Programming language: Go
Tags: Utilities    
Latest version: v2.9.7

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Online demo if you need to minify files now.

Command line tool that minifies concurrently and watches file changes.

Releases of CLI for various platforms. See CLI for more installation instructions.

Did you know that the shortest valid piece of HTML5 is <!doctype html><title>x</title>? See for yourself at the W3C Validator!

Minify is a minifier package written in Go. It provides HTML5, CSS3, JS, JSON, SVG and XML minifiers and an interface to implement any other minifier. Minification is the process of removing bytes from a file (such as whitespace) without changing its output and therefore shrinking its size and speeding up transmission over the internet and possibly parsing. The implemented minifiers are designed for high performance.

The core functionality associates mimetypes with minification functions, allowing embedded resources (like CSS or JS within HTML files) to be minified as well. Users can add new implementations that are triggered based on a mimetype (or pattern), or redirect to an external command (like ClosureCompiler, UglifyCSS, ...).


There are no sponsors yet. Please see https://www.patreon.com/tdewolff for ways to contribute, otherwise please contact me directly!

Table of Contents


  • [ ] Use ASM/SSE to further speed-up core parts of the parsers/minifiers
  • [x] Improve JS minifiers by shortening variables and proper semicolon omission
  • [ ] Speed-up SVG minifier, it is very slow
  • [x] Proper parser error reporting and line number + column information
  • [ ] Generation of source maps (uncertain, might slow down parsers too much if it cannot run separately nicely)
  • [ ] Create a cmd to pack webfiles (much like webpack), ie. merging CSS and JS files, inlining small external files, minification and gzipping. This would work on HTML files.


Minifiers or bindings to minifiers exist in almost all programming languages. Some implementations are merely using several regular expressions to trim whitespace and comments (even though regex for parsing HTML/XML is ill-advised, for a good read see Regular Expressions: Now You Have Two Problems). Some implementations are much more profound, such as the YUI Compressor and Google Closure Compiler for JS. As most existing implementations either use JavaScript, use regexes, and don't focus on performance, they are pretty slow.

This minifier proves to be that fast and extensive minifier that can handle HTML and any other filetype it may contain (CSS, JS, ...). It is usually orders of magnitude faster than existing minifiers.


With modules enabled (GO111MODULES=auto or GO111MODULES=on), add the following imports and run the project with go get

import (

See CLI tool for installation instructions of the binary.

API stability

There is no guarantee for absolute stability, but I take issues and bugs seriously and don't take API changes lightly. The library will be maintained in a compatible way unless vital bugs prevent me from doing so. There has been one API change after v1 which added options support and I took the opportunity to push through some more API clean up as well. There are no plans whatsoever for future API changes.


For all subpackages and the imported parse package, test coverage of 100% is pursued. Besides full coverage, the minifiers are fuzz tested using github.com/dvyukov/go-fuzz, see the wiki for the most important bugs found by fuzz testing. These tests ensure that everything works as intended and that the code does not crash (whatever the input). If you still encounter a bug, please file a bug report!


The benchmarks directory contains a number of standardized samples used to compare performance between changes. To give an indication of the speed of this library, I've ran the tests on my Thinkpad T460 (i5-6300U quad-core 2.4GHz running Arch Linux) using Go 1.15.

