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A JavaScript mangler/compressor toolkit for ES6+.

*note*: <s>You can support this project on patreon: [link]</s> The Terser Patreon is shutting down in favor of opencollective. Check out PATRONS.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 is *being removed* from terser, we recommend you use prettier.

Find the changelog in CHANGELOG.md

[npm-image]: https://img.shields.io/npm/v/terser.svg

[npm-url]: https://npmjs.org/package/terser

[downloads-image]: https://img.shields.io/npm/dm/terser.svg

[downloads-url]: https://npmjs.org/package/terser

[ci-image]: https://github.com/terser/terser/actions/workflows/ci.yml/badge.svg

[ci-url]: https://github.com/terser/terser/actions/workflows/ci.yml

[opencollective-contributors]: https://opencollective.com/terser/tiers/badge.svg

[opencollective-url]: https://opencollective.com/terser

Why choose terser?

------------------

uglify-es is no longer maintained and uglify-js does not support ES6+.

terser is a fork of uglify-es that mostly retains API and CLI compatibility

with uglify-es and uglify-js@3.

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 terser -g

From NPM for programmatic use:

npm install terser

Command line usage

```

terser [input files] [options]

```

Terser can take multiple input files. It's recommended that you pass the

input files first, then pass the options. Terser 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.

Command line arguments that take options (like --parse, --compress, --mangle and

--format) can take in a comma-separated list of default option overrides. For

instance:

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 with

a double dash to prevent input files being used as option arguments:

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

Command line options

```

-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 output

goes to STDOUT.

CLI source map options

Terser can generate a source map file, which is highly useful for

debugging your compressed JavaScript. To get a source map, pass

--source-map --output output.js (source map will be written out to

output.js.map).

Additional options:

--source-map "filename='<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 HTTP X-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 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). Terser has an

option to take an input source map. Assuming you have a mapping from

CoffeeScript → compiled JS, Terser can generate a map from CoffeeScript →

compressed JS by mapping every token in the compiled JS to its original

location.

To use this feature pass --source-map "content='/path/to/input/source.map'"

or --source-map "content=inline" if the source map is included inline with

the sources.

CLI compress options

You need to pass --compress (-c) to enable the compressor. Optionally

you can pass a comma-separated list of compress options.

Options are in the form foo=bar, or just foo (the latter implies

a boolean option that you want to set true; it's effectively a

shortcut for foo=true).

Example:

terser file.js -c toplevel,sequences=false

CLI mangle options

To enable the mangler you need to pass --mangle (-m). The following

(comma-separated) options are supported:

toplevel (default false) -- mangle names declared in the top level scope.

eval (default false) -- mangle names visible in scopes where eval or with are used.

When mangling is enabled but you want to prevent certain names from being

mangled, you can declare those names with --mangle reserved — pass a

comma-separated list of names. For example:

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

to prevent the require, exports and $ names from being changed.

CLI mangling property names (`--mangle-props`)

Note: THIS WILL 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 dangerous

way to use this is to use the regex option like so:

```

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

```

This will mangle all properties that end with an

underscore. So you can use it to mangle internal methods.

By default, it will mangle all properties in the

input code with the exception of built in DOM properties and properties

in 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 like defineProperty or hasOwnProperty, because they refer to object properties using strings and will break your code if you don't know what you are doing.

An example:

```javascript

// example.js

var x = {

baz_: 0,

foo_: 1,

calc: function() {

return this.foo_ + this.baz_;

}

};

x.bar_ = 2;

x["baz_"] = 3;

console.log(x.calc());

```

Mangle all properties (except for JavaScript builtins) (very unsafe):

```bash

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

```

```javascript

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

```

Mangle all properties except for reserved properties (still very unsafe):

```bash

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

```

```javascript

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

```

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

```bash

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

```

```javascript

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

```

Combining mangle properties options:

```bash

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

```

```javascript

var x={o:3,t:1,calc:function(){return this.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 DOM

API properties by default (--mangle-props builtins to override).

A regular expression can be used to define which property names should be

mangled. For example, --mangle-props regex=/^_/ will only mangle property

names that start with an underscore.

When you compress multiple files using this option, in order for them to

work together in the end we need to ensure somehow that one property gets

mangled to the same name in all of them. For this, pass --name-cache filename.json

and Terser will maintain these mappings in a file which can then be reused.

