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A JavaScript mangler/compressor toolkit for ES6+.

note:You can support this project on patreon: [link]The Terser Patreon is shutting down in favor of opencollective. Check outPATRONS.md for our first-tier patrons.

Terser recommends you use RollupJS to bundle your modules, as that produces smaller code overall.

Beautification has been undocumented and isbeing removed from terser, we recommend you useprettier.

Find the changelog inCHANGELOG.md

uglify-es isno longer maintained anduglify-js does not support ES6+.

terser is a fork ofuglify-es that mostly retains API and CLI compatibilitywithuglify-es anduglify-js@3.

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

From NPM for use as a command line app:

npm install terser -g

From NPM for programmatic use:

npm install terser

terser [input files] [options]

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

Command line arguments that take options (like --parse, --compress, --mangle and--format) can take in a comma-separated list of default option overrides. Forinstance:

terser input.js --compress ecma=2015,computed_props=false

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

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

terser --compress --mangle -- input.js

    -h, --help                  Print usage information.                                `--help options` for details on available options.    -V, --version               Print version number.    -p, --parse <options>       Specify parser options:                                `acorn`  Use Acorn for parsing.                                `bare_returns`  Allow return outside of functions.                                                Useful when minifying CommonJS                                                modules and Userscripts that may                                                be anonymous function wrapped (IIFE)                                                by the .user.js engine `caller`.                                `expression`  Parse a single expression, rather than                                              a program (for parsing JSON).                                `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 and DOM                                            API props.                                `debug`  Add debug prefix and suffix.                                `keep_quoted`  Only mangle unquoted properties, quoted                                               properties are automatically reserved.                                               `strict` disables quoted properties                                               being automatically reserved.                                `regex`  Only mangle matched property names.                                `only_annotated` Only mangle properties defined with /*@__MANGLE_PROP__*/.                                `reserved`  List of names that should not be mangled.    -f, --format [options]      Specify format options.                                `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 parenthesis. Note: you may                                             want to disable `negate_iife` under                                             compressor options.                                `wrap_func_args`  Wrap function arguments in parenthesis.    -o, --output <file>         Output file path (default STDOUT). Specify `ast` or                                `spidermonkey` to write Terser or SpiderMonkey AST                                as JSON to STDOUT respectively.    --comments [filter]         Preserve copyright comments in the output. By                                default this works like Google Closure, keeping                                JSDoc-style comments that contain e.g. "@license",                                or start with "!". You can optionally pass one of the                                following arguments to this flag:                                - "all" to keep all comments                                - `false` to omit comments in the output                                - a valid JS RegExp like `/foo/` or `/^!/` to                                keep only matching comments.                                Note that currently not *all* comments can be                                kept when compression is on, because of dead                                code removal or cascading statements into                                sequences.    --config-file <file>        Read `minify()` options from JSON file.    -d, --define <expr>[=value] Global definitions.    --ecma <version>            Specify ECMAScript release: 5, 2015, 2016, etc.    -e, --enclose [arg[:value]] Embed output in a big function with configurable                                arguments and values.    --ie8                       Support non-standard Internet Explorer 8.                                Equivalent to setting `ie8: true` in `minify()`                                for `compress`, `mangle` and `format` options.                                By default Terser will not try to be IE-proof.    --keep-classnames           Do not mangle/drop class names.    --keep-fnames               Do not mangle/drop function names.  Useful for                                code relying on Function.prototype.name.    --module                    Input is an ES6 module. If `compress` or `mangle` is                                enabled then the `toplevel` option, as well as strict mode,                                will be enabled.    --name-cache <file>         File to hold mangled name mappings.    --safari10                  Support non-standard Safari 10/11.                                Equivalent to setting `safari10: true` in `minify()`                                for `mangle` and `format` options.                                By default `terser` will not work around                                Safari 10/11 bugs.    --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`  Name and/or location of the output source.                                `includeSources`  Pass this flag if you want to include                                                  the content of source files in the                                                  source map as sourcesContent property.                                `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.    --wrap <name>               Embed everything in a big function, making the                                “exports” and “global” variables available. You                                need to pass an argument to this option to                                specify the name that your module will take                                when included in, say, a browser.

Specify--output (-o) to declare the output file. Otherwise the outputgoes to STDOUT.

Terser can generate a source map file, which is highly useful fordebugging your compressed JavaScript. To get a source map, pass--source-map --output output.js (source map will be written out tooutput.js.map).

