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UglifyJS 2 ![logo](http://trigen.pro/colalogo.png)
==========
[![Build Status](https://travis-ci.org/mishoo/UglifyJS2.png)](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]
--source-map-include-sources
Pass this flag if you want to include the content of
source files in the source map as sourcesContent
property. [boolean]
--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 `with` 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) ColaScript is a language that compiles in JavaScript. This language is similar to Dart, CoffeeScript, Python and PHP, with some original ideas. Compiler based on [UglifyJS2](https://github.com/mishoo/UglifyJS2). In present time compiler in development. Play with language you can in `lib/index.html`.
- `conditionals` -- apply optimizations for `if`-s and conditional # to do:
expressions
- `comparisons` -- apply certain optimizations to binary nodes, for example: - semicolon is always required, status: done
`!(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 ## Operators:
- `booleans` -- various optimizations for boolean context, for example `!!a ### Unary
? b : c → a ? b : c` - `varname?`
- `loops` -- optimizations for `do`, `while` and `for` loops when we can bool exist = SOME?;
statically determine the condition
- `unused` -- drop unreferenced functions and variables
- `hoist_funs` -- hoist function declarations ### Binary
- `**`, status: done
- `hoist_vars` (default: false) -- hoist `var` declarations (this is `false` int pow = 5 ** 2; // 25
by default because it seems to increase the size of the output in general)
- `%%`, status: done
- `if_return` -- optimizations for if/return and if/continue int modulo = 5 %% 3; // 2
- `?=`, status: done
- `join_vars` -- join consecutive `var` statements var undef, def = 5;
def ?= undef; // def == 5
undef = 6;
def ?= undef; // def == 6
- `a ? b`
- `cascade` -- small optimization for sequences, transform `x, x` into `x` int a = 5, b = 3;
and `x = something(), x` into `x = something()`
a = a > 10 ? b; // a == 5
- `warnings` -- display warnings when dropping unreachable code or unused a = 11;
declarations etc. a = a > 10 ? b; // a == 3
- `negate_iife` -- negate "Immediately-Called Function Expressions" - `is`, status: done
where the return value is discarded, to avoid the parens that the
code generator would insert. bool isRegExp = /[^\d]+/g is RegExp; // true
- `pure_getters` -- the default is `false`. If you pass `true` for - `isnt`, status: done
this, UglifyJS will assume that object property access
(e.g. `foo.bar` or `foo["bar"]`) doesn't have any side effects. bool isntString = 3.14 isnt String; // true
- `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 ### Multiple
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 Object profile = {
function won't produce any side effect, in which case the whole name : "dan",
statement would get discarded. The current implementation adds some nick : "dan",
overhead (compression will be slower). friends : [
{ name : "eric", nick : "eric" }
- `drop_console` -- default `false`. Pass `true` to discard calls to ],
`console.*` functions. "info" : "coder"
}
### The `unsafe` option ..name += "iil"
..nick += "green"
It enables some transformations that *might* break code logic in certain ..friends[0]:
contrived cases, but should be fine for most code. You might want to try it ..name = profile.friends[0].name.capitalize()
on your own code, it should reduce the minified size. Here's what happens ..nick += "bro";
when this flag is on: ..info += ", student";
- `new Array(1, 2, 3)` or `Array(1, 2, 3)` → `[1, 2, 3 ]`
- `new Object()` → `{}` - `a > b > c`
- `String(exp)` or `exp.toString()` → `"" + exp`
- `new Object/RegExp/Function/Error/Array (...)` → we discard the `new` if( 0 < x < 100 ) console.log("x E (0; 100)");
- `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 ### Compilator
reduced to a single character). - `@require`
### Conditional compilation @require "./library/jquery.js", "./library/underscore.js"
You can use the `--define` (`-d`) switch in order to declare global - `@use`
variables that UglifyJS will assume to be constants (unless defined in
scope). For example if you pass `--define DEBUG=false` then, coupled with @use strict, typing
dead code removal UglifyJS will discard the following from the output: @use asmjs
```javascript
if (DEBUG) { - `@if @end_if @else`
console.log("debug stuff");
} @if target == 'web'
``` @require './main.cola'
@else
UglifyJS will warn about the condition being always false and about dropping @require './mobile/main.cola'
unreachable code; for now there is no option to turn off only this specific @end_if
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 ## Expressins
separate file and include it into the build. For example you can have a - `switch` assigmention
`build/defines.js` file with the following:
```javascript String weather = switch(temperature){
const DEBUG = false; case -10: 'cold';
const PRODUCTION = true; case 20: 'normal';
// etc. case 35: 'hot';
``` };
and build your code like this: - `with` scoping, status: its need??