It should be initially empty. Example:

```bash

$ rm -f /tmp/cache.json # start fresh

$ 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 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 Terser.

Mangling unquoted names (`--mangle-props keep_quoted`)

Using quoted property name (o["foo"]) reserves the property name (foo)

so that it is not mangled throughout the entire script even when used in an

unquoted style (o.foo). Example:

```javascript

// stuff.js

var o = {

"foo": 1,

bar: 3

};

o.foo += o.bar;

console.log(o.foo);

```

```bash

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

```

```javascript

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

```

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 o.foo

would mangle to o._$foo$_ with this option. This allows property mangling

of a large codebase while still being able to debug the code and identify

where mangling is breaking things.

```bash

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

```

```javascript

var o={_$foo$_:1,_$bar$_:3};o._$foo$_+=o._$bar$_,console.log(o._$foo$_);

```

You can also pass a custom suffix using --mangle-props debug=XYZ. This would then

mangle o.foo to o._$foo$XYZ_. You can change this each time you compile a

script to identify how a property got mangled. One technique is to pass a

random number on every compile to simulate mangling changing with different

inputs (e.g. as you update the input script with new properties), and to help

identify mistakes like writing mangled keys to storage.

API Reference

Assuming installation via NPM, you can load Terser in your application

like this:

```javascript

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

```

Or,

```javascript

import { minify } from "terser";

```

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

```html

```

There is an async high level function, async minify(code, options),

which will perform all minification phases in a configurable

manner. By default minify() will enable compress

and mangle. Example:

```javascript

var code = "function add(first, second) { return first + second; }";

var result = await minify(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 a minify_sync() alternative version of it, which returns instantly.

You can minify more than one JavaScript file at a time by using an object

for the first argument where the keys are file names and the values are source

code:

```javascript

var code = {

"file1.js": "function add(first, second) { return first + second; }",

"file2.js": "console.log(add(1 + 2, 3 + 4));"

};

var result = await minify(code);

console.log(result.code);

// function add(d,n){return d+n}console.log(add(3,7));

```

The toplevel option:

```javascript

var code = {

"file1.js": "function add(first, second) { return first + second; }",

"file2.js": "console.log(add(1 + 2, 3 + 4));"

};

var options = { toplevel: true };

var result = await minify(code, options);

console.log(result.code);

// console.log(3+7);

```

The nameCache option:

```javascript

var options = {

mangle: {

toplevel: true,

nameCache: {}

};

var result1 = await minify({

"file1.js": "function add(first, second) { return first + second; }"

}, options);

var result2 = await minify({

"file2.js": "console.log(add(1 + 2, 3 + 4));"

}, options);

console.log(result1.code);

// function n(n,r){return n+r}

console.log(result2.code);

// console.log(n(3,7));

```

You may persist the name cache to the file system in the following way:

```javascript

var cacheFileName = "/tmp/cache.json";

var options = {

mangle: {

properties: true,

nameCache: JSON.parse(fs.readFileSync(cacheFileName, "utf8"))

};

fs.writeFileSync("part1.js", await minify({

"file1.js": fs.readFileSync("file1.js", "utf8"),

"file2.js": fs.readFileSync("file2.js", "utf8")

}, options).code, "utf8");

fs.writeFileSync("part2.js", await minify({

"file3.js": fs.readFileSync("file3.js", "utf8"),

"file4.js": fs.readFileSync("file4.js", "utf8")

}, options).code, "utf8");

fs.writeFileSync(cacheFileName, JSON.stringify(options.nameCache), "utf8");

```

An example of a combination of minify() options:

```javascript

var code = {

"file1.js": "function add(first, second) { return first + second; }",

"file2.js": "console.log(add(1 + 2, 3 + 4));"

};

var options = {

toplevel: true,

compress: {

global_defs: {

"@console.log": "alert"

passes: 2

format: {

preamble: "/* minified */"

}

};

var result = await minify(code, options);

console.log(result.code);

// /* minified */

// alert(10);"

```

An error example:

```javascript

try {

const result = await minify({"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

}

```

Minify options

ecma (default undefined) - pass 5, 2015, 2016, etc to override

compress and format's ecma options.

enclose (default false) - pass true, or a string in the format

of "args[:values]", where args and values are comma-separated

argument names and values, respectively, to embed the output in a big

function with the configurable arguments and values.

parse (default {}) — pass an object if you wish to specify some

additional parse options.

compress (default {}) — pass false to skip compressing entirely.