name                              time/op
CSS/sample_bootstrap.css-4          2.70ms ± 0%
CSS/sample_gumby.css-4              3.57ms ± 0%
CSS/sample_fontawesome.css-4         767µs ± 0%
CSS/sample_normalize.css-4          85.5µs ± 0%
HTML/sample_amazon.html-4           15.2ms ± 0%
HTML/sample_bbc.html-4              3.90ms ± 0%
HTML/sample_blogpost.html-4          420µs ± 0%
HTML/sample_es6.html-4              15.6ms ± 0%
HTML/sample_stackoverflow.html-4    3.73ms ± 0%
HTML/sample_wikipedia.html-4        6.60ms ± 0%
JS/sample_ace.js-4                  28.7ms ± 0%
JS/sample_dot.js-4                   357µs ± 0%
JS/sample_jquery.js-4               10.0ms ± 0%
JS/sample_jqueryui.js-4             20.4ms ± 0%
JS/sample_moment.js-4               3.47ms ± 0%
JSON/sample_large.json-4            3.25ms ± 0%
JSON/sample_testsuite.json-4        1.74ms ± 0%
JSON/sample_twitter.json-4          24.2µs ± 0%
SVG/sample_arctic.svg-4             34.7ms ± 0%
SVG/sample_gopher.svg-4              307µs ± 0%
SVG/sample_usa.svg-4                57.4ms ± 0%
SVG/sample_car.svg-4                18.0ms ± 0%
SVG/sample_tiger.svg-4              5.61ms ± 0%
XML/sample_books.xml-4              54.7µs ± 0%
XML/sample_catalog.xml-4            33.0µs ± 0%
XML/sample_omg.xml-4                7.17ms ± 0%

name                              speed
CSS/sample_bootstrap.css-4        50.7MB/s ± 0%
CSS/sample_gumby.css-4            52.1MB/s ± 0%
CSS/sample_fontawesome.css-4      61.2MB/s ± 0%
CSS/sample_normalize.css-4        70.8MB/s ± 0%
HTML/sample_amazon.html-4         31.1MB/s ± 0%
HTML/sample_bbc.html-4            29.5MB/s ± 0%
HTML/sample_blogpost.html-4       49.8MB/s ± 0%
HTML/sample_es6.html-4            65.6MB/s ± 0%
HTML/sample_stackoverflow.html-4  55.0MB/s ± 0%
HTML/sample_wikipedia.html-4      67.5MB/s ± 0%
JS/sample_ace.js-4                22.4MB/s ± 0%
JS/sample_dot.js-4                14.5MB/s ± 0%
JS/sample_jquery.js-4             24.8MB/s ± 0%
JS/sample_jqueryui.js-4           23.0MB/s ± 0%
JS/sample_moment.js-4             28.6MB/s ± 0%
JSON/sample_large.json-4           234MB/s ± 0%
JSON/sample_testsuite.json-4       394MB/s ± 0%
JSON/sample_twitter.json-4        63.0MB/s ± 0%
SVG/sample_arctic.svg-4           42.4MB/s ± 0%
SVG/sample_gopher.svg-4           19.0MB/s ± 0%
SVG/sample_usa.svg-4              17.8MB/s ± 0%
SVG/sample_car.svg-4              29.3MB/s ± 0%
SVG/sample_tiger.svg-4            12.2MB/s ± 0%
XML/sample_books.xml-4            81.0MB/s ± 0%
XML/sample_catalog.xml-4          58.6MB/s ± 0%
XML/sample_omg.xml-4               159MB/s ± 0%


HTML (with JS and CSS) minification typically shaves off about 10%.

The HTML5 minifier uses these minifications:

  • strip unnecessary whitespace and otherwise collapse it to one space (or newline if it originally contained a newline)
  • strip superfluous quotes, or uses single/double quotes whichever requires fewer escapes
  • strip default attribute values and attribute boolean values
  • strip some empty attributes
  • strip unrequired tags (html, head, body, ...)
  • strip unrequired end tags (tr, td, li, ... and often p)
  • strip default protocols (http:, https: and javascript:)
  • strip all comments (including conditional comments, old IE versions are not supported anymore by Microsoft)
  • shorten doctype and meta charset
  • lowercase tags, attributes and some values to enhance gzip compression


  • KeepConditionalComments preserve all IE conditional comments such as <!--[if IE 6]><![endif]--> and <![if IE 6]><![endif]>, see https://msdn.microsoft.com/en-us/library/ms537512(v=vs.85).aspx#syntax
  • KeepDefaultAttrVals preserve default attribute values such as <script type="application/javascript">
  • KeepDocumentTags preserve html, head and body tags
  • KeepEndTags preserve all end tags
  • KeepQuotes preserve quotes around attribute values
  • KeepWhitespace preserve whitespace between inline tags but still collapse multiple whitespace characters into one

After recent benchmarking and profiling it became really fast and minifies pages in the 10ms range, making it viable for on-the-fly minification.