uglifyjs build/defines.js js/foo.js js/bar.js... -c with(document.body.querySelector('ul').childNodes){
var txt = 'text';
UglifyJS will notice the constants and, since they cannot be altered, it
will evaluate references to them to the value itself and drop unreachable forEach((li){
code as usual. The possible downside of this approach is that the build li.innerHTML = txt;
will contain the `const` declarations. });
}
<a name="codegen-options"></a>
## Beautifier options console.log(txt); // undefined
The code generator tries to output shortest code possible by default. In
case you want beautified output, pass `--beautify` (`-b`). Optionally you ## Vars
can pass additional arguments that control the code output: - declaration with type
- `beautify` (default `true`) -- whether to actually beautify the output. int b = 584;
Passing `-b` will set this to true, but you might need to pass `-b` even Array arr = [];
when you want to generate minified code, in order to specify additional Object obj = {};
arguments, so you can use `-b beautify=false` to override it. String str = "";
- `indent-level` (default 4)
- `indent-start` (default 0) -- prefix all lines by that many spaces - multiple assigment
- `quote-keys` (default `false`) -- pass `true` to quote all keys in literal
objects [a, b, c] = [b, c, a];
- `space-colon` (default `true`) -- insert a space after the colon signs {poet: {String name, address: [street, city]}} = futurists;
- `ascii-only` (default `false`) -- escape Unicode characters in strings and [a, ..., b] = someArray;
regexps
- `inline-script` (default `false`) -- escape the slash in occurrences of
`</script` in strings ### bool
- `width` (default 80) -- only takes effect when beautification is on, this - aliases, status: done
specifies an (orientative) line width that the beautifier will try to
obey. It refers to the width of the line text (excluding indentation). yes === on === true;
It doesn't work very well currently, but it does make the code generated no === off === false;
by UglifyJS more readable.
- `max-line-len` (default 32000) -- maximum line length (for uglified code)
- `bracketize` (default `false`) -- always insert brackets in `if`, `for`, ### String
`do`, `while` or `with` statements, even if their body is a single - \` new string \`, status: done
statement.
- `semicolons` (default `true`) -- separate statements with semicolons. If String name = `dangreen`;
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 - multiline, status: done
gzip could be smaller; size after gzip insignificantly larger).
- `preamble` (default `null`) -- when passed it must be a string and String ml = "
it will be prepended to the output literally. The source map will
adjust for this text. Can be used to insert a comment containing Lorem ipsum,
licensing information, for example. Lorem ipsum.
### Keeping copyright notices or other comments ";
You can pass `--comments` to retain certain comments in the output. By - raw, status: done
default it will keep JSDoc-style comments that contain "@preserve",
"@license" or "@cc_on" (conditional compilation for IE). You can pass String str = r"\n \r"; // "\\n \\r"
`--comments all` to keep all the comments, or a valid JavaScript regexp to
keep only comments that match this regexp. For example `--comments - templating, status : done
'/foo|bar/'` will keep only comments that contain "foo" or "bar".
String name = "dan";
Note, however, that there might be situations where comments are lost. For
example: console.log("My name is @name."); // My name is dan.
```javascript name = "eric";
function f() { console.log("My name is @name."); // My name is eric.
/** @preserve Foo Bar */ console.log("My name is @{name.capitalize()} or {{name.capitalize()}}"); // My name is Eric.
function g() {
// this function is never called ### RegExp
} - multiline ( and x flag ), status: done
return something();
} RegExp re = /
``` ([^\d]+)-
(\w+)
Even though it has "@preserve", the comment will be lost because the inner /gx;
function `g` (which is the AST node to which the comment is attached to) is
discarded by the compressor as not referenced.
### Arrays
The safest comments where to place copyright information (or other info that - pushing
needs to be kept in the output) are comments attached to toplevel nodes.