Pass an object to specify custom compress options.

mangle (default true) — pass false to skip mangling names, or pass

an object to specify mangle options (see below).

- mangle.properties (default false) — a subcategory of the mangle option.

Pass an object to specify custom mangle property options.

module (default false) — Use when minifying an ES6 module. "use strict"

is implied and names can be mangled on the top scope. If compress or

mangle is enabled then the toplevel option will be enabled.

format or output (default null) — pass an object if you wish to specify

additional format options. The defaults are optimized

for best compression.

sourceMap (default false) - pass an object if you wish to specify

source map options.

toplevel (default false) - set to true if you wish to enable top level

variable and function name mangling and to drop unused variables and functions.

nameCache (default null) - pass an empty object {} or a previously

used nameCache object if you wish to cache mangled variable and

property names across multiple invocations of minify(). Note: this is

a read/write property. minify() will read the name cache state of this

object and update it during minification so that it may be

reused or externally persisted by the user.

ie8 (default false) - set to true to support IE8.

keep_classnames (default: undefined) - pass true to prevent discarding or mangling

of class names. Pass a regular expression to only keep class names matching that regex.

keep_fnames (default: false) - pass true to prevent discarding or mangling

of function names. Pass a regular expression to only keep function names matching that regex.

Useful for code relying on Function.prototype.name. If the top level minify option

keep_classnames is undefined it will be overridden with the value of the top level

minify option keep_fnames.

safari10 (default: false) - pass true to work around Safari 10/11 bugs in

loop scoping and await. See safari10 options in mangle

and format for details.

Minify options structure

```javascript

{

parse: {

// parse options

compress: {

// compress options

mangle: {

// mangle options

properties: {

// 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 object

safari10: false,

toplevel: false

}

```

Source map options

To generate a source map:

```javascript

var result = await minify({"file1.js": "var a = function() {};"}, {

sourceMap: {

filename: "out.js",

url: "out.js.map"

}

});

console.log(result.code); // minified output

console.log(result.map); // source map

```

Note that the source map is not saved in a file, it's just returned in

result.map. The value passed for sourceMap.url is only used to set

//# sourceMappingURL=out.js.map in result.code. The value of

filename is only used to set file attribute (see the spec)

in source map file.

You can set option sourceMap.url to be "inline" and source map will

be appended to code.

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

```javascript

var result = await minify({"file1.js": "var a = function() {};"}, {

sourceMap: {

root: "http://example.com/src",

url: "out.js.map"

}

});

```

If you're compressing compiled JavaScript and have a source map for it, you

can use sourceMap.content:

```javascript

var result = await minify({"compiled.js": "compiled code"}, {

sourceMap: {

content: "content from compiled.js.map",

url: "minified.js.map"

}

});

// same as before, it returns code and map

```

If you're using the X-SourceMap header instead, you can just omit sourceMap.url.

If you happen to need the source map as a raw object, set sourceMap.asObject to true.

Parse options

bare_returns (default false) -- support top level return statements

html5_comments (default true)

shebang (default true) -- support #!command as the first line

spidermonkey (default false) -- accept a Spidermonkey (Mozilla) AST

Compress options

defaults (default: true) -- Pass false to disable most default

enabled compress transforms. Useful when you only want to enable a few

compress options while disabling the rest.

arrows (default: true) -- Class and object literal methods are converted

will also be converted to arrow expressions if the resultant code is shorter:

m(){return x} becomes m:()=>x. To do this to regular ES5 functions which

don't use this or arguments, see unsafe_arrows.

arguments (default: false) -- replace arguments[index] with function

parameter 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, also

makes 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 when unsafe_comps), attempts to negate binary

nodes, e.g. a = !b && !c && !d && !e → a=!(b||c||d||e) etc. Note: comparisons

works best with lhs_constants enabled.

computed_props (default: true) -- Transforms constant computed properties

into regular ones: {["computed"]: 1} is converted to {computed: 1}.

conditionals (default: true) -- apply optimizations for if-s and conditional

expressions

dead_code (default: true) -- remove unreachable code

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

drop_console (default: false) -- Pass true to discard calls to

console.* functions. If you only want to discard a portion of console,

you can pass an array like this ['log', 'info'], which will only discard console.log、 console.info.

drop_debugger (default: true) -- remove debugger; statements

ecma (default: 5) -- Pass 2015 or greater to enable compress options that

will transform ES5 code into smaller ES6+ equivalent forms.

