uglify-js

What is uglify-js?

The uglify-js npm package is a JavaScript parser, minifier, compressor, and beautifier toolkit. It is commonly used to reduce the size of JavaScript files by removing unnecessary characters (like whitespace, comments, etc.) without changing their functionality. This helps in decreasing the load time and bandwidth usage of web applications.

What are uglify-js's main functionalities?

Minification

Minification is the process of removing unnecessary characters and whitespace from source code without changing its functionality. The code sample demonstrates how to read a JavaScript file, minify it using uglify-js, and then write the minified code to a new file.

const uglifyJS = require('uglify-js');
const fs = require('fs');

const code = fs.readFileSync('example.js', 'utf8');
const minifiedCode = uglifyJS.minify(code).code;

fs.writeFileSync('example.min.js', minifiedCode);

Compression

Compression is a form of optimization that reduces code size by performing various transformations, such as removing dead code or simplifying expressions. The code sample shows how to use uglify-js to compress a JavaScript file by removing console statements and then writing the compressed code to a new file.

const uglifyJS = require('uglify-js');
const fs = require('fs');

const options = {
  compress: {
    drop_console: true
  }
};

const code = fs.readFileSync('example.js', 'utf8');
const compressedCode = uglifyJS.minify(code, options).code;

fs.writeFileSync('example.compressed.js', compressedCode);

Mangling

Mangling is the process of shortening variable and function names to reduce file size and potentially obfuscate code. The code sample illustrates how to use uglify-js to mangle a JavaScript file and then save the mangled code to a new file.

const uglifyJS = require('uglify-js');
const fs = require('fs');

const options = {
  mangle: true
};

const code = fs.readFileSync('example.js', 'utf8');
const mangledCode = uglifyJS.minify(code, options).code;

fs.writeFileSync('example.mangled.js', mangledCode);

Beautification

Beautification is the process of reformatting minified or obfuscated code into a more readable and maintainable format. The code sample demonstrates how to use uglify-js to beautify a JavaScript file and then write the beautified code to a new file.

const uglifyJS = require('uglify-js');
const fs = require('fs');

const options = {
  output: {
    beautify: true
  }
};

const code = fs.readFileSync('example.js', 'utf8');
const beautifiedCode = uglifyJS.minify(code, options).code;

fs.writeFileSync('example.beautified.js', beautifiedCode);

Other packages similar to uglify-js

terser

Terser is a modern fork of uglify-js that supports the ES6+ syntax. It offers similar functionalities such as minification, compression, and mangling. Terser is often preferred for projects using modern JavaScript syntax.

google-closure-compiler

Google Closure Compiler is a JavaScript optimizing compiler that provides advanced optimizations including dead code removal, function inlining, and variable renaming. It is known for its aggressive optimization techniques and can produce smaller output files compared to uglify-js, but it may require more configuration.

babel-minify

Babel-minify is a minifier based on the Babel toolchain. It leverages Babel's understanding of the latest JavaScript syntax to provide minification and compression. It is suitable for projects that are already using Babel and need to support newer JavaScript features.

esbuild

Esbuild is an extremely fast JavaScript bundler and minifier. It is written in Go and focuses on speed, boasting compile times significantly faster than other bundlers and minifiers. Esbuild also supports modern JavaScript and TypeScript out of the box.

README

UglifyJS 3

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

Note:

• uglify-js supports JavaScript and most language features in ECMAScript.
• For more exotic parts of ECMAScript, process your source file with transpilers like Babel before passing onto uglify-js.
• uglify-js@3 has a simplified API and CLI that is not backwards compatible with uglify-js@2.

Install

First make sure you have installed the latest version of node.js (You may need to restart your computer after this step).

From NPM for use as a command line app:

npm install uglify-js -g

From NPM for programmatic use:

npm install uglify-js

Command line usage

uglifyjs [input files] [options]

UglifyJS can take multiple input files. It's recommended that you pass the input files first, then pass the options. UglifyJS will parse input files in sequence and apply any compression options. The files are parsed in the same global scope, that is, a reference from a file to some variable/function declared in another file will be matched properly.

If no input file is specified, UglifyJS will read from STDIN.