var arr = [3, 5, 6, 7];
## Support for the SpiderMonkey AST arr[] = 4; // [3, 5, 6, 7, 4]
UglifyJS2 has its own abstract syntax tree format; for - part assigment
[practical reasons](http://lisperator.net/blog/uglifyjs-why-not-switching-to-spidermonkey-ast/)
we can't easily change to using the SpiderMonkey AST internally. However, arr[0..2] = [0,1]; // [0, 1, 7, 4]
UglifyJS now has a converter which can import a SpiderMonkey AST. arr[0..2] = []; // [7, 4]
For example [Acorn][acorn] is a super-fast parser that produces a - inline array ranging
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 arr = [10..1]; // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
compress that: arr = [1..10]; // [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
acorn file.js | uglifyjs --spidermonkey -m -c ### Functions
- without `function` keyword
The `--spidermonkey` option tells UglifyJS that all input files are not
JavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore we void main(){
don't use our own parser in this case, but just transform that AST into our console.log('Hello World!');
internal AST. }
### Use Acorn for parsing - binding toplevel `main` functions to onload event
More for fun, I added the `--acorn` option which will use Acorn to do all // lib.cola
the parsing. If you pass this option, UglifyJS will `require("acorn")`.
main(){
Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), but console.log('Hello World from lib.cola!');
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.
// main.cola
API Reference
------------- require "./lib.cola";
Assuming installation via NPM, you can load UglifyJS in your application main(){
like this: console.log('Hello World!');
```javascript }
var UglifyJS = require("uglify-js");
``` - arrow functions
It exports a lot of names, but I'll discuss here the basics that are needed print(str) => console.log(str);
for parsing, mangling and compressing a piece of code. The sequence is (1)
parse, (2) compress, (3) mangle, (4) generate output code.
- named arguments
### The simple way
hello(String name:) => console.log("Hello @name!");
There's a single toplevel function which combines all the steps. If you hello(name: 'dangreen'); // Hello dangreen!
don't need additional customization, you might want to go with `minify`.
Example: hello(name: "World") => console.log("Hello @name!");
```javascript hello(); // Hello World!
var result = UglifyJS.minify("/path/to/file.js");
console.log(result.code); // minified output - defaults for positional arguments
// if you need to pass code instead of file name
var result = UglifyJS.minify("var b = function () {};", {fromString: true}); hello(String name = "World!") => console.log("Hello @name!");
``` hello('dangreen'); // Hello dangreen!
hello(); // Hello World!
You can also compress multiple files:
```javascript - some arguments into array
var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ]);
console.log(result.code); main(name, skills...){
``` console.log("My name is @name, my skills:");
skills.forEach((skill) => console.log("@skill,"));
To generate a source map: }
```javascript
var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ], {
outSourceMap: "out.js.map" ## Classes
}); - classes
console.log(result.code); // minified output - singletones
console.log(result.map); - injectors
```
class A {
Note that the source map is not saved in a file, it's just returned in
`result.map`. The value passed for `outSourceMap` is only used to set the private int a = 123;
`file` attribute in the source map (see [the spec][sm-spec]). protected var o = {};
You can also specify sourceRoot property to be included in source map: readonly String about = "class";
```javascript
var result = UglifyJS.minify([ "file1.js", "file2.js", "file3.js" ], { A(a){
outSourceMap: "out.js.map", about = "some else";
sourceRoot: "http://example.com/src" }
});
``` static Hello() => "hello!";
If you're compressing compiled JavaScript and have a source map for it, you public String about() => about;
can use the `inSourceMap` argument: }
```javascript
var result = UglifyJS.minify("compiled.js", { class B extends A {
inSourceMap: "compiled.js.map",
outSourceMap: "minified.js.map" B(){
}); parent();
// same as before, it returns `code` and `map` about += "!";
``` }
The `inSourceMap` is only used if you also request `outSourceMap` (it makes B.anotherConstructor(){
no sense otherwise). about = "ups!";
}
Other options:
get some => "some " + about;
- `warnings` (default `false`) — pass `true` to display compressor warnings. set some(val) => about += val;
}
- `fromString` (default `false`) — if you pass `true` then you can pass
JavaScript source code, rather than file names. singleton S { // in fact this is object
int x = 45;
- `mangle` — pass `false` to skip mangling names. String s = "txt";
- `output` (default `null`) — pass an object if you wish to specify say(some){
additional [output options][codegen]. The defaults are optimized alert(some);
for best compression. }
- `compress` (default `{}`) — pass `false` to skip compressing entirely. int operator[](int index) => index + 584;
Pass an object to specify custom [compressor options][compressor]. operator[]=(int index, int val) => x = index + val;
We could add more options to `UglifyJS.minify` — if you need additional String operator.(String key) => key + "!";
functionality please suggest! operator.(String key, String value) => s = "@key @value";
### The hard way }
Following there's more detailed API info, in case the `minify` function is injector String {
too simple for your needs. String replaceAll(a, b){
String res = this;
#### The parser while(res.indexOf(a) != -1) res = res.replace(a, b);
```javascript return res;
var toplevel_ast = UglifyJS.parse(code, options); }
``` }
`options` is optional and if present it must be an object. The following // or
properties are available:
String String::replaceAll(a, b){
- `strict` — disable automatic semicolon insertion and support for trailing String res = this;
comma in arrays and objects while(res.indexOf(a) != -1) res = res.replace(a, b);
- `filename` — the name of the file where this code is coming from return res;
- `toplevel` — a `toplevel` node (as returned by a previous invocation of }
`parse`)
### Statistic
The last two options are useful when you'd like to minify multiple files and
get a single file as the output and a proper source map. Our CLI tool does - 30 feature ( without classes )
something like this: - 12 done
```javascript
var toplevel = null;
files.forEach(function(file){
var code = fs.readFileSync(file, "utf8");
toplevel = UglifyJS.parse(code, {
filename: file,
toplevel: toplevel
});
});
```
After this, we have in `toplevel` a big AST containing all our files, with
each token having proper information about where it came from.