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

expression (default: false) -- Pass true to preserve completion values

from terminal statements without return, e.g. in bookmarklets.

global_defs (default: {}) -- see conditional compilation

hoist_funs (default: false) -- hoist function declarations

hoist_props (default: true) -- hoist properties from constant object and

array 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 to f(1, 2);. Note: hoist_props

works best with mangle enabled, the compress option passes set to 2 or higher,

and the compress option toplevel enabled.

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

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

- false -- same as 0

- 0 -- disabled inlining

- 1 -- inline simple functions

- 2 -- inline functions with arguments

- 3 -- inline functions with arguments and variables

- true -- same as 3

join_vars (default: true) -- join consecutive var, let and const statements

keep_classnames (default: false) -- Pass true to prevent the compressor from

discarding class names. Pass a regular expression to only keep class names matching

that regex. See also: the keep_classnames mangle option.

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. Pass a regular expression to only keep

function names matching that regex. Useful for code relying on Function.prototype.name.

See also: the keep_fnames mangle option.

keep_infinity (default: false) -- Pass true to prevent Infinity from

being compressed into 1/0, which may cause performance issues on Chrome.

lhs_constants (default: true) -- Moves constant values to the left-hand side

of binary nodes. foo == 42 → 42 == foo

loops (default: true) -- optimizations for do, while and for loops

when we can statically determine the condition.

module (default false) -- Pass true when compressing an ES6 module. Strict

mode is implied and the toplevel option as well.

negate_iife (default: true) -- negate "Immediately-Called Function Expressions"

where the return value is discarded, to avoid the parens that the

code 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 in

mind more passes will take more time.

properties (default: true) -- rewrite property access using the dot notation, for

example foo["bar"] → foo.bar

pure_funcs (default: null) -- You can pass an array of names and

Terser 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, Terser 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).

pure_getters (default: "strict") -- If you pass true for

this, Terser will assume that object property access

(e.g. foo.bar or foo["bar"]) doesn't have any side effects.

Specify "strict" to treat foo.bar as side-effect-free only when

foo is certain to not throw, i.e. not null or undefined.

pure_new (default: false) -- Set to true to assume new X() never has

side effects.

reduce_vars (default: true) -- Improve optimization on variables assigned with and

used as constant values.

reduce_funcs (default: true) -- Inline single-use functions when

possible. Depends on reduce_vars being enabled. Disabling this option

sometimes improves performance of the output code.

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.

side_effects (default: true) -- Remove expressions which have no side effects

and whose results aren't used.

switches (default: true) -- de-duplicate and remove unreachable switch branches

toplevel (default: false) -- drop unreferenced functions ("funcs") and/or

variables ("vars") in the top level scope (false by default, true to drop

both unreferenced functions and variables)

top_retain (default: null) -- prevent specific toplevel functions and

variables from unused removal (can be array, comma-separated, RegExp or

function. Implies toplevel)

typeofs (default: true) -- Transforms typeof foo == "undefined" into

foo === void 0. Note: recommend to set this value to false for IE10 and

earlier versions due to known issues.

unsafe (default: false) -- apply "unsafe" transformations

(details).

unsafe_arrows (default: false) -- Convert ES5 style anonymous function

expressions to arrow functions if the function body does not reference this.

Note: it is not always safe to perform this conversion if code relies on the

the function having a prototype, which arrow functions lack.

This transform requires that the ecma compress option is set to 2015 or greater.

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. Or if one of two operands is NaN, the result is always

false. Compression only works if both comparisons and

unsafe_comps are both set to true.

unsafe_Function (default: false) -- compress and mangle Function(args, code)

when both args and code are string literals.

unsafe_math (default: false) -- optimize numerical expressions like

2 * x * 3 into 6 * x, which may give imprecise floating point results.

unsafe_symbols (default: false) -- removes keys from native Symbol

declarations, e.g Symbol("kDog") becomes Symbol().

unsafe_methods (default: false) -- Converts { m: function(){} } to

{ m(){} }. ecma must be set to 6 or greater to enable this transform.

If unsafe_methods is a RegExp then key/value pairs with keys matching the

RegExp will be converted to concise methods.