Additional options:

• --source-map "filename='<NAME>'" to specify the name of the source map.

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

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

For example:

terser 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 manglefile1.js andfile2.js, will drop theoutput infoo.min.js and the source map infoo.min.js.map. The sourcemapping will refer tohttp://foo.com/src/js/file1.js andhttp://foo.com/src/js/file2.js (in fact it will listhttp://foo.com/srcas the source map root, and the original files asjs/file1.js andjs/file2.js).

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

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 withthe sources.

You need to pass--compress (-c) to enable the compressor. Optionallyyou can pass a comma-separated list ofcompress options.

Options are in the formfoo=bar, or justfoo (the latter impliesa boolean option that you want to settrue; it's effectively ashortcut forfoo=true).

Example:

terser file.js -c toplevel,sequences=false

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

• toplevel (defaultfalse) -- mangle names declared in the top level scope.

• eval (defaultfalse) -- mangle names visible in scopes whereeval orwith are used.

When mangling is enabled but you want to prevent certain names from beingmangled, you can declare those names with--mangle reserved — pass acomma-separated list of names. For example:

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

to prevent therequire,exports and$ names from being changed.

Note: THISWILL BREAK YOUR CODE. A good rule of thumb is not to use this unless you know exactly what you're doing and how this works and read this section until the end.

Mangling property names is a separate step, different from variable name mangling. Pass--mangle-props to enable it. The least dangerousway to use this is to use theregex option like so:

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

This will mangle all properties that end with anunderscore. So you can use it to mangle internal methods.

By default, it will mangle all properties in theinput code with the exception of built in DOM properties and propertiesin core JavaScript classes, which is what will break your code if you don't:

1. Control all the code you're mangling
2. Avoid using a module bundler, as they usually will call Terser on each file individually, making it impossible to pass mangled objects between modules.
3. Avoid calling functions likedefineProperty orhasOwnProperty, because they refer to object properties using strings and will break your code if you don't know what you are doing.

An example:

// example.jsvarx={baz_:0,foo_:1,calc:function(){returnthis.foo_+this.baz_;}};x.bar_=2;x["baz_"]=3;console.log(x.calc());

Mangle all properties (except for JavaScriptbuiltins) (very unsafe):

$ terser example.js -c passes=2 -m --mangle-props

varx={o:3,t:1,i:function(){returnthis.t+this.o},s:2};console.log(x.i());

Mangle all properties except forreserved properties (still very unsafe):

$ terser example.js -c passes=2 -m --mangle-props reserved=[foo_,bar_]

varx={o:3,foo_:1,t:function(){returnthis.foo_+this.o},bar_:2};console.log(x.t());

Mangle all properties matching aregex (not as unsafe but still unsafe):

$ terser example.js -c passes=2 -m --mangle-props regex=/_$/

varx={o:3,t:1,calc:function(){returnthis.t+this.o},i:2};console.log(x.calc());

Combining mangle properties options:

$ terser example.js -c passes=2 -m --mangle-props regex=/_$/,reserved=[bar_]

varx={o:3,t:1,calc:function(){returnthis.t+this.o},bar_:2};console.log(x.calc());

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

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

When you compress multiple files using this option, in order for them towork together in the end we need to ensure somehow that one property getsmangled to the same name in all of them. For this, pass--name-cache filename.jsonand Terser 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$ terser file1.js file2.js --mangle-props --name-cache /tmp/cache.json -o part1.js$ terser file3.js file4.js --mangle-props --name-cache /tmp/cache.json -o part2.js

Now,part1.js andpart2.js will be consistent with each other in termsof mangled property names.

Using the name cache is not necessary if you compress all your files in asingle call to Terser.

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 anunquoted style (o.foo). Example:

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

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

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

You can also pass--mangle-props debug in order to mangle property nameswithout completely obscuring them. For example the propertyo.foowould mangle too._$foo$_ with this option. This allows property manglingof a large codebase while still being able to debug the code and identifywhere mangling is breaking things.

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

varo={_$foo$_: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 thenmangleo.foo too._$foo$XYZ_. You can change this each time you compile ascript to identify how a property got mangled. One technique is to pass arandom number on every compile to simulate mangling changing with differentinputs (e.g. as you update the input script with new properties), and to helpidentify mistakes like writing mangled keys to storage.

