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 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

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uglify-es is no longer maintained and uglify-js does not support ES6+.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

// 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):

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

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):

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

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):

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

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

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:

$ 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:

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

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

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.

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

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:

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

Or,

import { minify } from "terser";

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

<script src="https://cdn.jsdelivr.net/npm/source-map@0.7.3/dist/source-map.js"></script>
<script src="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 minification phases in a configurable manner. By default minify() will enable compress and mangle. Example:

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:

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:

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:

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:

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:

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:

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

{
    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:

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:

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:

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. 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". See also: the safari10 format option.

Examples:

// test.js
var globalVar;
function funcName(firstLongName, anotherLongName) {
    var myVariable = firstLongName +  anotherLongName;
}

var code = fs.readFileSync("test.js", "utf8");

await minify(code).code;
// 'function funcName(a,n){}var globalVar;'

await minify(code, { mangle: { reserved: ['firstLongName'] } }).code;
// 'function funcName(firstLongName,a){}var globalVar;'

await minify(code, { mangle: { toplevel: true } }).code;
// 'function n(n,a){}var a;'

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. See also: the safari10 mangle option.

• semicolons (default true) -- separate statements with semicolons. If you pass false then whenever possible we will use a newline instead of a semicolon, leading to more readable output of minified code (size before gzip could be smaller; size after gzip insignificantly larger).

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

• spidermonkey (default false) -- produce a Spidermonkey (Mozilla) AST

• webkit (default false) -- enable workarounds for WebKit bugs. PhantomJS users should set this option to true.

• wrap_iife (default false) -- pass true to wrap immediately invoked function expressions. See #640 for more details.

• wrap_func_args (default true) -- pass false if you do not want to wrap function expressions that are passed as arguments, in parenthesis. See OptimizeJS for more details.

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:

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:

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:

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:

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:

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

Otherwise it would be replaced as string literal:

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:

/*@__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.

To enable fast minify mode from the CLI use:

terser file.js -m

To enable fast minify mode with the API use:

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.

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:

  "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.

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: You can support this project on patreon: [link] 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!

• CKEditor
• 38elements

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]

Changelog

v5.39.0

• Remove unnecessary console.assert calls (#1590)

v5.38.2

• internal: Flatten inheritance tree for object/class members

v5.38.1

• Fix inlining non-call expressions into an optional_call?.()

v5.38.0

• Remove console method-of-method calls (eg console.log.apply()) when drop_console option is used (#1585)
• Remove more object spreads, such as { ...void !0 } (#1142)

v5.37.0

• Reserved object properties from chrome extensions (domprops)
• Fix semicolon insertion between a class property without a semicolon a and a computed class property ["prop"]

v5.36.0

• Support import attributes with syntax

v5.35.0

• Ensure parent directory exists when using --output on CLI (#1530)

v5.34.1

• bump the rollup devDependency to disable CVE warnings (Terser was not affected)

v5.34.0

• internal: stop assigning properties to objects they don't belong in
• internal: run compress tests in parallel
• drop_console: emit an empty function if the return value of console.METHOD(...) may be called.

v5.33.0

• reduce_vars improved when dealing with hoisted function definitions (#1544)

v5.32.0

• import("module") can now be input and output from ESTree AST (#1557)
• BigInt literals can now be input and output from ESTree AST (#1555)
• typeof an object or array (typeof {} and typeof []) can now be statically evaluated. (#1546)

v5.31.6

• Retain side effects in a case when the expression is a sequence (comma) expression

v5.31.5

• Revert v5.31.4, which created mysterious issues #1548, #1549

v5.31.4 (reverted)

• drop_unused: drop classes which only have side effects in the extends part

v5.31.3

• drop_unused: drop unused parameters from IIFEs in some more situations.

v5.31.2

• drop_unused: scan variables in self-referential class declarations that contain side effects.
• Don't add parens to arrow function when it's the default for an argument (#1540)
• Update domprops (#1538)

v5.31.1

• Allow drop-unused to drop the whole assignment (not just the assigned name) in more situations, in order to avoid duplication of long strings.

v5.31.0

• Sync up property mangler exceptions with current contents of Firefox and Chrome environments
• Add more webcomponent properties to property mangler exceptions (#1525)
• Drop non-nullish constants in ...spreads in objects (#1141)

v5.30.4

• Fix parsing #private in ... when next to other operators

v5.30.3

• Fix precedence of #private in ... operator

v5.30.2

• Avoid optimizations inside computed keys, because they can cause js-engine-specific bugs.