Note: if enabled there is a risk of getting a "<method name> is not a

constructor" TypeError should any code try to new the former function.

unsafe_proto (default: false) -- optimize expressions like

Array.prototype.slice.call(a) into [].slice.call(a)

unsafe_regexp (default: false) -- enable substitutions of variables with

RegExp values the same way as if they are constants.

unsafe_undefined (default: false) -- substitute void 0 if there is a

variable named undefined in scope (variable name will be mangled, typically

reduced to a single character)

unused (default: true) -- drop unreferenced functions and variables (simple

direct variable assignments do not count as references unless set to "keep_assign")

Mangle options

eval (default false) -- Pass true to mangle names visible in scopes

where eval or with are used.

keep_classnames (default false) -- Pass true to not mangle class names.

Pass a regular expression to only keep class names matching that regex.

See also: the keep_classnames compress option.

keep_fnames (default false) -- Pass true to not mangle function names.

Pass a regular expression to only keep function names matching that regex.

Useful for code relying on Function.prototype.name. See also: the keep_fnames

compress option.

module (default false) -- Pass true an ES6 modules, where the toplevel

scope is not the global scope. Implies toplevel and assumes input code is strict mode JS.

nth_identifier (default: an internal mangler that weights based on character

frequency analysis) -- Pass an object with a get(n) function that converts an

ordinal into the nth most favored (usually shortest) identifier.

Optionally also provide reset(), sort(), and consider(chars, delta) to

use character frequency analysis of the source code.

reserved (default []) -- Pass an array of identifiers that should be

excluded from mangling. Example: ["foo", "bar"].

toplevel (default false) -- Pass true to mangle names declared in the

top level scope.

safari10 (default false) -- Pass true to work around the Safari 10 loop

iterator bug

"Cannot declare a let variable twice".

Mangle properties options

builtins (default: false) — Use true to allow the mangling of builtin

DOM 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": ...} and obj["prop"]) controls what gets mangled.

- "strict" (recommended) -- obj.prop is mangled.

- false -- obj["prop"] is mangled.

- true -- obj.prop is mangled unless there is obj["prop"] elsewhere in the code.

nth_identifier (default: an internal mangler that weights based on character

frequency analysis) -- Pass an object with a get(n) function that converts an

ordinal into the nth most favored (usually shortest) identifier.

Optionally also provide reset(), sort(), and consider(chars, delta) to

use character frequency analysis of the source code.

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

reserved (default: []) — Do not mangle property names listed in the

reserved array.

undeclared (default: false) - Mangle those names when they are accessed

as properties of known top level variables but their declarations are never

found in input code. May be useful when only minifying parts of a project.

See #397 for more details.

Format options

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

as "output options".

ascii_only (default false) -- escape Unicode characters in strings and

regexps (affects directives with non-ascii characters becoming invalid)

beautify (default false) -- (DEPRECATED) whether to beautify the output.

When using the legacy -b CLI flag, this is set to true by default.

braces (default false) -- always insert braces in if, for,

do, while or with statements, even if their body is a single

statement.

comments (default "some") -- by default it keeps JSDoc-style comments

that contain "@license", "@copyright", "@preserve" or start with !, pass true

or "all" to preserve all comments, false to omit comments in the output,

a regular expression string (e.g. /^!/) or a function.

ecma (default 5) -- set desired EcmaScript standard version for output.

Set ecma to 2015 or greater to emit shorthand object properties - i.e.:

{a} instead of {a: a}. The ecma option will only change the output in

direct control of the beautifier. Non-compatible features in your input will

still be output as is. For example: an ecma setting of 5 will not

convert modern code to ES5.

indent_level (default 4)

indent_start (default 0) -- prefix all lines by that many spaces

inline_script (default true) -- escape HTML comments and the slash in

occurrences of </script> in strings

keep_numbers (default false) -- keep number literals as it was in original code

(disables optimizations like converting 1000000 into 1e6)

keep_quoted_props (default false) -- when turned on, prevents stripping

quotes from property names in object literals.

max_line_len (default false) -- maximum line length (for minified code)

preamble (default null) -- when passed it must be a string and

it will be prepended to the output literally. The source map will

adjust for this text. Can be used to insert a comment containing

licensing information, for example.

quote_keys (default false) -- pass true to quote all keys in literal

objects

quote_style (default 0) -- preferred quote style for strings (affects

quoted property names and directives as well):

- 0 -- prefers double quotes, switches to single quotes when there are

more double quotes in the string itself. 0 is best for gzip size.