Assuming installation via NPM, you can load Terser in your applicationlike this:

const{ minify}=require("terser");

Or,

import{minify}from"terser";

Browser loading is also supported. It exposes a global variableTerser containing a.minify property:

<scriptsrc="https://cdn.jsdelivr.net/npm/source-map@0.7.3/dist/source-map.js"></script><scriptsrc="https://cdn.jsdelivr.net/npm/terser/dist/bundle.min.js"></script>

There is an async high level function,async minify(code, options),which will perform all minificationphases in a configurablemanner. By defaultminify() will enablecompressandmangle. Example:

varcode="function add(first, second) { return first + second; }";varresult=awaitminify(code,{sourceMap:true});console.log(result.code);// minified output: function add(n,d){return n+d}console.log(result.map);// source map

There is also aminify_sync() alternative version of it, which returns instantly.

You canminify more than one JavaScript file at a time by using an objectfor the first argument where the keys are file names and the values are sourcecode:

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

Thetoplevel option:

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

ThenameCache option:

varoptions={mangle:{toplevel:true,},nameCache:{}};varresult1=awaitminify({"file1.js":"function add(first, second) { return first + second; }"},options);varresult2=awaitminify({"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:

varcacheFileName="/tmp/cache.json";varoptions={mangle:{properties:true,},nameCache:JSON.parse(fs.readFileSync(cacheFileName,"utf8"))};fs.writeFileSync("part1.js",awaitminify({"file1.js":fs.readFileSync("file1.js","utf8"),"file2.js":fs.readFileSync("file2.js","utf8")},options).code,"utf8");fs.writeFileSync("part2.js",awaitminify({"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 ofminify() options:

varcode={"file1.js":"function add(first, second) { return first + second; }","file2.js":"console.log(add(1 + 2, 3 + 4));"};varoptions={toplevel:true,compress:{global_defs:{"@console.log":"alert"},passes:2},format:{preamble:"/* minified */"}};varresult=awaitminify(code,options);console.log(result.code);// /* minified */// alert(10);"

An error example:

try{constresult=awaitminify({"foo.js" :"if (0) else console.log(1);"});// Do something with result}catch(error){const{ message, filename, line, col, pos}=error;// Do something with error}

• ecma (defaultundefined) - pass5,2015,2016, etc to overridecompress andformat'secma options.

• enclose (defaultfalse) - passtrue, or a string in the formatof"args[:values]", whereargs andvalues are comma-separatedargument names and values, respectively, to embed the output in a bigfunction with the configurable arguments and values.

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

• compress (default{}) — passfalse to skip compressing entirely.Pass an object to specify customcompress options.

• mangle (defaulttrue) — passfalse to skip mangling names, or passan object to specifymangle options (see below).

  • mangle.properties (defaultfalse) — a subcategory of the mangle option.Pass an object to specify custommangle property options.

• module (defaultfalse) — Use when minifying an ES6 module. "use strict"is implied and names can be mangled on the top scope. Ifcompress ormangle is enabled then thetoplevel option will be enabled.

• format oroutput (defaultnull) — pass an object if you wish to specifyadditionalformat options. The defaults are optimizedfor best compression.

• sourceMap (defaultfalse) - pass an object if you wish to specifysource map options.

• toplevel (defaultfalse) - set totrue if you wish to enable top levelvariable and function name mangling and to drop unused variables and functions.

• nameCache (defaultnull) - pass an empty object{} or a previouslyusednameCache object if you wish to cache mangled variable andproperty names across multiple invocations ofminify(). Note: this isa read/write property.minify() will read the name cache state of thisobject and update it during minification so that it may bereused or externally persisted by the user.

• ie8 (defaultfalse) - set totrue to support IE8.

• keep_classnames (default:undefined) - passtrue to prevent discarding or manglingof class names. Pass a regular expression to only keep class names matching that regex.

• keep_fnames (default:false) - passtrue to prevent discarding or manglingof function names. Pass a regular expression to only keep function names matching that regex.Useful for code relying onFunction.prototype.name. If the top level minify optionkeep_classnames isundefined it will be overridden with the value of the top levelminify optionkeep_fnames.

• safari10 (default:false) - passtrue to work around Safari 10/11 bugs inloop scoping andawait. Seesafari10 options inmangleandformat for details.