v5.30.1

• Removed useless \ escapes for non-ascii characters
• Make modern identifier characters quoted for older environments (#1512)

v5.30.0

• Improve removal of classes referring to themselves

v5.29.2

• Make sure 'computed_props' creates string keys
• Take into account the evaluated size when inlining

v5.29.1

• fix optimisation of all-bits mask check

v5.29.0

• Re-releases previously reverted 5.28.0
• Fix crash while optimizing some bitwise ops
• (internal) Remove needless wrapper for from_moz (#1499)

v5.28.1

(hotfix release)

• Reverts v5.28.0

v5.28.0

• Optimise redundant or shrinkable bitwise operations (|, ^, &, >>, <<)
• Evaluate some BigInt math operations

v5.27.2

• Recognise this as a reference to the surrounding class in drop_unused. Closes #1472

v5.27.1

• Fixed case where collapse_vars inlines await expressions into non-async functions.

v5.27.0

• Created minify_sync() alternative to minify() since there's no async code left.

v5.26.0

• Do not take the /*#__PURE__*/ annotation into account when the side_effects compress option is off.
• The preserve_annotations option now automatically opts annotation comments in, instead of requiring the comments option to be configured for this.
• Refuse to parse empty parenthesized expressions (())

v5.25.0

• Regex properties added to reserved property mangler (#1471)
• pure_new option added to drop unused new expressions.

v5.24.0

• Improve formatting performance in V8 by keeping a small work string and a large output string

v5.23.0

• When top_retain will keep a variable assignment around, inline the assignee when it's shorter than the name (#1434)
• Remove empty class static {} blocks.

v5.22.0

• Do not unsafely shorten expressions like a?.toString() when they're conditional.
• Avoid running drop_unused in nodes that aren't scopes. Fixes a rare crash.
• When 'module' is enabled, assume strict mode when figuring out scopes.

v5.21.0

• Do not inline functions that would be retained in the toplevel (as this would cause code duplication).
• Fix precedence of arrow function and ternary operator when formatting output.

v5.20.0

• Passing minify() zero files will now throw a clean exception (#1450)
• drop_console supports passing in an array of console.* method names (#1445)
• New DOM properties from the WebGPU API have been added for use in the property mangler (#1436)
• Internal code simplification (#1437)

v5.19.4

• Prevent creating very deeply nested ternaries from a long list of if..return
• Prevent inlining classes into other functions, to avoid constructors being compared.

v5.19.3

• Fix side effect detection of optional?.chains.
• Add roundRect to domprops.js (#1426)

v5.19.2

• fix performance hit from avoiding HTML comments in the output

v5.19.1

• Better avoid outputting </script> and HTML comments.
• Fix unused variables in class static blocks not being dropped correctly.
• Fix sourcemap names of methods that are async or static

v5.19.0

• Allow /*@__MANGLE_PROP__*/ annotation in object.property, in addition to property declarations.

v5.18.2

• Stop using recursion in hoisted defuns fix.

v5.18.1

• Fix major performance issue caused by hoisted defuns' scopes bugfix.

v5.18.0

• Add new /*@__MANGLE_PROP__*/ annotation, to mark properties that should be mangled.

v5.17.7

• Update some dependencies
• Add consistent sorting for v RegExp flag
• Add inert DOM attribute to domprops

v5.17.6

• Fixes to mozilla AST input and output, for class properties, private properties and static blocks
• Fix outputting a shorthand property in quotes when safari10 and ecma=2015 options are enabled
• configurable and enumerable, used in Object.defineProperty, added to domprops (#1393)

v5.17.5

• Take into account the non-deferred bits of a class, such as static properties, while dropping unused code.

v5.17.4

• Fix crash when trying to negate a class (!class{})
• Avoid outputting comments between yield/await and its argument
• Fix detection of left-hand-side of assignment, to avoid optimizing it like any other expression in some edge cases

v5.17.3

• Fix issue with trimming a static class property's contents accessing the class as this.

