terser
Last updated
Last updated
A JavaScript parser and mangler/compressor toolkit for ES6+.
note: You can support this project on patreon: . 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
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is no longer maintained and uglify-js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:
From NPM for programmatic use:
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.
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:
Specify --output
(-o
) to declare the output file. Otherwise the output goes to STDOUT.
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:
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
).
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.
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:
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:
to prevent the require
, exports
and $
names from being changed.
--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:
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:
Control all the code you're mangling
Avoid using a module bundler, as they usually will call Terser on each file individually, making it impossible to pass mangled objects between modules.
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:
Mangle all properties (except for JavaScript builtins
) (very unsafe):
Mangle all properties except for reserved
properties (still very unsafe):
Mangle all properties matching a regex
(not as unsafe but still unsafe):
Combining mangle properties options:
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:
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.
--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:
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.
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.
Assuming installation via NPM, you can load Terser in your application like this:
Browser loading is also supported:
There is a single high level function, minify(code, options)
, which will perform all minification phases in a configurable manner. By default minify()
will enable the options compress
and mangle
. Example:
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:
The toplevel
option:
The nameCache
option:
You may persist the name cache to the file system in the following way:
An example of a combination of minify()
options:
To produce warnings:
An error example:
Note: unlike uglify-js@2.x
, the Terser API does not throw errors. To achieve a similar effect one could do the following:
ecma
(default undefined
) - pass 5
, 2015
, 2016
, etc to override parse
, compress
and output
's ecma
options.
warnings
(default false
) — pass true
to return compressor warnings in result.warnings
. Use the value "verbose"
for more detailed warnings.
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.
output
(default null
) — pass an object if you wish to specify additional output 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 class 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
.
To generate a 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:
If you're compressing compiled JavaScript and have a source map for it, you can use sourceMap.content
:
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
.
bare_returns
(default false
) -- support top level return
statements
ecma
(default: 2017
) -- specify one of 5
, 2015
, 2016
or 2017
. Note: this setting is not presently enforced except for ES8 optional trailing commas in function parameter lists and calls with ecma
2017
.
html5_comments
(default true
)
shebang
(default true
) -- support #!command
as the first line
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
) -- Pass false
to disable potentially dropping function calls marked as "pure". A function call is marked as "pure" if a comment annotation /*@__PURE__*/
or /*#__PURE__*/
immediately precedes the call. For example: /*@__PURE__*/foo();
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"
)
warnings
(default: false
) -- display warnings when dropping unreachable code or unused declarations etc.
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
output option.
Examples:
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.
The code generator tries to output shortest code possible by default. In case you want beautified output, pass --beautify
(-b
). Optionally you can pass additional arguments that control the code output:
ascii_only
(default false
) -- escape Unicode characters in strings and regexps (affects directives with non-ascii characters becoming invalid)
beautify
(default true
) -- whether to actually beautify the output. Passing -b
will set this to true, but you might need to pass -b
even when you want to generate minified code, in order to specify additional arguments, so you can use -b beautify=false
to override it.
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" or "@preserve", 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 output printing mode. 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 the abstract syntax tree will still be output as is. For example: an ecma
setting of 5
will not convert ES6+ 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)
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.
You can pass --comments
to retain certain comments in the output. By default it will keep JSDoc-style comments that contain "@preserve", "@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:
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.
unsafe
compress
optionIt 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"
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:
You can specify nested constants in the form of --define env.DEBUG=false
.
Terser will warn about the condition being always false and about dropping unreachable code; for now there is no option to turn off only this specific warning, you can pass warnings=false
to turn off all warnings.
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:
and build your code like this:
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.
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:
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:
Otherwise it would be replaced as string literal:
minify()
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:
Traversal and transformation of the native AST can be performed through TreeWalker
and TreeTransformer
respectively.
Largely compatible native AST examples can be found in the original UglifyJS documentation. See: tree walker and tree transform.
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:
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.
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.
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:
To enable fast minify mode with the API use:
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
.
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.
https://www.npmjs.com/browse/depended/terser
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:
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:
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 make good issues.
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