{parse:{// parse options},compress:{// compress options},mangle:{// mangle optionsproperties:{// mangle property options}},format:{// format options (can also use `output` for backwards compatibility)},sourceMap:{// source map options},ecma:5,// specify one of: 5, 2015, 2016, etc.enclose:false,// or specify true, or "args:values"keep_classnames:false,keep_fnames:false,ie8:false,module:false,nameCache:null,// or specify a name cache objectsafari10:false,toplevel:false}

To generate a source map:

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

Note that the source map is not saved in a file, it's just returned inresult.map. The value passed forsourceMap.url is only used to set//# sourceMappingURL=out.js.map inresult.code. The value offilename is only used to setfile attribute (seethe spec)in source map file.

You can set optionsourceMap.url to be"inline" and source map willbe appended to code.

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

varresult=awaitminify({"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, youcan usesourceMap.content:

varresult=awaitminify({"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 theX-SourceMap header instead, you can just omitsourceMap.url.

If you happen to need the source map as a raw object, setsourceMap.asObject totrue.

• bare_returns (defaultfalse) -- support top levelreturn statements

• html5_comments (defaulttrue)

• shebang (defaulttrue) -- support#!command as the first line

• spidermonkey (defaultfalse) -- accept a Spidermonkey (Mozilla) AST

• defaults (default:true) -- Passfalse to disable most defaultenabledcompress transforms. Useful when you only want to enable a fewcompress options while disabling the rest.

• arrows (default:true) -- Class and object literal methods are convertedwill also be converted to arrow expressions if the resultant code is shorter:m(){return x} becomesm:()=>x. To do this to regular ES5 functions whichdon't usethis orarguments, seeunsafe_arrows.

• arguments (default:false) -- replacearguments[index] with functionparameter name whenever possible.

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

• booleans_as_integers (default:false) -- Turn booleans into 0 and 1, alsomakes comparisons with booleans use== and!= instead of=== and!==.

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

• comparisons (default:true) -- apply certain optimizations to binary nodes,e.g.!(a <= b) → a > b (only whenunsafe_comps), attempts to negate binarynodes, e.g.a = !b && !c && !d && !e → a=!(b||c||d||e) etc. Note:comparisonsworks best withlhs_constants enabled.

• computed_props (default:true) -- Transforms constant computed propertiesinto regular ones:{["computed"]: 1} is converted to{computed: 1}.

• conditionals (default:true) -- apply optimizations forif-s and conditionalexpressions

• dead_code (default:true) -- remove unreachable code

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

• drop_console (default:false) -- Passtrue to discard calls toconsole.* functions. If you only want to discard a portion of console,you can pass an array like this['log', 'info'], which will only discardconsole.log、console.info.

• drop_debugger (default:true) -- removedebugger; statements

• ecma (default:5) -- Pass2015 or greater to enablecompress options thatwill transform ES5 code into smaller ES6+ equivalent forms.

• evaluate (default:true) -- attempt to evaluate constant expressions

• expression (default:false) -- Passtrue to preserve completion valuesfrom terminal statements withoutreturn, e.g. in bookmarklets.

• global_defs (default:{}) -- seeconditional compilation

• hoist_funs (default:false) -- hoist function declarations

• hoist_props (default:true) -- hoist properties from constant object andarray literals into regular variables subject to a set of constraints. For example:var o={p:1, q:2}; f(o.p, o.q); is converted tof(1, 2);. Note:hoist_propsworks best withmangle enabled, thecompress optionpasses set to2 or higher,and thecompress optiontoplevel enabled.

• hoist_vars (default:false) -- hoistvar declarations (this isfalseby default because it seems to increase the size of the output in general)

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

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

  • false -- same as0
  • 0 -- disabled inlining
  • 1 -- inline simple functions
  • 2 -- inline functions with arguments
  • 3 -- inline functions with arguments and variables
  • true -- same as3

• join_vars (default:true) -- join consecutivevar,let andconst statements

• keep_classnames (default:false) -- Passtrue to prevent the compressor fromdiscarding class names. Pass a regular expression to only keep class names matchingthat regex. See also: thekeep_classnamesmangle option.

• keep_fargs (default:true) -- Prevents the compressor from discarding unusedfunction arguments. You need this for code which relies onFunction.length.

• keep_fnames (default:false) -- Passtrue to prevent thecompressor from discarding function names. Pass a regular expression to only keepfunction names matching that regex. Useful for code relying onFunction.prototype.name.See also: thekeep_fnamesmangle option.

