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

$3

``
-h, --help Print usage information.
--help options for details on available options.
-V, --version Print version number.
-p, --parse 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 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 Read minify() options from JSON file.
-d, --define [=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 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 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=''" to specify the name of the source map. The value of
filename is only used to set file attribute (see [the spec][sm-spec])
in source map file.

- --source-map "root=''" 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=''" 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).

$3

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.

$3

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:

`javascript
// 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):
`bash
$ uglifyjs example.js -c -m --mangle-props
`
`javascript
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:
`bash
$ uglifyjs example.js -c -m --mangle-props reserved=[foo_,bar_]
`
`javascript
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:
`bash
$ uglifyjs example.js -c -m --mangle-props regex=/_$/
`
`javascript
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:
`bash
$ uglifyjs example.js -c -m --mangle-props regex=/_$/,reserved=[bar_]
`
`javascript
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:

`bash
$ 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.

$3

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:

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

`bash
$ uglifyjs stuff.js --mangle-props keep_quoted -c -m -O keep_quoted_props
`
`javascript
var o={"foo":1,o:3};o.foo+=o.o,console.log(o.foo);
`

$3

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.

`bash
$ uglifyjs stuff.js --mangle-props debug -c -m
`
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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

`javascript
{
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,
}
`

$3

To generate a source map:
`javascript
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][sm-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:
`javascript
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:
`javascript
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 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

$3

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:
`javascript
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.

$3

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

$3

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:
`javascript
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:
`javascript
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.

$3

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:

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

Otherwise it would be replaced as string literal:
`javascript
UglifyJS.minify("alert('hello');", {
compress: {
global_defs: {
"alert": "console.log"
}
}
}).code;
// returns: '"console.log"("hello");'
`

$3


`javascript
// 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
`
`javascript
// 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.
`

$3

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

$3

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][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.

$3

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.

[acorn]: https://github.com/ternjs/acorn
[sm-spec]: https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k

$3

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

$3

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.

$3

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:
`javascript
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:
`javascript
({
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
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:
`javascript
(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:
`javascript
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:
`javascript
(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:
`javascript
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:
`javascript
"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:
`javascript
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:
`javascript
alert(42 ? null ? "FAIL" : "PASS" : "FAIL");
// Expected: "PASS"
// Actual: SyntaxError: Expected: :
``
UglifyJS may modify the input which in turn may suppress those errors.