If you wish to pass your options before the input files, separate the two with a double dash to prevent input files being used as option arguments:

uglifyjs --compress --mangle -- input.js

Command line options

    -h, --help                  Print usage information.
                                `--help options` for details on available options.
    -V, --version               Print version number.
    -p, --parse <options>       Specify parser options:
                                `acorn`  Use Acorn for parsing.
                                `bare_returns`  Allow return outside of functions.
                                                Useful when minifying CommonJS
                                                modules and Userscripts that may
                                                be anonymous function wrapped (IIFE)
                                                by the .user.js engine `caller`.
                                `spidermonkey`  Assume input files are SpiderMonkey
                                                AST format (as JSON).
    -c, --compress [options]    Enable compressor/specify compressor options:
                                `pure_funcs`  List of functions that can be safely
                                              removed when their return values are
                                              not used.
    -m, --mangle [options]      Mangle names/specify mangler options:
                                `reserved`  List of names that should not be mangled.
    --mangle-props [options]    Mangle properties/specify mangler options:
                                `builtins`  Mangle property names that overlaps
                                            with standard JavaScript globals.
                                `debug`  Add debug prefix and suffix.
                                `domprops`  Mangle property names that overlaps
                                            with DOM properties.
                                `keep_quoted`  Only mangle unquoted properties.
                                `regex`  Only mangle matched property names.
                                `reserved`  List of names that should not be mangled.
    -b, --beautify [options]    Beautify output/specify output options:
                                `beautify`  Enabled with `--beautify` by default.
                                `preamble`  Preamble to prepend to the output. You
                                            can use this to insert a comment, for
                                            example for licensing information.
                                            This will not be parsed, but the source
                                            map will adjust for its presence.
                                `quote_style`  Quote style:
                                               0 - auto
                                               1 - single
                                               2 - double
                                               3 - original
                                `wrap_iife`  Wrap IIFEs in parentheses. Note: you may
                                             want to disable `negate_iife` under
                                             compressor options.
    -O, --output-opts [options] Specify output options (`beautify` disabled by default).
    -o, --output <file>         Output file path (default STDOUT). Specify `ast` or
                                `spidermonkey` to write UglifyJS or SpiderMonkey AST
                                as JSON to STDOUT respectively.
    --annotations               Process and preserve comment annotations.
                                (`/*@__PURE__*/` or `/*#__PURE__*/`)
    --no-annotations            Ignore and discard comment annotations.
    --comments [filter]         Preserve copyright comments in the output. By
                                default this works like Google Closure, keeping
                                JSDoc-style comments that contain "@license" or
                                "@preserve". You can optionally pass one of the
                                following arguments to this flag:
                                - "all" to keep all comments
                                - a valid JS RegExp like `/foo/` or `/^!/` to
                                keep only matching comments.
                                Note that currently not *all* comments can be
                                kept when compression is on, because of dead
                                code removal or cascading statements into
                                sequences.
    --config-file <file>        Read `minify()` options from JSON file.
    -d, --define <expr>[=value] Global definitions.
    -e, --enclose [arg[:value]] Embed everything in a big function, with configurable
                                argument(s) & value(s).
    --expression                Parse a single expression, rather than a program
                                (for parsing JSON).
    --ie                        Support non-standard Internet Explorer.
                                Equivalent to setting `ie: true` in `minify()`
                                for `compress`, `mangle` and `output` options.
                                By default UglifyJS will not try to be IE-proof.
    --keep-fargs                Do not mangle/drop function arguments.
    --keep-fnames               Do not mangle/drop function names.  Useful for
                                code relying on Function.prototype.name.
    --module                    Process input as ES module (implies --toplevel)
    --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 output goes to STDOUT.

CLI source map options

UglifyJS can generate a source map file, which is highly useful for debugging your compressed JavaScript. To get a source map, pass --source-map --output output.js (source map will be written out to output.js.map).

Additional options:

• --source-map "filename='<NAME>'" to specify the name of the source map. The value of filename is only used to set file attribute (see the spec) in source map file.

• --source-map "root='<URL>'" to pass the URL where the original files can be found.

• --source-map "names=false" to omit symbol names if you want to reduce size of the source map file.

• --source-map "url='<URL>'" to specify the URL where the source map can be found. Otherwise UglifyJS assumes HTTP X-SourceMap is being used and will omit the //# sourceMappingURL= directive.

For example:

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

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

Composed source map

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

To use this feature pass --source-map "content='/path/to/input/source.map'" or --source-map "content=inline" if the source map is included inline with the sources.