• keep_infinity (default:false) -- Passtrue to preventInfinity frombeing compressed into1/0, which may cause performance issues on Chrome.

• lhs_constants (default:true) -- Moves constant values to the left-hand sideof binary nodes.foo == 42 → 42 == foo

• loops (default:true) -- optimizations fordo,while andfor loopswhen we can statically determine the condition.

• module (defaultfalse) -- Passtrue when compressing an ES6 module. Strictmode is implied and thetoplevel option as well.

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

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

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

• pure_funcs (default:null) -- You can pass an array of names andTerser will assume that those functions do not produce sideeffects. DANGER: will not check if the name is redefined in scope.An example case here, for instancevar q = Math.floor(a/b). Ifvariableq is not used elsewhere, Terser will drop it, but willstill keep theMath.floor(a/b), not knowing what it does. You canpasspure_funcs: [ 'Math.floor' ] to let it know that thisfunction won't produce any side effect, in which case the wholestatement would get discarded. The current implementation adds someoverhead (compression will be slower).

• pure_getters (default:"strict") -- If you passtrue forthis, Terser will assume that object property access(e.g.foo.bar orfoo["bar"]) doesn't have any side effects.Specify"strict" to treatfoo.bar as side-effect-free only whenfoo is certain to not throw, i.e. notnull orundefined.

• pure_new (default:false) -- Set totrue to assumenew X() never hasside effects.

• reduce_vars (default:true) -- Improve optimization on variables assigned with andused as constant values.

• reduce_funcs (default:true) -- Inline single-use functions whenpossible. Depends onreduce_vars being enabled. Disabling this optionsometimes improves performance of the output code.

• sequences (default:true) -- join consecutive simple statements using thecomma operator. May be set to a positive integer to specify the maximum numberof consecutive comma sequences that will be generated. If this option is set totrue then the defaultsequences limit is200. Set option tofalse or0to disable. The smallestsequences length is2. Asequences value of1is grandfathered to be equivalent totrue and as such means200. On rareoccasions the default sequences limit leads to very slow compress times in whichcase a value of20 or less is recommended.

• side_effects (default:true) -- Remove expressions which have no side effectsand whose results aren't used.

• switches (default:true) -- de-duplicate and remove unreachableswitch branches

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

• top_retain (default:null) -- prevent specific toplevel functions andvariables fromunused removal (can be array, comma-separated, RegExp orfunction. Impliestoplevel)

• typeofs (default:true) -- Transformstypeof foo == "undefined" intofoo === void 0. Note: recommend to set this value tofalse for IE10 andearlier versions due to known issues.

• unsafe (default:false) -- apply "unsafe" transformations(details).

• unsafe_arrows (default:false) -- Convert ES5 style anonymous functionexpressions to arrow functions if the function body does not referencethis.Note: it is not always safe to perform this conversion if code relies on thethe function having aprototype, which arrow functions lack.This transform requires that theecma compress option is set to2015 or greater.

• unsafe_comps (default:false) -- Reverse< and<= to> and>= toallow improved compression. This might be unsafe when an at least one of twooperands is an object with computed values due the use of methods likeget,orvalueOf. This could cause change in execution order after operands in thecomparison are switching. Compression only works if bothcomparisons andunsafe_comps are both set to true.

• unsafe_Function (default:false) -- compress and mangleFunction(args, code)when bothargs andcode are string literals.

• unsafe_math (default:false) -- optimize numerical expressions like2 * x * 3 into6 * x, which may give imprecise floating point results.

• unsafe_symbols (default:false) -- removes keys from native Symboldeclarations, e.gSymbol("kDog") becomesSymbol().

• unsafe_methods (default: false) -- Converts{ m: function(){} } to{ m(){} }.ecma must be set to6 or greater to enable this transform.Ifunsafe_methods is a RegExp then key/value pairs with keys matching theRegExp will be converted to concise methods.Note: if enabled there is a risk of getting a "<method name> is not aconstructor" TypeError should any code try tonew the former function.

• unsafe_proto (default:false) -- optimize expressions likeArray.prototype.slice.call(a) into[].slice.call(a)

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

• unsafe_undefined (default:false) -- substitutevoid 0 if there is avariable namedundefined in scope (variable name will be mangled, typicallyreduced to a single character)

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

• eval (defaultfalse) -- Passtrue to mangle names visible in scopeswhereeval orwith are used.