However, be careful when doing on-the-fly minification. Minification typically trims off 10% and does this at worst around about 20MB/s. This means users have to download slower than 2MB/s to make on-the-fly minification worthwhile. This may or may not apply in your situation. Rather use caching!

Whitespace removal

The whitespace removal mechanism collapses all sequences of whitespace (spaces, newlines, tabs) to a single space. If the sequence contained a newline or carriage return it will collapse into a newline character instead. It trims all text parts (in between tags) depending on whether it was preceded by a space from a previous piece of text and whether it is followed up by a block element or an inline element. In the former case we can omit spaces while for inline elements whitespace has significance.

Make sure your HTML doesn't depend on whitespace between block elements that have been changed to inline or inline-block elements using CSS. Your layout should not depend on those whitespaces as the minifier will remove them. An example is a menu consisting of multiple <li> that have display:inline-block applied and have whitespace in between them. It is bad practise to rely on whitespace for element positioning anyways!


Minification typically shaves off about 10%-15%. This CSS minifier will not do structural changes to your stylesheets. Although this could result in smaller files, the complexity is quite high and the risk of breaking website is high too.

The CSS minifier will only use safe minifications:

  • remove comments and unnecessary whitespace (but keep /*! ... */ which usually contains the license)
  • remove trailing semicolons
  • optimize margin, padding and border-width number of sides
  • shorten numbers by removing unnecessary + and zeros and rewriting with/without exponent
  • remove dimension and percentage for zero values
  • remove quotes for URLs
  • remove quotes for font families and make lowercase
  • rewrite hex colors to/from color names, or to three digit hex
  • rewrite rgb(, rgba(, hsl( and hsla( colors to hex or name
  • use four digit hex for alpha values (transparent#0000)
  • replace normal and bold by numbers for font-weight and font
  • replace none0 for border, background and outline
  • lowercase all identifiers except classes, IDs and URLs to enhance gzip compression
  • shorten MS alpha function
  • rewrite data URIs with base64 or ASCII whichever is shorter
  • calls minifier for data URI mediatypes, thus you can compress embedded SVG files if you have that minifier attached
  • shorten aggregate declarations such as background and font

It does purposely not use the following techniques:

  • (partially) merge rulesets
  • (partially) split rulesets
  • collapse multiple declarations when main declaration is defined within a ruleset (don't put font-weight within an already existing font, too complex)
  • remove overwritten properties in ruleset (this not always overwrites it, for example with !important)
  • rewrite properties into one ruleset if possible (like margin-top, margin-right, margin-bottom and margin-leftmargin)
  • put nested ID selector at the front (body > div#elem p#elem p)
  • rewrite attribute selectors for IDs and classes (div[id=a]div#a)
  • put space after pseudo-selectors (IE6 is old, move on!)

There are a couple of comparison tables online, such as CSS Minifier Comparison, CSS minifiers comparison and CleanCSS tests. Comparing speed between each, this minifier will usually be between 10x-300x faster than existing implementations, and even rank among the top for minification ratios. It falls short with the purposely not implemented and often unsafe techniques.


  • KeepCSS2 prohibits using CSS3 syntax (such as exponents in numbers, or rgba(rgb(), might be incomplete
  • Precision number of significant digits to preserve for numbers, 0 means no trimming


The JS minifier typically shaves off about 35% -- 65% of filesize depening on the file, which is a compression close to many other minifiers. Common speeds of PHP and JS implementations are about 100-300kB/s (see Uglify2, Adventures in PHP web asset minimization). This implementation is orders of magnitude faster at around ~25MB/s.

