Bobby L. Craig

UglifyJS 2

UglifyJS 2 ========== [![Build Status](]( 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]( There's also an [in-browser online demo]( (for Firefox, Chrome and probably Safari). #### Note: - `uglify-js` only supports ECMAScript 5 (ES5). - Support for `const` is [present but incomplete](#support-for-const), and may not be transformed properly. - Those wishing to minify ES2015+ (ES6+) should use the `npm` package [**uglify-es**]( 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 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. 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 The available options are: ``` --source-map Specify an output file where to generate source map. --source-map-root The path to the original source to be included in the source map. --source-map-url The path to the source map to be added in //# sourceMappingURL. Defaults to the value passed with --source-map. --source-map-include-sources Pass this flag if you want to include the content of source files in the source map as sourcesContent property. --source-map-inline Write base64-encoded source map to the end of js output. --in-source-map 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 inline with the sources. --screw-ie8 Use this flag if you don't wish to support Internet Explorer 6/7/8. By default UglifyJS will not try to be IE-proof. --support-ie8 Use this flag to support Internet Explorer 6/7/8. Equivalent to setting `screw_ie8: false` in `minify()` for `compress`, `mangle` and `output` options. --expr Parse a single expression, rather than a program (for parsing JSON) -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. -o, --output Output file (default STDOUT). -b, --beautify Beautify output/specify output options. -m, --mangle Mangle names/pass mangler options. -r, --reserved Reserved names to exclude from mangling. -c, --compress Enable compressor/pass compressor options, e.g. `-c 'if_return=false,pure_funcs=["Math.pow","console.log"]'` Use `-c` with no argument to enable default compression options. -d, --define Global definitions -e, --enclose Embed everything in a big function, with a configurable parameter/argument list. --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 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. --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. --acorn Use Acorn for parsing. --spidermonkey Assume input files are SpiderMonkey AST format (as JSON). --self Build itself (UglifyJS2) as a library (implies --wrap=UglifyJS --export-all) --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. --export-all Only used when --wrap, this tells UglifyJS to add code to automatically export all globals. --lint Display some scope warnings -v, --verbose Verbose -V, --version Print version number and exit. --noerr Don't throw an error for unknown options in -c, -b or -m. --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`. --keep-fnames Do not mangle/drop function names. Useful for code relying on --reserved-file File containing reserved names --reserve-domprops Make (most?) DOM properties reserved for --mangle-props --mangle-props Mangle property names (default `0`). Set to `true` or `1` to mangle all property names. Set to `unquoted` or `2` to only mangle unquoted property names. Mode `2` also enables the `keep_quoted_props` beautifier option to preserve the quotes around property names and disables the `properties` compressor option to prevent rewriting quoted properties with dot notation. You can override these by setting them explicitly on the command line. --mangle-regex Only mangle property names matching the regex --name-cache File to hold mangled names mappings --pure-funcs Functions that can be safely removed if their return value is not used, e.g. `--pure-funcs Math.floor` (requires `--compress`) ``` 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` (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 \ --source-map-root \ -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 ``. The source mapping will refer to `` and `` (in fact it will list `` 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/` or `--in-source-map inline` if the source map is included inline with the sources. 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: - `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. ### 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`. It will mangle all properties that are seen in some object literal, or that are assigned to. For example: ```js var x = { foo: 1 }; = 2; x["baz"] = 3; x[condition ? "moo" : "boo"] = 4; console.log(x.something()); ``` In the above code, `foo`, `bar`, `baz`, `moo` and `boo` will be replaced with single characters, while `something()` will be left as is. In order for this to be of any use, we should avoid mangling standard JS names. For instance, if your code would contain `x.length = 10`, then `length` becomes a candidate for mangling and it will be mangled throughout the code, regardless if it's being used as part of your own objects or accessing an array's length. To avoid that, you can use `--reserved-file` to pass a filename that should contain the names to be excluded