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A JavaScript mangler/compressor toolkit for ES6+.

note: You can support this project on patreon: [link] The Terser Patreon is shutting down in favor of opencollective. Check 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uglify-es is no longer maintained and uglify-js does not support ES6+.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.


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
    --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).


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;

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;
$ 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");


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);
// 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(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);
// function n(n,r){return n+r}
// 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);
// /* 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.logconsole.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.


// 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_identifer (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.


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

Otherwise it would be replaced as string literal:

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


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

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

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

Here are some examples on how to use them:



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.

d3.js size gzip size time (s)
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:

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


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

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


Code Contributors

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

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