The following features are implemented:

  • remove superfluous whitespace
  • remove superfluous semicolons
  • shorten true, false, and undefined to !0, !1 and void 0
  • rename variables and functions to shorter names (not in global scope)
  • move var declarations to the top of the global/function scope (if more than one)
  • collapse if/else statements to expressions
  • minify conditional expressions to simpler ones
  • merge sequential expression statements to one, including into return and throw
  • remove superfluous grouping in expressions
  • shorten or remove string escapes
  • convert object key or index expression from string to identifier or decimal
  • merge concatenated strings
  • rewrite numbers (binary, octal, decimal, hexadecimal) to shorter representations

Comparison with other tools

Performance is measured with time [command] ran 10 times and selecting the fastest one, on a Thinkpad T460 (i5-6300U quad-core 2.4GHz running Arch Linux) using Go 1.15.

  • minify: minify -o script.min.js script.js
  • esbuild: esbuild --minify --outfile=script.min.js script.js
  • terser: terser script.js --compress --mangle -o script.min.js
  • UglifyJS: uglifyjs --compress --mangle -o script.min.js script.js
  • Closure Compiler: closure-compiler -O SIMPLE --js script.js --js_output_file script.min.js --language_in ECMASCRIPT_NEXT -W QUIET --jscomp_off=checkVars optimization level SIMPLE instead of ADVANCED to make similar assumptions as do the other tools (do not rename/assume anything of global level variables)

Compression ratio (lower is better)

All tools give very similar results, although UglifyJS compresses slightly better.

Tool ace.js dot.js jquery.js jqueryui.js moment.js
minify 53.7% 64.8% 34.2% 51.3% 34.8%
esbuild 53.8% 66.3% 34.4% 53.1% 34.8%
terser 53.2% 65.2% 34.2% 51.8% 34.7%
UglifyJS 53.1% 64.7% 33.8% 50.7% 34.2%
Closure Compiler 53.4% 64.0% 35.7% 53.6% 34.3%

Time (lower is better)

Most tools are extremely slow, with minify and esbuild being orders of magnitudes faster.

Tool ace.js dot.js jquery.js jqueryui.js moment.js
minify 49ms 5ms 22ms 35ms 13ms
esbuild 64ms 9ms 31ms 51ms 17ms
terser 2900s 180ms 1400ms 2200ms 730ms
UglifyJS 3900ms 210ms 2000ms 3100ms 910ms
Closure Compiler 6100ms 2500ms 4400ms 5300ms 3500ms


Minification typically shaves off about 15% of filesize for common indented JSON such as generated by JSON Generator.

The JSON minifier only removes whitespace, which is the only thing that can be left out, and minifies numbers (1000 => 1e3).


  • Precision number of significant digits to preserve for numbers, 0 means no trimming


The SVG minifier uses these minifications:

  • trim and collapse whitespace between all tags
  • strip comments, empty doctype, XML prelude, metadata
  • strip SVG version
  • strip CDATA sections wherever possible
  • collapse tags with no content to a void tag
  • minify style tag and attributes with the CSS minifier
  • minify colors
  • shorten lengths and numbers and remove default px unit
  • shorten path data
  • use relative or absolute positions in path data whichever is shorter


  • convert attributes to style attribute whenever shorter
  • merge path data? (same style and no intersection -- the latter is difficult)


  • Precision number of significant digits to preserve for numbers, 0 means no trimming


The XML minifier uses these minifications:

  • strip unnecessary whitespace and otherwise collapse it to one space (or newline if it originally contained a newline)
  • strip comments
  • collapse tags with no content to a void tag
  • strip CDATA sections wherever possible


  • KeepWhitespace preserve whitespace between inline tags but still collapse multiple whitespace characters into one


Any input stream is being buffered by the minification functions. This is how the underlying buffer package inherently works to ensure high performance. The output stream however is not buffered. It is wise to preallocate a buffer as big as the input to which the output is written, or otherwise use bufio to buffer to a streaming writer.