#### Scope information
UglifyJS contains a scope analyzer that you need to call manually before
compressing or mangling. Basically it augments various nodes in the AST
with information about where is a name defined, how many times is a name
referenced, if it is a global or not, if a function is using `eval` or the
`with` statement etc. I will discuss this some place else, for now what's
important to know is that you need to call the following before doing
anything with the tree:
```javascript
toplevel.figure_out_scope()
```
#### Compression
Like this:
```javascript
var compressor = UglifyJS.Compressor(options);
var compressed_ast = toplevel.transform(compressor);
```
The `options` can be missing. Available options are discussed above in
“Compressor options”. Defaults should lead to best compression in most
scripts.
The compressor is destructive, so don't rely that `toplevel` remains the
original tree.
#### Mangling
After compression it is a good idea to call again `figure_out_scope` (since
the compressor might drop unused variables / unreachable code and this might
change the number of identifiers or their position). Optionally, you can
call a trick that helps after Gzip (counting character frequency in
non-mangleable words). Example:
```javascript
compressed_ast.figure_out_scope();
compressed_ast.compute_char_frequency();
compressed_ast.mangle_names();
```
#### Generating output
AST nodes have a `print` method that takes an output stream. Essentially,
to generate code you do this:
```javascript
var stream = UglifyJS.OutputStream(options);
compressed_ast.print(stream);
var code = stream.toString(); // this is your minified code
```
or, for a shortcut you can do:
```javascript
var code = compressed_ast.print_to_string(options);
```
As usual, `options` is optional. The output stream accepts a lot of otions,
most of them documented above in section “Beautifier options”. The two
which we care about here are `source_map` and `comments`.
#### Keeping comments in the output
In order to keep certain comments in the output you need to pass the
`comments` option. Pass a RegExp or a function. If you pass a RegExp, only
those comments whose body matches the regexp will be kept. Note that body
means without the initial `//` or `/*`. If you pass a function, it will be
called for every comment in the tree and will receive two arguments: the
node that the comment is attached to, and the comment token itself.
The comment token has these properties:
- `type`: "comment1" for single-line comments or "comment2" for multi-line
comments
- `value`: the comment body
- `pos` and `endpos`: the start/end positions (zero-based indexes) in the
original code where this comment appears
- `line` and `col`: the line and column where this comment appears in the
original code
- `file` — the file name of the original file
- `nlb` — true if there was a newline before this comment in the original
code, or if this comment contains a newline.
Your function should return `true` to keep the comment, or a falsy value
otherwise.
#### Generating a source mapping
You need to pass the `source_map` argument when calling `print`. It needs
to be a `SourceMap` object (which is a thin wrapper on top of the
[source-map][source-map] library).
Example:
```javascript
var source_map = UglifyJS.SourceMap(source_map_options);
var stream = UglifyJS.OutputStream({
...
source_map: source_map
});
compressed_ast.print(stream);
var code = stream.toString();
var map = source_map.toString(); // json output for your source map
```
The `source_map_options` (optional) can contain the following properties:
- `file`: the name of the JavaScript output file that this mapping refers to
- `root`: the `sourceRoot` property (see the [spec][sm-spec])
- `orig`: the "original source map", handy when you compress generated JS
and want to map the minified output back to the original code where it
came from. It can be simply a string in JSON, or a JSON object containing
the original source map.
[acorn]: https://github.com/marijnh/acorn
[source-map]: https://github.com/mozilla/source-map
[sm-spec]: https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit
[codegen]: http://lisperator.net/uglifyjs/codegen
[compressor]: http://lisperator.net/uglifyjs/compress