- 1 -- always use single quotes

- 2 -- always use double quotes

- 3 -- always use the original quotes

preserve_annotations -- (default false) -- Preserve Terser annotations in the output.

safari10 (default false) -- set this option to true to work around

the Safari 10/11 await bug.

Miscellaneous

Keeping copyright notices or other comments

You can pass --comments to retain certain comments in the output. By

default it will keep comments starting with "!" and JSDoc-style comments that

contain "@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 to

keep only comments that match this regexp. For example --comments /^!/

will keep comments like /*! Copyright Notice */.

Note, however, that there might be situations where comments are lost. For

example:

```javascript

function f() {

/** @preserve Foo Bar */

function g() {

// this function is never called

}

return something();

}

```

Even though it has "@preserve", the comment will be lost because the inner

function g (which is the AST node to which the comment is attached to) is

discarded by the compressor as not referenced.

The safest comments where to place copyright information (or other info that

needs to be kept in the output) are comments attached to toplevel nodes.

The `unsafe` `compress` option

It enables some transformations that *might* break code logic in certain

contrived cases, but should be fine for most code. It assumes that standard

built-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) or Array(1, 2, 3) → [ 1, 2, 3 ]
Array.from([1, 2, 3]) → [1, 2, 3]
new Object() → {}
String(exp) or exp.toString() → "" + exp
new Object/RegExp/Function/Error/Array (...) → we discard the new
"foo bar".substr(4) → "bar"

Conditional compilation

You can use the --define (-d) switch in order to declare global

variables that Terser will assume to be constants (unless defined in

scope). For example if you pass --define DEBUG=false then, coupled with

dead code removal Terser 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.

Another way of doing that is to declare your globals as constants in a

separate file and include it into the build. For example you can have a

build/defines.js file with the following:

```javascript

var DEBUG = false;

var PRODUCTION = true;

// etc.

```

and build your code like this:

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

Terser 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:

```javascript

var result = await minify(fs.readFileSync("input.js", "utf8"), {

compress: {

dead_code: true,

global_defs: {

DEBUG: false

}

});

```

To replace an identifier with an arbitrary non-constant expression it is

necessary to prefix the global_defs key with "@" to instruct Terser

to parse the value as an expression:

```javascript

await minify("alert('hello');", {

compress: {

global_defs: {

"@alert": "console.log"

}

}).code;

// returns: 'console.log("hello");'

```

Otherwise it would be replaced as string literal:

```javascript

await minify("alert('hello');", {

compress: {

global_defs: {

"alert": "console.log"

}

}).code;

// returns: '"console.log"("hello");'

```

Annotations

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:

```javascript

/*@__INLINE__*/

function_always_inlined_here()

/*#__NOINLINE__*/

function_cant_be_inlined_into_here()

const x = /*#__PURE__*/i_am_dropped_if_x_is_not_used()

function lookup(object, key) { return object[key]; }

lookup({ i_will_be_mangled_too: "bar" }, /*@__KEY__*/ "i_will_be_mangled_too");

```

ESTree / SpiderMonkey AST

Terser has its own abstract syntax tree format; for

practical reasons

we can't easily change to using the SpiderMonkey AST internally. However,

Terser now has a converter which can import a SpiderMonkey AST.

For example Acorn is a super-fast parser that produces a

SpiderMonkey AST. It has a small CLI utility that parses one file and dumps

the AST in JSON on the standard output. To use Terser to mangle and

compress that:

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

The -p spidermonkey option tells Terser that all input files are not

JavaScript, but JS code described in SpiderMonkey AST in JSON. Therefore we

don't use our own parser in this case, but just transform that AST into our

internal AST.

spidermonkey is also available in minify as parse and format options to

accept and/or produce a spidermonkey AST.

Use Acorn for parsing

More for fun, I added the -p acorn option which will use Acorn to do all

the parsing. If you pass this option, Terser will require("acorn").

Acorn is really fast (e.g. 250ms instead of 380ms on some 650K code), but

converting the SpiderMonkey tree that Acorn produces takes another 150ms so

in total it's a bit more than just using Terser's own parser.