v5.17.2

• Be less conservative when detecting use-before-definition of var in hoisted functions.
• Support unusual (but perfectly valid) initializers of for-in and for-of loops.
• Fix issue where hoisted function would be dropped if it was after a continue statement

v5.17.1

• Fix evaluating .length when the source array might've been mutated

v5.17.0

• Drop vestigial = undefined default argument in IIFE calls (#1366)
• Evaluate known arrays' .length property when statically determinable
• Add @__KEY__ annotation to mangle string literals (#1365)

v5.16.9

• Fix parentheses in output of optional chains (a?.b) (#1374)
• More documentation on source maps (#1368)
• New lhs_constants option, allowing to stop Terser from swapping comparison operands (#1361)

v5.16.8

• Become even less conservative around function definitions for reduce_vars
• Fix parsing context of import.meta expressions such that method calls are allowed

v5.16.6

• Become less conservative with analyzing function definitions for reduce_vars
• Parse import.meta as a real AST node and not an object.property

v5.16.5

• Correctly handle AST transform functions that mutate children arrays
• Don't mutate the options object passed to Terser (#1342)
• Do not treat BigInt like a number

v5.16.4

• Keep (defaultArg = undefined) => ..., because default args don't count for function length
• Prevent inlining variables into ?. optional chains
• Avoid removing unused arguments while transforming
• Optimize iterating AST node lists
• Make sure catch and finally aren't children of try in the AST
• Use modern unicode property escapes (\p{...}) to parse identifiers when available

v5.16.3

• Ensure function definitions, don't assume the values of variables defined after them.

v5.16.2

• Fix sourcemaps with non-ascii characters (#1318)
• Support string module name and export * as (#1336)
• Do not move let out of for initializers, as it can change scoping
• Fix a corner case that would generate the invalid syntax if (something) let x ("let" in braceless if body)
• Knowledge of more native object properties (#1330)
• Got rid of Travis (#1323)
• Added semi-secret asObject sourcemap option to typescript defs (#1321)

v5.16.1

• Properly handle references in destructurings (const { [reference]: val } = ...)
• Allow parsing of .#privatefield in nested classes
• Do not evaluate operations that return large strings if that would make the output code larger
• Make collapse_vars handle block scope correctly
• Internal improvements: Typos (#1311), more tests, small-scale refactoring

v5.16.0

• Disallow private fields in object bodies (#1011)
• Parse #privatefield in object (#1279)
• Compress #privatefield in object

v5.15.1

• Fixed missing parentheses around optional chains
• Avoid bare let or const as the bodies of if statements (#1253)
• Small internal fixes (#1271)
• Avoid inlining a class twice and creating two equivalent but !== classes.

v5.15.0

• Basic support for ES2022 class static initializer blocks.
• Add AudioWorkletNode constructor options to domprops list (#1230)
• Make identity function inliner not inline id(...expandedArgs)

v5.14.2

• Security fix for RegExps that should not be evaluated (regexp DDOS)
• Source maps improvements (#1211)
• Performance improvements in long property access evaluation (#1213)

v5.14.1

• keep_numbers option added to TypeScript defs (#1208)
• Fixed parsing of nested template strings (#1204)

v5.14.0

• Switched to @jridgewell/source-map for sourcemap generation (#1190, #1181)
• Fixed source maps with non-terminated segments (#1106)
• Enabled typescript types to be imported from the package (#1194)
• Extra DOM props have been added (#1191)
• Delete the AST while generating code, as a means to save RAM

v5.13.1

• Removed self-assignments (varname=varname) (closes #1081)
• Separated inlining code (for inlining things into references, or removing IIFEs)
• Allow multiple identifiers with the same name in var destructuring (eg var { a, a } = x) (#1176)

v5.13.0

• All calls to eval() were removed (#1171, #1184)
• source-map was updated to 0.8.0-beta.0 (#1164)
• NavigatorUAData was added to domprops to avoid property mangling (#1166)

v5.12.1

• Fixed an issue with function definitions inside blocks (#1155)
• Fixed parens of new in some situations (closes #1159)

v5.12.0

• TERSER_DEBUG_DIR environment variable
• @copyright comments are now preserved with the comments="some" option (#1153)

v5.11.0

• Unicode code point escapes (\u{abcde}) are not emitted inside RegExp literals anymore (#1147)
• acorn is now a regular dependency