Retrieve a minifier struct which holds a map of mediatype → minifier functions.

m := minify.New()

The following loads all provided minifiers.

m := minify.New()
m.AddFunc("text/css", css.Minify)
m.AddFunc("text/html", html.Minify)
m.AddFunc("image/svg+xml", svg.Minify)
m.AddFuncRegexp(regexp.MustCompile("^(application|text)/(x-)?(java|ecma)script$"), js.Minify)
m.AddFuncRegexp(regexp.MustCompile("[/+]json$"), json.Minify)
m.AddFuncRegexp(regexp.MustCompile("[/+]xml$"), xml.Minify)

You can set options to several minifiers.

m.Add("text/html", &html.Minifier{
    KeepDefaultAttrVals: true,
    KeepWhitespace: true,

From reader

Minify from an io.Reader to an io.Writer for a specific mediatype.

if err := m.Minify(mediatype, w, r); err != nil {

From bytes

Minify from and to a []byte for a specific mediatype.

b, err = m.Bytes(mediatype, b)
if err != nil {

From string

Minify from and to a string for a specific mediatype.

s, err = m.String(mediatype, s)
if err != nil {

To reader

Get a minifying reader for a specific mediatype.

mr := m.Reader(mediatype, r)
if _, err := mr.Read(b); err != nil {

To writer

Get a minifying writer for a specific mediatype. Must be explicitly closed because it uses an io.Pipe underneath.

mw := m.Writer(mediatype, w)
if mw.Write([]byte("input")); err != nil {
if err := mw.Close(); err != nil {


Minify resources on the fly using middleware. It passes a wrapped response writer to the handler that removes the Content-Length header. The minifier is chosen based on the Content-Type header or, if the header is empty, by the request URI file extension. This is on-the-fly processing, you should preferably cache the results though!

fs := http.FileServer(http.Dir("www/"))
http.Handle("/", m.Middleware(fs))

Custom minifier

Add a minifier for a specific mimetype.

type CustomMinifier struct {
    KeepLineBreaks bool

func (c *CustomMinifier) Minify(m *minify.M, w io.Writer, r io.Reader, params map[string]string) error {
    // ...
    return nil

m.Add(mimetype, &CustomMinifier{KeepLineBreaks: true})
// or
m.AddRegexp(regexp.MustCompile("/x-custom$"), &CustomMinifier{KeepLineBreaks: true})

Add a minify function for a specific mimetype.

m.AddFunc(mimetype, func(m *minify.M, w io.Writer, r io.Reader, params map[string]string) error {
    // ...
    return nil
m.AddFuncRegexp(regexp.MustCompile("/x-custom$"), func(m *minify.M, w io.Writer, r io.Reader, params map[string]string) error {
    // ...
    return nil

Add a command cmd with arguments args for a specific mimetype.

m.AddCmd(mimetype, exec.Command(cmd, args...))
m.AddCmdRegexp(regexp.MustCompile("/x-custom$"), exec.Command(cmd, args...))


Using the params map[string]string argument one can pass parameters to the minifier such as seen in mediatypes (type/subtype; key1=val2; key2=val2). Examples are the encoding or charset of the data. Calling Minify will split the mimetype and parameters for the minifiers for you, but MinifyMimetype can be used if you already have them split up.

Minifiers can also be added using a regular expression. For example a minifier with image/.* will match any image mime.


