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UglifyJS 2
==========
[](https://travis-ci.org/mishoo/UglifyJS2)
UglifyJS is a JavaScript parser, minifier, compressor or beautifier toolkit.
This page documents the command line utility. For
[API and internals documentation see my website](http://lisperator.net/uglifyjs/).
There's also an
[in-browser online demo](http://lisperator.net/uglifyjs/#demo) (for Firefox,
Chrome and probably Safari).
Install
-------
First make sure you have installed the latest version of [node.js](http://nodejs.org/)
(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
From Git:
git clone git://github.com/mishoo/UglifyJS2.git
cd UglifyJS2
npm link .
Usage
-----
uglifyjs [input files] [options]
UglifyJS2 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 you want to read from STDIN instead, pass a single dash instead of input
files.
The available options are:
```
--source-map Specify an output file where to generate source map.
[string]
--source-map-root The path to the original source to be included in the
source map. [string]
--source-map-url The path to the source map to be added in //#
sourceMappingURL. Defaults to the value passed with
--source-map. [string]
--in-source-map Input source map, useful if you're compressing JS that was
generated from some other original code.
--screw-ie8 Pass this flag if you don't care about full compliance
with Internet Explorer 6-8 quirks (by default UglifyJS
will try to be IE-proof). [boolean]
--expr Parse a single expression, rather than a program (for
parsing JSON) [boolean]
-p, --prefix Skip prefix for original filenames that appear in source
maps. For example -p 3 will drop 3 directories from file
names and ensure they are relative paths. You can also
specify -p relative, which will make UglifyJS figure out
itself the relative paths between original sources, the
source map and the output file. [string]
-o, --output Output file (default STDOUT).
-b, --beautify Beautify output/specify output options. [string]
-m, --mangle Mangle names/pass mangler options. [string]
-r, --reserved Reserved names to exclude from mangling.
-c, --compress Enable compressor/pass compressor options. Pass options
like -c hoist_vars=false,if_return=false. Use -c with no
argument to use the default compression options. [string]
-d, --define Global definitions [string]
-e, --enclose Embed everything in a big function, with a configurable
parameter/argument list. [string]
--comments 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 (needs to start with a slash) to keep
only comments that match.
Note that currently not *all* comments can be kept when
compression is on, because of dead code removal or
cascading statements into sequences. [string]
--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.
--stats Display operations run time on STDERR. [boolean]
--acorn Use Acorn for parsing. [boolean]
--spidermonkey Assume input files are SpiderMonkey AST format (as JSON).
[boolean]
--self Build itself (UglifyJS2) as a library (implies
--wrap=UglifyJS --export-all) [boolean]
--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.
[string]
--export-all Only used when --wrap, this tells UglifyJS to add code to
automatically export all globals. [boolean]
--lint Display some scope warnings [boolean]
-v, --verbose Verbose [boolean]
-V, --version Print version number and exit. [boolean]
```
Specify `--output` (`-o`) to declare the output file. Otherwise the output
goes to STDOUT.
## Source map options
UglifyJS2 can generate a source map file, which is highly useful for
debugging your compressed JavaScript. To get a source map, pass
`--source-map output.js.map` (full path to the file where you want the
source map dumped).
Additionally you might need `--source-map-root` to pass the URL where the
original files can be found. In case you are passing full paths to input
files to UglifyJS, you can use `--prefix` (`-p`) to specify the number of
directories to drop from the path prefix when declaring files in the source
map.
For example:
uglifyjs /home/doe/work/foo/src/js/file1.js \
/home/doe/work/foo/src/js/file2.js \
-o foo.min.js \
--source-map foo.min.js.map \
--source-map-root http://foo.com/src \
-p 5 -c -m
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 you need to pass `--in-source-map
/path/to/input/source.map`. Normally the input source map should also point
to the file containing the generated JS, so if that's correct you can omit
input files from the command line.
## Mangler options
To enable the mangler you need to pass `--mangle` (`-m`). The following
(comma-separated) options are supported:
- `sort` — to assign shorter names to most frequently used variables. This
saves a few hundred bytes on jQuery before gzip, but the output is
_bigger_ after gzip (and seems to happen for other libraries I tried it
on) therefore it's not enabled by default.
- `toplevel` — mangle names declared in the toplevel scope (disabled by
default).
- `eval` — mangle names visible in scopes where `eval` or `when` are used
(disabled by default).
When mangling is enabled but you want to prevent certain names from being
mangled, you can declare those names with `--reserved` (`-r`) — pass a
comma-separated list of names. For example:
uglifyjs ... -m -r '$,require,exports'
to prevent the `require`, `exports` and `$` names from being changed.
## Compressor options
You need to pass `--compress` (`-c`) to enable the compressor. Optionally
you can pass a comma-separated list of 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`).
- `sequences` -- join consecutive simple statements using the comma operator
- `properties` -- rewrite property access using the dot notation, for
example `foo["bar"] → foo.bar`
- `dead_code` -- remove unreachable code
- `drop_debugger` -- remove `debugger;` statements
- `unsafe` (default: false) -- apply "unsafe" transformations (discussion below)
- `conditionals` -- apply optimizations for `if`-s and conditional
expressions
- `comparisons` -- apply certain optimizations to binary nodes, for example:
`!(a <= b) → a > b` (only when `unsafe`), attempts to negate binary nodes,
e.g. `a = !b && !c && !d && !e → a=!(b||c||d||e)` etc.
- `evaluate` -- attempt to evaluate constant expressions
- `booleans` -- various optimizations for boolean context, for example `!!a
? b : c → a ? b : c`
- `loops` -- optimizations for `do`, `while` and `for` loops when we can
statically determine the condition
- `unused` -- drop unreferenced functions and variables
- `hoist_funs` -- hoist function declarations
- `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` -- optimizations for if/return and if/continue
- `join_vars` -- join consecutive `var` statements
- `cascade` -- small optimization for sequences, transform `x, x` into `x`
and `x = something(), x` into `x = something()`
- `warnings` -- display warnings when dropping unreachable code or unused
declarations etc.
- `negate_iife` -- negate "Immediately-Called Function Expressions"
where the return value is discarded, to avoid the parens that the
code generator would insert.
- `pure_getters` -- the default is `false`. If you pass `true` for
this, UglifyJS will assume that object property access
(e.g. `foo.bar` or `foo["bar"]`) doesn't have any side effects.
- `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).
### The `unsafe` 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`
- `typeof foo == "undefined"` → `foo === void 0`
- `void 0` → `undefined` (if there is a variable named "undefined" in
scope; we do it because the variable name will be mangled, typically
reduced to a single character).
### 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:
```javascript
if (DEBUG) {
console.log("debug stuff");
}
```
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
const DEBUG = false;
const 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 possible downside of this approach is that the build
will contain the `const` declarations.
## Beautifier 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:
- `beautify` (default `true`) -- whether to actually beautify the output.
Passing `-b` will set this to true, but you might need to pass `-b` even
when you want to generate minified code, in order to specify additional
arguments, so you can use `-b beautify=false` to override it.
- `indent-level` (default 4)
- `indent-start` (default 0) -- prefix all lines by that many spaces
- `quote-keys` (default `false`) -- pass `true` to quote all keys in literal
objects
- `space-colon` (default `true`) -- insert a space after the colon signs
- `ascii-only` (default `false`) -- escape Unicode characters in strings and
regexps
- `inline-script` (default `false`) -- escape the slash in occurrences of
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