CLI compress options

You need to pass --compress (-c) to enable the compressor. Optionally you can pass a comma-separated list of compress options.

Options are in the form foo=bar, or just foo (the latter implies a boolean option that you want to set true; it's effectively a shortcut for foo=true).

Example:

uglifyjs file.js -c toplevel,sequences=false

CLI mangle options

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

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

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

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

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

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;
console.log(x.calc());

Mangle all properties (except for JavaScript builtins):

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

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

Mangle all properties except for reserved properties:

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

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

Mangle all properties matching a regex:

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

var x={o:0,_:1,calc:function(){return this._+this.o}};x.l=2,x.o=3,console.log(x.calc());

Combining mangle properties options:

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

var x={o:0,_:1,calc:function(){return this._+this.o}};x.bar_=2,x.o=3,console.log(x.calc());

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

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

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

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

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

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

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

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 (o.foo). Example:

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

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

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

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

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

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

Debugging property name mangling

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

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

var o={_$foo$_:1,_$bar$_:3};o._$foo$_+=o._$bar$_,console.log(o._$foo$_);

You can also pass a custom suffix using --mangle-props debug=XYZ. This would then mangle o.foo to o._$foo$XYZ_. You can change this each time you compile a script to identify how a property got mangled. One technique is to pass a random number on every compile to simulate mangling changing with different inputs (e.g. as you update the input script with new properties), and to help identify mistakes like writing mangled keys to storage.

API Reference

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

var UglifyJS = require("uglify-js");

There is a single high level function, minify(code, options), which will perform all minification phases in a configurable manner. By default minify() will enable the options compress and mangle. Example:

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

You can minify more than one JavaScript file at a time by using an object for the first argument where the keys are file names and the values are source code:

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

The toplevel option:

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

The nameCache option:

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

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

var cacheFileName = "/tmp/cache.json";
var options = {
    mangle: {
        properties: true,
    },
    nameCache: JSON.parse(fs.readFileSync(cacheFileName, "utf8"))
};
fs.writeFileSync("part1.js", UglifyJS.minify({
    "file1.js": fs.readFileSync("file1.js", "utf8"),
    "file2.js": fs.readFileSync("file2.js", "utf8")
}, options).code, "utf8");
fs.writeFileSync("part2.js", UglifyJS.minify({
    "file3.js": fs.readFileSync("file3.js", "utf8"),
    "file4.js": fs.readFileSync("file4.js", "utf8")
}, options).code, "utf8");
fs.writeFileSync(cacheFileName, JSON.stringify(options.nameCache), "utf8");

An example of a combination of minify() options:

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

To produce warnings:

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

An error example:

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

Note: unlike 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

• annotations — pass false to ignore all comment annotations and elide them from output. Useful when, for instance, external tools incorrectly applied /*@__PURE__*/ or /*#__PURE__*/. Pass true to both compress and retain comment annotations in output to allow for further processing downstream.

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

• expression (default: false) — parse as a single expression, e.g. JSON.

• ie (default: false) — enable workarounds for Internet Explorer bugs.

• keep_fargs (default: false) — pass true to prevent discarding or mangling of function arguments.

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

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

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

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

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

• output (default: null) — pass an object if you wish to specify additional output options. The defaults are optimized for best compression.

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

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

• toplevel (default: false) — set to true if you wish to enable top level variable and function name mangling and to drop unused variables and functions.

• v8 (default: false) — enable workarounds for Chrome & Node.js bugs.

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

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

Minify options structure

{
    parse: {
        // parse options
    },
    compress: {
        // compress options
    },
    mangle: {
        // mangle options

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

Source map options

To generate a source map:

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

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

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

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

var result = UglifyJS.minify({"file1.js": "var a = function() {};"}, {
    sourceMap: {
        root: "http://example.com/src",
        url: "out.js.map"
    }
});

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

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

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

If you wish to reduce file size of the source map, set option sourceMap.names to be false and all symbol names will be omitted.

Parse options

• bare_returns (default: false) — support top level return statements

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

• module (default: false) — set to true if you wish to process input as ES module, i.e. implicit "use strict"; and support for top-level await.