v5.10.0

• Massive optimization to max_line_len (#1109)
• Basic support for import assertions
• Marked ES2022 Object.hasOwn as a pure function
• Fix delete optional?.property
• New CI/CD pipeline with github actions (#1057)
• Fix reordering of switch branches (#1092), (#1084)
• Fix error when creating a class property called get
• Acorn dependency is now an optional peerDependency
• Fix mangling collision with exported variables (#1072)
• Fix an issue with return someVariable = (async () => { ... })() (#1073)

v5.9.0

• Collapsing switch cases with the same bodies (even if they're not next to each other) (#1070).
• Fix evaluation of optional chain expressions (#1062)
• Fix mangling collision in ESM exports (#1063)
• Fix issue with mutating function objects after a second pass (#1047)
• Fix for inlining object spread { ...obj } (#1071)
• Typescript typings fix (#1069)

v5.8.0

• Fixed shadowing variables while moving code in some cases (#1065)
• Stop mangling computed & quoted properties when keep_quoted is enabled.
• Fix for mangling private getter/setter and .#private access (#1060, #1068)
• Array.from has a new optimization when the unsafe option is set (#737)
• Mangle/propmangle let you generate your own identifiers through the nth_identifier option (#1061)
• More optimizations to switch statements (#1044)

v5.7.2

• Fixed issues with compressing functions defined in global_defs option (#1036)
• New recipe for using Terser in gulp was added to RECIPES.md (#1035)
• Fixed issues with ?? and ?. (#1045)
• Future reserved words such as package no longer require you to disable strict mode to be used as names.
• Refactored huge compressor file into multiple more focused files.
• Avoided unparenthesized in operator in some for loops (it breaks parsing because of for..in loops)
• Improved documentation (#1021, #1025)
• More type definitions (#1021)

v5.7.1

• Avoided collapsing assignments together if it would place a chain assignment on the left hand side, which is invalid syntax (a?.b = c)
• Removed undefined from object expansions ({ ...void 0 } -> {})
• Fix crash when checking if something is nullish or undefined (#1009)
• Fixed comparison of private class properties (#1015)
• Minor performance improvements (#993)
• Fixed scope of function defs in strict mode (they are block scoped)

v5.7.0

• Several compile-time evaluation and inlining fixes
• Allow reduce_funcs to be disabled again.
• Add spidermonkey options to parse and format (#974)
• Accept {get = "default val"} and {set = "default val"} in destructuring arguments.
• Change package.json export map to help require.resolve (#971)
• Improve docs
• Fix export default of an anonymous class with extends

v5.6.1

• Mark assignments to the .prototype of a class as pure
• Parenthesize await on the left of ** (while accepting legacy non-parenthesised input)
• Avoided outputting NUL bytes in optimized RegExps, to stop the output from breaking other tools
• Added exports to domprops (#939)
• Fixed a crash when spreading ...this
• Fixed the computed size of arrow functions, which improves their inlining

v5.6.0

• Added top-level await
• Beautify option has been removed in #895
• Private properties, getters and setters have been added in #913 and some more commits
• Docs improvements: #896, #903, #916

v5.5.1

• Fixed object properties with unicode surrogates on safari.

v5.5.0

• Fixed crash when inlining uninitialized variable into template string.
• The sourcemap for dist was removed for being too large.

v5.4.0

• Logical assignment
• Change let x = undefined to just let x
• Removed some optimizations for template strings, placing them behind unsafe options. Reason: adding strings is not equivalent to template strings, due to valueOf differences.
• The AST_Token class was slimmed down in order to use less memory.

v5.3.8

• Restore node 13 support

v5.3.7

Hotfix release, fixes package.json "engines" syntax

v5.3.6

• Fixed parentheses when outputting ?? mixed with || and &&
• Improved hygiene of the symbol generator

v5.3.5

• Avoid moving named functions into default exports.
• Enabled transform() for chain expressions. This allows AST transformers to reach inside chain expressions.

v5.3.4

• Fixed a crash when hoisting (with hoist_vars) a destructuring variable declaration

v5.3.3

• source-map library has been updated, bringing memory usage and CPU time improvements when reading input source maps (the SourceMapConsumer is now WASM based).
• The wrap_func_args option now also wraps arrow functions, as opposed to only function expressions.

v5.3.2

• Prevented spread operations from being expanded when the expanded array/object contains getters, setters, or array holes.
• Fixed very slow self-recursion in some cases of removing extraneous parentheses from + operations.