• keep_classnames (defaultfalse) -- Passtrue to not mangle class names.Pass a regular expression to only keep class names matching that regex.See also: thekeep_classnamescompress option.

• keep_fnames (defaultfalse) -- Passtrue to not mangle function names.Pass a regular expression to only keep function names matching that regex.Useful for code relying onFunction.prototype.name. See also: thekeep_fnamescompress option.

• module (defaultfalse) -- Passtrue an ES6 modules, where the toplevelscope is not the global scope. Impliestoplevel and assumes input code is strict mode JS.

• nth_identifier (default: an internal mangler that weights based on characterfrequency analysis) -- Pass an object with aget(n) function that converts anordinal into the nth most favored (usually shortest) identifier.Optionally also providereset(),sort(), andconsider(chars, delta) touse character frequency analysis of the source code.

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

• toplevel (defaultfalse) -- Passtrue to mangle names declared in thetop level scope.

• safari10 (defaultfalse) -- Passtrue to work around the Safari 10 loopiteratorbug"Cannot declare a let variable twice".See also: thesafari10format option.

Examples:

// test.jsvarglobalVar;functionfuncName(firstLongName,anotherLongName){varmyVariable=firstLongName+anotherLongName;}

varcode=fs.readFileSync("test.js","utf8");awaitminify(code).code;// 'function funcName(a,n){}var globalVar;'awaitminify(code,{mangle:{reserved:['firstLongName']}}).code;// 'function funcName(firstLongName,a){}var globalVar;'awaitminify(code,{mangle:{toplevel:true}}).code;// 'function n(n,a){}var a;'

• builtins (default:false) — Usetrue to allow the mangling of builtinDOM properties. Not recommended to override this setting.

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

• keep_quoted (default:false) — How quoting properties ({"prop": ...} andobj["prop"]) controls what gets mangled.

  • "strict" (recommended) --obj.prop is mangled.
  • false --obj["prop"] is mangled.
  • true --obj.prop is mangled unless there isobj["prop"] elsewhere in the code.

• nth_identifier (default: an internal mangler that weights based on characterfrequency analysis) -- Pass an object with aget(n) function that converts anordinal into the nth most favored (usually shortest) identifier.Optionally also providereset(),sort(), andconsider(chars, delta) touse character frequency analysis of the source code.

• regex (default:null) — Pass aRegExp literal or pattern string to only mangle property matching the regular expression.

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

• undeclared (default:false) - Mangle those names when they are accessedas properties of known top level variables but their declarations are neverfound in input code. May be useful when only minifying parts of a project.See#397 for more details.

These options control the format of Terser's output code. Previously knownas "output options".

• ascii_only (defaultfalse) -- escape Unicode characters in strings andregexps (affects directives with non-ascii characters becoming invalid)

• beautify (defaultfalse) -- (DEPRECATED) whether to beautify the output.When using the legacy-b CLI flag, this is set to true by default.

• braces (defaultfalse) -- always insert braces inif,for,do,while orwith statements, even if their body is a singlestatement.

• comments (default"some") -- by default it keeps JSDoc-style commentsthat contain "@license", "@copyright", "@preserve" or start with!, passtrueor"all" to preserve all comments,false to omit comments in the output,a regular expression string (e.g./^!/) or a function.

• ecma (default5) -- set desired EcmaScript standard version for output.Setecma to2015 or greater to emit shorthand object properties - i.e.:{a} instead of{a: a}. Theecma option will only change the output indirect control of the beautifier. Non-compatible features in your input willstill be output as is. For example: anecma setting of5 willnotconvert modern code to ES5.

• indent_level (default4)

• indent_start (default0) -- prefix all lines by that many spaces

• inline_script (defaulttrue) -- escape HTML comments and the slash inoccurrences of</script> in strings

• keep_numbers (defaultfalse) -- keep number literals as it was in original code(disables optimizations like converting1000000 into1e6)

• keep_quoted_props (defaultfalse) -- when turned on, prevents strippingquotes from property names in object literals.

• max_line_len (defaultfalse) -- maximum line length (for minified code)

• preamble (defaultnull) -- when passed it must be a string andit will be prepended to the output literally. The source map willadjust for this text. Can be used to insert a comment containinglicensing information, for example.

