XUtils

Terser

parser, mangler and compressor toolkit for ES6+


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 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.

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.
    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.

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.

  • 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 wish to drop a specific function call such as console.info and/or retain side effects from function arguments after dropping the function call then use pure_funcs instead.

  • 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 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.

  • 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.

  • reduce_funcs (legacy option, safely ignored for backwards compatibility).

  • reduce_vars (default: true) – Improve optimization on variables assigned with and used as constant values.

  • 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 class 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.

  • 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) — Only mangle unquoted property names.

    • true – Quoted property names are automatically reserved and any unquoted property names will not be mangled.
    • "strict" – Advanced, all unquoted property names are mangled unless explicitly reserved.
  • 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.

Miscellaneous

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 ]
  • 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 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.

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()

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.

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.

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

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