v5.3.1

• An issue with destructuring declarations when pure_getters is enabled has been fixed
• Fixed a crash when chain expressions need to be shallowly compared
• Made inlining functions more conservative to make sure a function that contains a reference to itself isn't moved into a place that can create multiple instances of itself.

v5.3.0

• Fixed a crash when compressing object spreads in some cases
• Fixed compiletime evaluation of optional chains (caused typeof a?.b to always return "object")
• domprops has been updated to contain every single possible prop

v5.2.1

• The parse step now doesn't accept an ecma option, so that all ES code is accepted.
• Optional dotted chains now accept keywords, just like dotted expressions (foo?.default)

v5.2.0

• Optional chaining syntax is now supported.
• Consecutive await expressions don't have unnecessary parens
• Taking the variable name's length (after mangling) into consideration when deciding to inline

v5.1.0

• import.meta is now supported
• Typescript typings have been improved

v5.0.0

• in operator now taken into account during property mangle.
• Fixed infinite loop in face of a reference loop in some situations.
• Kept exports and imports around even if there's something which will throw before them.
• The main exported bundle for commonjs, dist/bundle.min.js is no longer minified.

v5.0.0-beta.0

• BREAKING: minify() is now async and rejects a promise instead of returning an error.
• BREAKING: Internal AST is no longer exposed, so that it can be improved without releasing breaking changes.
• BREAKING: Lowest supported node version is 10
• BREAKING: There are no more warnings being emitted
• Module is now distributed as a dual package - You can import and require() too.
• Inline improvements were made

v4.8.1 (backport)

• Security fix for RegExps that should not be evaluated (regexp DDOS)

v4.8.0

• Support for numeric separators (million = 1_000_000) was added.
• Assigning properties to a class is now assumed to be pure.
• Fixed bug where yield wasn't considered a valid property key in generators.

v4.7.0

• A bug was fixed where an arrow function would have the wrong size
• arguments object is now considered safe to retrieve properties from (useful for length, or 0) even when pure_getters is not set.
• Fixed erroneous const declarations without value (which is invalid) in some corner cases when using collapse_vars.

v4.6.13

• Fixed issue where ES5 object properties were being turned into ES6 object properties due to more lax unicode rules.
• Fixed parsing of BigInt with lowercase e in them.

v4.6.12

• Fixed subtree comparison code, making it see that [1,[2, 3]] is different from [1, 2, [3]]
• Printing of unicode identifiers has been improved

v4.6.11

• Read unused classes' properties and method keys, to figure out if they use other variables.
• Prevent inlining into block scopes when there are name collisions
• Functions are no longer inlined into parameter defaults, because they live in their own special scope.
• When inlining identity functions, take into account the fact they may be used to drop this in function calls.
• Nullish coalescing operator (x ?? y), plus basic optimization for it.
• Template literals in binary expressions such as + have been further optimized

v4.6.10

• Do not use reduce_vars when classes are present

v4.6.9

• Check if block scopes actually exist in blocks

v4.6.8

• Take into account "executed bits" of classes like static properties or computed keys, when checking if a class evaluation might throw or have side effects.

v4.6.7

• Some new performance gains through a AST_Node.size() method which measures a node's source code length without printing it to a string first.
• An issue with setting --comments to false in the CLI has been fixed.
• Fixed some issues with inlining
• unsafe_symbols compress option was added, which turns Symbol("name") into just Symbol()
• Brought back compress performance improvement through the AST_Node.equivalent_to(other) method (which was reverted in v4.6.6).

v4.6.6

(hotfix release)

• Reverted code to 4.6.4 to allow for more time to investigate an issue.

v4.6.5 (REVERTED)

• Improved compress performance through using a new method to see if two nodes are equivalent, instead of printing them to a string.

v4.6.4

• The "some" value in the comments output option now preserves @lic and other important comments when using //
• </script> is now better escaped in regex, and in comments, when using the inline_script output option
• Fixed an issue when transforming new RegExp into /.../ when slashes are included in the source
• AST_Node.prototype.constructor now exists, allowing for easier debugging of crashes
• Multiple if statements with the same consequents are now collapsed
• Typescript typings improvements
• Optimizations while looking for surrogate pairs in strings

v4.6.3

• Annotations such as /*#__NOINLINE__*/ and /*#__PURE__*/ may now be preserved using the preserve_annotations output option
• A TypeScript definition update for the keep_quoted output option.