• quote_keys (defaultfalse) -- passtrue to quote all keys in literalobjects

• quote_style (default0) -- preferred quote style for strings (affectsquoted property names and directives as well):

  • 0 -- prefers double quotes, switches to single quotes when there aremore 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

• preserve_annotations -- (defaultfalse) -- PreserveTerser annotations in the output.

• safari10 (defaultfalse) -- set this option totrue to work aroundtheSafari 10/11 await bug.See also: thesafari10mangle option.

• semicolons (defaulttrue) -- separate statements with semicolons. Ifyou passfalse then whenever possible we will use a newline instead of asemicolon, leading to more readable output of minified code (size beforegzip could be smaller; size after gzip insignificantly larger).

• shebang (defaulttrue) -- preserve shebang#! in preamble (bash scripts)

• spidermonkey (defaultfalse) -- produce a Spidermonkey (Mozilla) AST

• webkit (defaultfalse) -- enable workarounds for WebKit bugs.PhantomJS users should set this option totrue.

• wrap_iife (defaultfalse) -- passtrue to wrap immediately invokedfunction expressions. See#640 for more details.

• wrap_func_args (defaulttrue) -- passfalse if you do not want to wrapfunction expressions that are passed as arguments, in parenthesis. SeeOptimizeJS for more details.

You can pass--comments to retain certain comments in the output. Bydefault it will keep comments starting with "!" and JSDoc-style comments thatcontain "@preserve", "@copyright", "@license" or "@cc_on" (conditional compilation for IE).You can pass--comments all to keep all the comments, or a valid JavaScript regexp tokeep 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. Forexample:

functionf(){/**@preserve Foo Bar */functiong(){// this function is never called}returnsomething();}

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

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

It enables some transformations thatmight break code logic in certaincontrived cases, but should be fine for most code. It assumes that standardbuilt-in ECMAScript functions and classes have not been altered or replaced.You might want to try it on your own code; it should reduce the minified size.Some examples of the optimizations made when this option is enabled:

• new Array(1, 2, 3) orArray(1, 2, 3) →[ 1, 2, 3 ]
• Array.from([1, 2, 3]) →[1, 2, 3]
• new Object() →{}
• String(exp) orexp.toString() →"" + exp
• new Object/RegExp/Function/Error/Array (...) → we discard thenew
• "foo bar".substr(4) →"bar"

You can use the--define (-d) switch in order to declare globalvariables that Terser will assume to be constants (unless defined inscope). For example if you pass--define DEBUG=false then, coupled withdead code removal Terser will discard the following from the output:

if(DEBUG){console.log("debug stuff");}

You can specify nested constants in the form of--define env.DEBUG=false.

Another way of doing that is to declare your globals as constants in aseparate file and include it into the build. For example you can have abuild/defines.js file with the following:

varDEBUG=false;varPRODUCTION=true;// etc.

and build your code like this:

terser build/defines.js js/foo.js js/bar.js... -c

Terser will notice the constants and, since they cannot be altered, itwill evaluate references to them to the value itself and drop unreachablecode as usual. The build will contain theconst declarations if you usethem. If you are targeting < ES6 environments which does not supportconst,usingvar withreduce_vars (enabled by default) should suffice.

You can also use conditional compilation via the programmatic API. With the difference that theproperty name isglobal_defs and is a compressor property:

varresult=awaitminify(fs.readFileSync("input.js","utf8"),{compress:{dead_code:true,global_defs:{DEBUG:false}}});

To replace an identifier with an arbitrary non-constant expression it isnecessary to prefix theglobal_defs key with"@" to instruct Terserto parse the value as an expression:

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

Otherwise it would be replaced as string literal:

awaitminify("alert('hello');",{compress:{global_defs:{"alert":"console.log"}}}).code;// returns: '"console.log"("hello");'

Annotations in Terser are a way to tell it to treat a certain function call differently. The following annotations are available:

• /*@__INLINE__*/ - forces a function to be inlined somewhere.
• /*@__NOINLINE__*/ - Makes sure the called function is not inlined into the call site.
• /*@__PURE__*/ - Marks a function call as pure. That means, it can safely be dropped.
• /*@__KEY__*/ - Marks a string literal as a property to also mangle it when mangling properties.
• /*@__MANGLE_PROP__*/ - Opts-in an object property (or class field) for mangling, when the property mangler is enabled.

You can use either a@ sign at the start, or a#.