from mangling. This file can be used both for excluding variable names and property names. It could look like this, for example: ```js { "vars": [ "define", "require", ... ], "props": [ "length", "prototype", ... ] } ``` `--reserved-file` can be an array of file names (either a single comma-separated argument, or you can pass multiple `--reserved-file` arguments) — in this case it will exclude names from all those files. A default exclusion file is provided in `tools/domprops.json` which should cover most standard JS and DOM properties defined in various browsers. Pass `--reserve-domprops` to read that in. You can also use a regular expression to define which property names should be mangled. For example, `--mangle-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=unquoted` or `--mangle-props=2`) Using quoted property name (`o["foo"]`) reserves the property name (`foo`) so that it is not mangled throughout the entire script even when used in an unquoted style (``). Example: ``` $ echo 'var o={"foo":1, bar:3}; +=; console.log(;' | uglifyjs --mangle-props=2 -mc var o={"foo":1,a:3};,console.log(; ``` #### 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 `` 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. You can also pass a custom suffix using `--mangle-props-debug=XYZ`. This would then mangle `` 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. ## 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` (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. - `properties` -- rewrite property access using the dot notation, for example `foo["bar"] →` - `dead_code` -- remove unreachable code - `drop_debugger` -- remove `debugger;` statements - `unsafe` (default: false) -- apply "unsafe" transformations (discussion below) - `unsafe_comps` (default: false) -- Reverse `<` and `<=` to `>` and `>=` to allow improved compression. This might be unsafe when an at least one of two operands is an object with computed values due the use of methods like `get`, or `valueOf`. This could cause change in execution order after operands in the comparison are switching. Compression only works if both `comparisons` and `unsafe_comps` are both set to true. - `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 `` into `[]` - `unsafe_regexp` (default: false) -- enable substitutions of variables with `RegExp` values the same way as if they are constants. - `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_comps`), 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 (simple direct variable assignments do not count as references unless set to `"keep_assign"`) - `toplevel` -- drop unreferenced functions (`"funcs"`) and/or variables (`"vars"`) in the toplevel scope (`false` by default, `true` to drop both unreferenced functions and variables) - `top_retain` -- prevent specific toplevel functions and variables from `unused` removal (can be array, comma-separated, RegExp or function. Implies `toplevel`) - `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()` - `collapse_vars` -- Collapse single-use `var` and `const` definitions when possible. - `reduce_vars` -- Improve optimization on variables assigned with and used as constant values. - `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. `` or `foo["bar"]`) doesn't have any side effects. Specify `"strict"` to treat `` as side-effect-free only when `foo` is certain to not throw, i.e. not `null` or `undefined`. - `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). - `drop_console` -- default `false`. Pass `true` to discard calls to `console.*` functions. If you wish to drop a specific function call such as `` and/or retain side effects from function arguments after dropping the function call then use `pure_funcs` instead. - `expression` -- default `false`. Pass `true` to preserve completion values from terminal statements without `return`, e.g. in bookmarklets. - `keep_fargs` -- default `true`. Prevents the compressor from discarding unused function arguments. You need this for code which relies on `Function.length`. - `keep_fnames` -- default `false`. Pass `true` to prevent the compressor from discarding function names. Useful for code relying on ``. See also: the `keep_fnames` [mangle option](#mangle). - `passes` -- default `1`. Number of times to run compress with a maximum of 3. In some cases more than one pass leads to further compressed code. Keep in mind more passes will take more time. - `keep_infinity` -- default `false`. Pass `true` to prevent `Infinity` from being compressed into `1/0`, which may cause performance issues on Chrome. - `side_effects` -- 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();` ### 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"); } ``` 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 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 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: ```js uglifyJS.minify([ "input.js"], { compress: { dead_code: true, global_defs: { DEBUG: false } } }); ``` ## 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 (affects directives with non-ascii characters becoming invalid) - `inline-script` (default `false`) -- escape the slash in occurrences of `