.

  • hoist_vars(default: false) -- hoist vardeclarations (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/ returnstatement:

    • false-- same as0
    • 0-- disabled inlining
    • 1-- inline simple functions
    • 2-- inline functions with arguments
    • 3-- inline functions with arguments and variables
    • true-- same as3
  • join_vars(default: true) -- join consecutive varstatements

  • keep_fargs(default: true) -- Prevents the compressor from discarding unused function arguments. You need this for code which relies on Function.length.

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

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

  • loops(default: true) -- optimizations for do, whileand forloops 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.

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

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

  • 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 exampleable for ex var q = Math.floor(a/b)p qnot 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 inich case the whole state. current implementation adds some overhead (compression will be slower). Make sure symbols under pure_funcs are also under mangle.reservedto avoid mangling.

  • pure_getters(default: "strict") -- If you pass truefor this, UglifyJS will assume that object property access (eg foo.baror foo["bar"]) doesn't have any side effects. Specify "strict"to treat foo.baras side-effect-free only noten foois cerie not isth nullor 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 v. negatively impact other major browsers.

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

  • sequencestrue_ true_ sequences_ 200_ to falseor 0 to disable. The smallest sequenceslength is 2. A sequencesvalue of 1 is grandfathered to be equivalent to trueand as such means 200. On rare occasions the default sequences limit leads to very slow com press times in whiched aase a times 20.

  • side_effects(default: true) -- Pass falseto 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 switchbranches

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

  • top_retain(default: null) -- prevent specific toplevel functions and variables from unusedremoval (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 falsefor IE10 and earlier versions due to known issues.

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

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

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

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

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

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

  • unsafe_undefined(default: false) -- substitute void 0if there is a variable named undefinedin 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")

  • warnings(default: false) -- display warnings when dropping unreachable code or unused declarations etc.

Mangle options

  • eval(default false) -- Pass trueto mangle names visible in scopes where evalor withare used.

  • keep_fnames(default false) -- Pass trueto 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 trueto mangle names declared in the top level scope.


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// test.js var globalVar; function funcName(firstLongName, anotherLongName) { var myVariable = firstLongName + anotherLongName; }
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var code = fs.readFileSync("test.js", "utf8"); UglifyJS.minify(code).code; // 'function funcName(a,n){}var globalVar;' UglifyJS.minify(code, { mangle: { reserved: ['firstLongName'] } }).code; // 'function funcName(firstLongName,a){}var globalVar;' UglifyJS.minify(code, { mangle: { toplevel: true } }).code; // 'function n(n,a){}var a;'

Mangle properties options

  • builtins(default: false) -- Use trueto 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 reservedarray.

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 -bwill set this to true, but you might need to pass -beven when you want to generate minified code, in order to specify additional arguments, so you can so so -b beautify=falseyouride .

  • braces(default false) -- always insert braces in if, for, do, whileor withstatements, even if their body is a single statement.

  • comments(default false) -- pass trueor "all"to preserve all comments, "some"to preserve some comments, a regular expression string (eg /^!/) 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 enslicing information, for example.

  • preserve_line(default false) -- pass trueto preserve lines, but it only works if beautifyis set to false.

  • quote_keys(default false) -- pass trueto 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. 0is best for gzip size.
    • 1-- always use single quotes
    • 2-- always use double quotes
    • 3-- always use the original quotes
  • semicolonstrue_ false_

  • 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 tr​​y to obey. It refers to the width of the line text (excluding indentation)。 currently, but it does make the code generated by UglifyJS more readable.

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


You can pass --commentsto 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 allto 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:

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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 compressoption

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。

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

Conditional compilation

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

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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=falseto 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.jsfile with the following:

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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 constdeclarations if you code as usual. The build will contain the declarations if you code as usual. The build will contain the declarations if you code. not support const, using varwith 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_defsand is a compressor property:

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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_defskey with "@"to instruct UglifyJS to parse the value as an expression:

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

Otherwise it would be replaced as string literal:

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

Using native Uglify AST withminify()

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// 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
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// 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 TreeWalkerand 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 spidermonkeyoption 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 AST.

Use Acorn for parsing

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

Acorn is really fast (eg 250ms instead of 380ms on some 650K code)

Uglify Fast Minify Mode

It's not well known compressmangleonly mode Uglify has comparable minify speeds and gzip sizes to butternut:

d3.js minify size gzip size minify time (seconds)
original 451,131 108,733 -
uglify-js@3.0.24 mangle=false, compress=false 316,600 85,245 0.70
uglify-js@3.0.24 mangle=true, compress=false 220,216 72,730 1.13
butternut@0.4.6 217,568 72,738 1.41
uglify-js@3.0.24 mangle=true, compress=true 212,511 71,560 3.36
babili@0.1.4 210,713 72,140 12.64

To enable fast minify mode from the CLI use:

uglifyjs file.js -m

To enable fast minify mode with the API use:

UglifyJS.minify(code, { compress: false, mangle: true });

Source maps and debugging

Various compresstransforms 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 ten s simplyt is simplyt oss. map debugging disable the Uglify compressoption 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, NaNand Infinityhave not been externally redefined.
  • arguments.callee, arguments.callerand Function.prototype.callerare not used.
  • The code doesn't expect the contents of Function.prototype.toString()or Error.prototype.stackto 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()).