v4.6.2

• A bug where functions were inlined into other functions with scope conflicts has been fixed.
• /*#__NOINLINE__*/ annotation fixed for more use cases where inlining happens.

v4.6.1

• Fixed an issue where a class is duplicated by reduce_vars when there's a recursive reference to the class.

v4.6.0

• Fixed issues with recursive class references.
• BigInt evaluation has been prevented, stopping Terser from evaluating BigInts like it would do regular numbers.
• Class property support has been added

v4.5.1

(hotfix release)

• Fixed issue where () => ({})[something] was not parenthesised correctly.

v4.5.0

• Inlining has been improved
• An issue where keep_fnames combined with functions declared through variables was causing name shadowing has been fixed
• You can now set the ES version through their year
• The output option keep_numbers has been added, which prevents Terser from turning 1000 into 1e3 and such
• Internal small optimisations and refactors

v4.4.3

• Number and BigInt parsing has been fixed
• /*#__INLINE__*/ annotation fixed for arrow functions with non-block bodies.
• Functional tests have been added, using this repository.
• A memory leak, where the entire AST lives on after compression, has been plugged.

v4.4.2

• Fixed a problem with inlining identity functions

v4.4.1

note: This introduced a feature, therefore it should have been a minor release.

• Fixed a crash when unsafe was enabled.
• An issue has been fixed where let statements might be collapsed out of their scope.
• Some error messages have been improved by adding quotes around variable names.

v4.4.0

• Added /*#__INLINE__*/ and /*#__NOINLINE__*/ annotations for calls. If a call has one of these, it either forces or forbids inlining.

v4.3.11

• Fixed a problem where window was considered safe to access, even though there are situations where it isn't (Node.js, workers...)
• Fixed an error where ++ and -- were considered side-effect free
• Number(x) now needs both unsafe and and unsafe_math to be compressed into +x because x might be a BigInt
• keep_fnames now correctly supports regexes when the function is in a variable declaration

v4.3.10

• Fixed syntax error when repeated semicolons were encountered in classes
• Fixed invalid output caused by the creation of empty sequences internally
• Scopes are now updated when scopes are inlined into them

v4.3.9

• Fixed issue with mangle's keep_fnames option, introduced when adding code to keep variable names of anonymous functions

v4.3.8

• Typescript typings fix

v4.3.7

• Parsing of regex options in the CLI (which broke in v4.3.5) was fixed.
• typescript definition updates

v4.3.6

(crash hotfix)

v4.3.5

• Fixed an issue with DOS line endings strings separated by \ and a new line.
• Improved fix for the output size regression related to unused references within the extends section of a class.
• Variable names of anonymous functions (eg: const x = () => { ... } or var func = function () {...}) are now preserved when keep_fnames is true.
• Fixed performance degradation introduced for large payloads in v4.2.0

v4.3.4

• Fixed a regression where the output size was increased when unused classes were referred to in the extends clause of a class.
• Small typescript typings fixes.
• Comments with @preserve, @license, @cc_on as well as comments starting with /*! and /**! are now preserved by default.

v4.3.3

• Fixed a problem where parsing template strings would mix up octal notation and a slash followed by a zero representing a null character.
• Started accepting the name async in destructuring arguments with default value.
• Now Terser takes into account side effects inside class extends clauses.
• Added parens whenever there's a comment between a return statement and the returned value, to prevent issues with ASI.
• Stopped using raw RegExp objects, since the spec is going to continue to evolve. This ensures Terser is able to process new, unknown RegExp flags and features. This is a breaking change in the AST node AST_RegExp.

v4.3.2

• Typescript typing fix
• Ensure that functions can't be inlined, by reduce_vars, into places where they're accessing variables with the same name, but from somewhere else.

v4.3.1

• Fixed an issue from 4.3.0 where any block scope within a for loop erroneously had its parent set to the function scopee
• Fixed an issue where compressing IIFEs with argument expansions would result in some parameters becoming undefined
• addEventListener options argument's properties are now part of the DOM properties list.