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

Compress options

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

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

• arrows (default: true) — apply optimizations to arrow functions

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

• awaits (default: true) — apply optimizations to await expressions

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

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

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

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

• dead_code (default: true) — remove unreachable code

• default_values (default: true) — drop overshadowed default values

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

• drop_console (default: false) — Pass true to discard calls to console.* functions. If you wish to drop a specific function call such as console.info and/or retain side effects from function arguments after dropping the function call then use pure_funcs instead.

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

• evaluate (default: true) — Evaluate expression for shorter constant representation. Pass "eager" to always replace function calls whenever possible, or a positive integer to specify an upper bound for each individual evaluation in number of characters.

• expression (default: false) — Pass true to preserve completion values from terminal statements without return, e.g. in bookmarklets.

• functions (default: true) — convert declarations from var to function whenever possible.

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

• hoist_exports (default: true) — hoist export statements to facilitate various compress and mangle optimizations.

• hoist_funs (default: false) — hoist function declarations

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

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

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

• imports (default: true) — drop unreferenced import symbols when used with unused

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

  • false — same as 0
  • 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 as 4

• join_vars (default: true) — join consecutive var statements

• keep_fargs (default: false) — discard unused function arguments except when unsafe to do so, e.g. code which relies on Function.prototype.length. Pass true to always retain function arguments.

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

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

• merge_vars (default: true) — combine and reuse variables.

• module (default: false) — set to true if you wish to process input as ES module, i.e. implicit "use strict";.

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

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

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

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

• pure_funcs (default: null) — You can pass an array of names and UglifyJS will assume that those functions do not produce side effects. DANGER: will not check if the name is redefined in scope. An example case here, for instance var q = Math.floor(a/b). If variable q is not used elsewhere, UglifyJS will drop it, but will still keep the Math.floor(a/b), not knowing what it does. You can pass pure_funcs: [ 'Math.floor' ] to let it know that this function won't produce any side effect, in which case the whole statement would get discarded. The current implementation adds some overhead (compression will be slower). Make sure symbols under pure_funcs are also under mangle.reserved to avoid mangling.

• pure_getters (default: "strict") — Pass true for UglifyJS to assume that object property access (e.g. foo.bar or a[42]) does not throw exception or alter program states via getter function. Pass "strict" to allow dropping or reordering foo.bar only if foo is not null or undefined and is safe to access as a variable. Pass false to retain all property accesses.

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

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

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

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

• side_effects (default: true) — drop extraneous code which does not affect outcome of runtime execution.

• spreads (default: true) — flatten spread expressions.

• strings (default: true) — compact string concatenations.

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

• templates (default: true) — compact template literals by embedding expressions and/or converting to string literals, e.g. `foo ${42}` → "foo 42"

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

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

• typeofs (default: true) — compress typeof expressions, e.g. typeof foo == "undefined" → void 0 === foo

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

• unsafe_comps (default: false) — assume operands cannot be (coerced to) NaN in numeric comparisons, e.g. a <= b. In addition, expressions involving in or instanceof would never throw.

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

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

• unsafe_proto (default: false) — optimize expressions like Array.prototype.slice.call(a) into [].slice.call(a)

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

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

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

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

• yields (default: true) — apply optimizations to yield expressions

Mangle options

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

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

• toplevel (default: false) — Pass true to mangle names declared in the top level scope.

Examples:

// test.js
var globalVar;
function funcName(firstLongName, anotherLongName) {
    var myVariable = firstLongName +  anotherLongName;
}

var code = fs.readFileSync("test.js", "utf8");

UglifyJS.minify(code).code;
// 'function funcName(a,n){}var globalVar;'

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

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

Mangle properties options

• builtins (default: false) — Use true to allow the mangling of built-in properties of JavaScript API. Not recommended to override this setting.

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

• domprops (default: false) — Use true to allow the mangling of properties commonly found in Document Object Model. Not recommended to override this setting.

• keep_fargs (default: false) — Use true to prevent mangling of function arguments.

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

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

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

Output options

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

• annotations (default: false) — pass true to retain comment annotations /*@__PURE__*/ or /*#__PURE__*/, otherwise they will be discarded even if comments is set.

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

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

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

• comments (default: false) — pass true or "all" to preserve all comments, "some" to preserve multi-line comments that contain @cc_on, @license, or @preserve (case-insensitive), a regular expression string (e.g. /^!/), or a function which returns boolean, e.g.

  function(node, comment) {
      return comment.value.indexOf("@type " + node.TYPE) >= 0;
  }
  

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

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

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

• indent_start (default: 0) — prefix all lines by whitespace sequence specified in the same format as indent_level.