Here are some examples on how to use them:

/*@__INLINE__*/function_always_inlined_here()/*#__NOINLINE__*/function_cant_be_inlined_into_here()constx=/*#__PURE__*/i_am_dropped_if_x_is_not_used()functionlookup(object,key){returnobject[key];}lookup({i_will_be_mangled_too:"bar"},/*@__KEY__*/"i_will_be_mangled_too");

Terser has its own abstract syntax tree format; forpractical reasonswe can't easily change to using the SpiderMonkey AST internally. However,Terser now has a converter which can import a SpiderMonkey AST.

For exampleAcorn is a super-fast parser that produces aSpiderMonkey AST. It has a small CLI utility that parses one file and dumpsthe AST in JSON on the standard output. To use Terser to mangle andcompress that:

acorn file.js | terser -p spidermonkey -m -c

The-p spidermonkey option tells Terser that all input files are notJavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore wedon't use our own parser in this case, but just transform that AST into ourinternal AST.

spidermonkey is also available inminify asparse andformat options toaccept and/or produce a spidermonkey AST.

More for fun, I added the-p acorn option which will use Acorn to do allthe parsing. If you pass this option, Terser willrequire("acorn").

Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), butconverting the SpiderMonkey tree that Acorn produces takes another 150ms soin total it's a bit more than just using Terser's own parser.

It's not well known, but whitespace removal and symbol mangling accountsfor 95% of the size reduction in minified code for most JavaScript - notelaborate code transforms. One can simply disablecompress to speed upTerser builds by 3 to 4 times.

To enable fast minify mode from the CLI use:

terser file.js -m

To enable fast minify mode with the API use:

awaitminify(code,{compress:false,mangle:true});

Variouscompress transforms that simplify, rearrange, inline and remove codeare known to have an adverse effect on debugging with source maps. This isexpected as code is optimized and mappings are often simply not possible assome code no longer exists. For highest fidelity in source map debuggingdisable thecompress option and just usemangle.

When debugging, make sure you enable the"map scopes" feature to map mangled variable names back to their original names.
Without this, all variable values will beundefined. See#1367 for more details.

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

• .toString() and.valueOf() don't have side effects, and for built-inobjects they have not been overridden.
• undefined,NaN andInfinity have not been externally redefined.
• arguments.callee,arguments.caller andFunction.prototype.caller are not used.
• The code doesn't expect the contents ofFunction.prototype.toString() orError.prototype.stack to be anything in particular.
• Getting and setting properties on a plain object does not cause other side effects(using.watch() orProxy).
• Object properties can be added, removed and modified (not prevented withObject.defineProperty(),Object.defineProperties(),Object.freeze(),Object.preventExtensions() orObject.seal()).
• document.all is not== null
• Assigning properties to a class doesn't have side effects and does not throw.

https://www.npmjs.com/browse/depended/terser

A number of JS bundlers and uglify wrappers are still using buggy versionsofuglify-es and have not yet upgraded toterser. If you are usingyarnyou can add the following alias to your project'spackage.json file:

"resolutions":{"uglify-es":"npm:terser"}

to useterser instead ofuglify-es in all deeply nested dependencieswithout changing any code.

Note: for this change to take effect you must run the following commandsto remove the existingyarn lock file and reinstall all packages:

$ rm -rf node_modules yarn.lock$ yarn

You're expected to provide a [minimal reproducible example] of input code that will demonstrate your issue.

To get to this example, you can remove bits of your code and stop if your issue ceases to reproduce.

Because users often don't control the call toawait minify() or its arguments, Terser provides aTERSER_DEBUG_DIR environment variable to make terser output some debug logs.

These logs will contain the input code and options of eachminify() call.

TERSER_DEBUG_DIR=/tmp/terser-log-dir command-that-uses-terserls /tmp/terser-log-dirterser-debug-123456.log

If you're not sure how to set an environment variable on your shell (the above example works in bash), you can try using cross-env:

> npx cross-env TERSER_DEBUG_DIR=/path/to/logs command-that-uses-terser

In the terser CLI we usesource-map-support to produce good error stacks. In your own app, you're expected to enable source-map-support (read their docs) to have nice stack traces that will help you write good issues.

note:You can support this project on patreon: [link]The Terser Patreon is shutting down in favor of opencollective. Check outPATRONS.md for our first-tier patrons.

These are the second-tier patrons. Great thanks for your support!

• CKEditor
• 38elements

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