Terser Fast Minify Mode

It's not well known, but whitespace removal and symbol mangling accounts

for 95% of the size reduction in minified code for most JavaScript - not

elaborate code transforms. One can simply disable compress to speed up

Terser builds by 3 to 4 times.

| --- | ---: | ---: | ---: |

| original | 451,131 | 108,733 | - |

| terser@3.7.5 mangle=false, compress=false | 316,600 | 85,245 | 0.82 |

| terser@3.7.5 mangle=true, compress=false | 220,216 | 72,730 | 1.45 |

| terser@3.7.5 mangle=true, compress=true | 212,046 | 70,954 | 5.87 |

| babili@0.1.4 | 210,713 | 72,140 | 12.64 |

| babel-minify@0.4.3 | 210,321 | 72,242 | 48.67 |

| babel-minify@0.5.0-alpha.01eac1c3 | 210,421 | 72,238 | 14.17 |

To enable fast minify mode from the CLI use:

```

terser file.js -m

```

To enable fast minify mode with the API use:

```js

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

```

#### Source maps and debugging

Various compress transforms that simplify, rearrange, inline and remove code

are known to have an adverse effect on debugging with source maps. This is

expected as code is optimized and mappings are often simply not possible as

some code no longer exists. For highest fidelity in source map debugging

disable the compress option and just use mangle.

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 be undefined. See https://github.com/terser/terser/issues/1367 for more details.

!image

Compiler assumptions

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

.toString() and .valueOf() don't have side effects, and for built-in

objects they have not been overridden.

undefined, NaN and Infinity have not been externally redefined.
arguments.callee, arguments.caller and Function.prototype.caller are not used.
The code doesn't expect the contents of Function.prototype.toString() or

Error.prototype.stack to be anything in particular.

Getting and setting properties on a plain object does not cause other side effects

(using .watch() or Proxy).

Object properties can be added, removed and modified (not prevented with

Object.defineProperty(), Object.defineProperties(), Object.freeze(),

Object.preventExtensions() or Object.seal()).

document.all is not == null
Assigning properties to a class doesn't have side effects and does not throw.

Build Tools and Adaptors using Terser

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

Replacing `uglify-es` with `terser` in a project using `yarn`

A number of JS bundlers and uglify wrappers are still using buggy versions

of uglify-es and have not yet upgraded to terser. If you are using yarn

you can add the following alias to your project's package.json file:

```js

"resolutions": {

"uglify-es": "npm:terser"

}

```

to use terser instead of uglify-es in all deeply nested dependencies

without changing any code.

Note: for this change to take effect you must run the following commands

to remove the existing yarn lock file and reinstall all packages:

```

$ rm -rf node_modules yarn.lock

$ yarn

```

Reporting issues

A minimal, reproducible example

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.

Obtaining the source code given to Terser

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

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

```bash

TERSER_DEBUG_DIR=/tmp/terser-log-dir command-that-uses-terser

ls /tmp/terser-log-dir

terser-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

```

Stack traces

In the terser CLI we use source-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.

README.md Patrons:

*note*: <s>You can support this project on patreon: [link]</s> The Terser Patreon is shutting down in favor of opencollective. Check out PATRONS.md for our first-tier patrons.

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

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Contributors

Code Contributors

This project exists thanks to all the people who contribute. [Contribute].

Financial Contributors

Become a financial contributor and help us sustain our community. [Contribute]

#### Individuals

#### Organizations

Support this project with your organization. Your logo will show up here with a link to your website. [Contribute]

README.​md

Command line usage

Command line options

CLI source map options

Composed source map

CLI compress options

CLI mangle options

CLI mangling property names (--mangle-props)

Mangling unquoted names (--mangle-props keep_quoted)

Debugging property name mangling

API Reference

Minify options

Minify options structure

Source map options

Parse options

Compress options

Mangle options

Mangle properties options

Format options

Miscellaneous

Keeping copyright notices or other comments

The unsafe compress option

Conditional compilation

Conditional compilation API

Annotations

ESTree / SpiderMonkey AST

Use Acorn for parsing

Terser Fast Minify Mode

Compiler assumptions

Build Tools and Adaptors using Terser

Replacing uglify-es with terser in a project using yarn

Reporting issues

A minimal, reproducible example

Obtaining the source code given to Terser

Stack traces

README.md Patrons:

Contributors

Code Contributors

Financial Contributors

README.md

CLI mangling property names (`--mangle-props`)

Mangling unquoted names (`--mangle-props keep_quoted`)

The `unsafe` `compress` option

Replacing `uglify-es` with `terser` in a project using `yarn`