v4.3.0

• Do not drop computed object keys with side effects
• Functions passed to other functions in calls are now wrapped in parentheses by default, which speeds up loading most modules
• Objects with computed properties are now less likely to be hoisted
• Speed and memory efficiency optimizations
• Fixed scoping issues with try and switch

v4.2.1

• Minor refactors
• Fixed a bug similar to #369 in collapse_vars
• Functions can no longer be inlined into a place where they're going to be compared with themselves.
• reduce_funcs option is now legacy, as using reduce_vars without reduce_funcs caused some weird corner cases. As a result, it is now implied in reduce_vars and can't be turned off without turning off reduce_vars.
• Bug which would cause a random stack overflow has now been fixed.

v4.2.0

• When the source map URL is inline, don't write it to a file.
• Fixed output parens when a lambda literal is the tag on a tagged template string.
• The mangle.properties.undeclared option was added. This enables the property mangler to mangle properties of variables which can be found in the name cache, but whose properties are not known to this Terser run.
• The v8 bug where the toString and source representations of regexes like RegExp("\\\n") includes an actual newline is now fixed.
• Now we're guaranteed to not have duplicate comments in the output
• Domprops updates

v4.1.4

• Fixed a crash when inlining a function into somewhere else when it has interdependent, non-removable variables.

v4.1.3

• Several issues with the reduce_vars option were fixed.
• Starting this version, we only have a dist/bundle.min.js

v4.1.2

• The hotfix was hotfixed

v4.1.1

• Fixed a bug where toplevel scopes were being mixed up with lambda scopes

v4.1.0

• Internal functions were replaced by Object.assign, Array.prototype.some, Array.prototype.find and Array.prototype.every.
• A serious issue where some ESM-native code was broken was fixed.
• Performance improvements were made.
• Support for BigInt was added.
• Inline efficiency was improved. Functions are now being inlined more proactively instead of being inlined only after another Compressor pass.

v4.0.2

(Hotfix release. Reverts unmapped segments PR #342, which will be put back on Terser when the upstream issue is resolved)

v4.0.1

• Collisions between the arguments of inlined functions and names in the outer scope are now being avoided while inlining
• Unmapped segments are now preserved when compressing a file which has source maps
• Default values of functions are now correctly converted from Mozilla AST to Terser AST
• JSON ⊂ ECMAScript spec (if you don't know what this is you don't need to)
• Export AST_* classes to library users
• Fixed issue with collapse_vars when functions are created with the same name as a variable which already exists
• Added MutationObserverInit (Object with options for initialising a mutation observer) properties to the DOM property list
• Custom Error subclasses are now internally used instead of old-school Error inheritance hacks.
• Documentation fixes
• Performance optimizations

v4.0.0

• breaking change: The variables property of all scopes has become a standard JavaScript Map as opposed to the old bespoke Dictionary object.
• Typescript definitions were fixed
• terser --help was fixed
• The public interface was cleaned up
• Fixed optimisation of Array and new Array
• Added the keep_quoted=strict mode to mangle_props, which behaves more like Google Closure Compiler by mangling all unquoted property names, instead of reserving quoted property names automatically.
• Fixed parent functions' parameters being shadowed in some cases
• Allowed Terser to run in a situation where there are custom functions attached to Object.prototype
• And more bug fixes, optimisations and internal changes

v3.17.0

• More DOM properties added to --mangle-properties's DOM property list
• Closed issue where if 2 functions had the same argument name, Terser would not inline them together properly
• Fixed issue with hasOwnProperty.call
• You can now list files to minify in a Terser config file
• Started replacing new Array(<number>) with an array literal
• Started using ES6 capabilities like Set and the includes method for strings and arrays

v3.16.1

• Fixed issue where Terser being imported with import would cause it not to work due to the __esModule property. (PR #254 was submitted, which was nice, but since it wasn't a pure commonJS approach I decided to go with my own solution)

v3.16.0

• No longer leaves names like Array or Object or window as a SimpleStatement (statement which is just a single expression).
• Add support for sections sourcemaps (IndexedSourceMapConsumer)
• Drops node.js v4 and starts using commonJS
• Is now built with rollup

v3.15.0

• Inlined spread syntax ([...[1, 2, 3], 4, 5] => [1, 2, 3, 4, 5]) in arrays and objects.
• Fixed typo in compressor warning
• Fixed inline source map input bug
• Fixed parsing of template literals with unnecessary escapes (Like \\a)