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

• keep_quoted_props (default: false) — when turned on, prevents stripping quotes from property names in object literals.

• max_line_len (default: false) — maximum line length (for uglified code)

• preamble (default: null) — when passed it must be a string and it will be prepended to the output literally. The source map will adjust for this text. Can be used to insert a comment containing licensing information, for example.

• preserve_line (default: false) — pass true to retain line numbering on a best effort basis.

• quote_keys (default: false) — pass true to quote all keys in literal objects

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

  • 0 — prefers double quotes, switches to single quotes when there are more double quotes in the string itself. 0 is best for gzip size.
  • 1 — always use single 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)

• 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.

Miscellaneous

Keeping copyright notices or other comments

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

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

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

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

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

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"
        }
    }
}).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");'

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 5 times.

d3.js	minify size	gzip size	minify time (seconds)
original	511,371	119,932	-
uglify-js@3.13.0 mangle=false, compress=false	363,988	95,695	0.56
uglify-js@3.13.0 mangle=true, compress=false	253,305	81,281	0.99
uglify-js@3.13.0 mangle=true, compress=true	244,436	79,854	5.30

To enable fast minify mode from the CLI use:

uglifyjs file.js -m

To enable fast minify mode with the API use:

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

Source maps and debugging

Various compress transforms that simplify, rearrange, inline and remove code are known to have an adverse effect on debugging with source maps. This is expected as code is optimized and mappings are often simply not possible as some code no longer exists. For highest fidelity in source map debugging disable the Uglify compress option and just use mangle.

Compiler assumptions

To allow for better optimizations, the compiler makes various assumptions:

• The code does not rely on preserving its runtime performance characteristics. Typically uglified code will run faster due to less instructions and easier inlining, but may be slower on rare occasions for a specific platform, e.g. see reduce_funcs.
• .toString() and .valueOf() don't have side effects, and for built-in objects they have not been overridden.
• undefined, NaN 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()).
• If array destructuring is present, index-like properties in Array.prototype have not been overridden:

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

• Earlier versions of JavaScript will throw SyntaxError with the following:

  ({
      p: 42,
      get p() {},
  });
  // 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 later versions of Chrome and Node.js may be altered.
• When toplevel is enabled, UglifyJS effectively assumes input code is wrapped within function(){ ... }, thus forbids aliasing of declared global variables:

  A = "FAIL";
  var B = "FAIL";
  // can be `global`, `self`, `window` etc.
  var top = function() {
      return this;
  }();
  // "PASS"
  top.A = "PASS";
  console.log(A);
  // "FAIL" after compress and/or mangle
  top.B = "PASS";
  console.log(B);
  

• Use of arguments alongside destructuring as function parameters, e.g. function({}, arguments) {} will result in SyntaxError in earlier versions of Chrome and Node.js - UglifyJS may modify the input which in turn may suppress those errors.
• Earlier versions of Chrome and Node.js will throw ReferenceError with the following:

  var a;
  try {
      throw 42;
  } catch ({
      [a]: b,
      // ReferenceError: a is not defined
  }) {
      let a;
  }
  

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

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

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

  try {
      // ...
  } catch ({ message: a }) {
      var a;
  }
  // 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 throw ReferenceError with the following:

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

  UglifyJS may modify the input which in turn may suppress those errors.
• Some arithmetic operations with BigInt may throw TypeError:

  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 throw SyntaxError with the following:

  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 throw SyntaxError with the following:

  try {} catch (e) {
      for (var e of []);
  }
  // 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 the following:

  console.log({
      ...{
          set 42(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 throw SyntaxError with the following:

  var await;
  class A {
      static p = await;
  }
  // SyntaxError: Unexpected reserved word
  

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

  var async;
  for (async of []);
  // 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 the following:

  console.log({
      ...console,
      get 42() {
          return "FAIL";
      },
      [42]: "PASS",
  }[42], {
      ...console,
      get 42() {
          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 throw TypeError with the following:

  (function() {
      {
          const a = "foo";
      }
      {
          const a = "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 the following:

  try {
      class A {
          static 42;
          static get 42() {}
      }
      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 the following:

  (async function(a) {
      (function() {
          var b = 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 the following:

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

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

  "use strict";
  console.log(function f() {
      return f = "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.