Common minifiers

Basic example that minifies from stdin to stdout and loads the default HTML, CSS and JS minifiers. Optionally, one can enable java -jar build/compiler.jar to run for JS (for example the ClosureCompiler). Note that reading the file into a buffer first and writing to a pre-allocated buffer would be faster (but would disable streaming).

package main

import (


func main() {
    m := minify.New()
    m.AddFunc("text/css", css.Minify)
    m.AddFunc("text/html", html.Minify)
    m.AddFunc("image/svg+xml", svg.Minify)
    m.AddFuncRegexp(regexp.MustCompile("^(application|text)/(x-)?(java|ecma)script$"), js.Minify)
    m.AddFuncRegexp(regexp.MustCompile("[/+]json$"), json.Minify)
    m.AddFuncRegexp(regexp.MustCompile("[/+]xml$"), xml.Minify)

    if err := m.Minify("text/html", os.Stdout, os.Stdin); err != nil {

External minifiers

Below are some examples of using common external minifiers.

Closure Compiler

See Closure Compiler Application. Not tested.

    exec.Command("java", "-jar", "build/compiler.jar"))


See UglifyJS.



See esbuild.

    exec.Command("esbuild", "$in.js", "--minify", "--outfile=$out.js"))

Custom minifier

Custom minifier showing an example that implements the minifier function interface. Within a custom minifier, it is possible to call any minifier function (through m minify.Minifier) recursively when dealing with embedded resources.

package main

import (


func main() {
    m := minify.New()
    m.AddFunc("text/plain", func(m *minify.M, w io.Writer, r io.Reader, _ map[string]string) error {
        // remove newlines and spaces
        rb := bufio.NewReader(r)
        for {
            line, err := rb.ReadString('\n')
            if err != nil && err != io.EOF {
                return err
            if _, errws := io.WriteString(w, strings.Replace(line, " ", "", -1)); errws != nil {
                return errws
            if err == io.EOF {
        return nil

    in := "Because my coffee was too cold, I heated it in the microwave."
    out, err := m.String("text/plain", in)
    if err != nil {
    // Output: Becausemycoffeewastoocold,Iheateditinthemicrowave.



func main() {
    m := minify.New()
    m.AddFunc("text/css", css.Minify)
    m.AddFunc("text/html", html.Minify)
    m.AddFunc("image/svg+xml", svg.Minify)
    m.AddFuncRegexp(regexp.MustCompile("^(application|text)/(x-)?(java|ecma)script$"), js.Minify)
    m.AddFuncRegexp(regexp.MustCompile("[/+]json$"), json.Minify)
    m.AddFuncRegexp(regexp.MustCompile("[/+]xml$"), xml.Minify)

    fs := http.FileServer(http.Dir("www/"))
    http.Handle("/", m.Middleware(fs))


func Serve(w http.ResponseWriter, r *http.Request) {
    mw := m.ResponseWriter(w, r)
    defer mw.Close()
    w = mw

    http.ServeFile(w, r, path.Join("www", r.URL.Path))

Custom response writer

ResponseWriter example which returns a ResponseWriter that minifies the content and then writes to the original ResponseWriter. Any write after applying this filter will be minified.

type MinifyResponseWriter struct {

func (m MinifyResponseWriter) Write(b []byte) (int, error) {
    return m.WriteCloser.Write(b)

// MinifyResponseWriter must be closed explicitly by calling site.
func MinifyFilter(mediatype string, res http.ResponseWriter) MinifyResponseWriter {
    m := minify.New()
    // add minfiers

    mw := m.Writer(mediatype, res)
    return MinifyResponseWriter{res, mw}
// Usage
func(w http.ResponseWriter, req *http.Request) {
    w = MinifyFilter("text/html", w)
    if _, err := io.WriteString(w, "<p class="message"> This HTTP response will be minified. </p>"); err != nil {
    if err := w.Close(); err != nil {
    // Output: <p class=message>This HTTP response will be minified.


Here's an example of a replacement for template.ParseFiles from template/html, which automatically minifies each template before parsing it.

Be aware that minifying templates will work in most cases but not all. Because the HTML minifier only works for valid HTML5, your template must be valid HTML5 of itself. Template tags are parsed as regular text by the minifier.

func compileTemplates(filenames ...string) (*template.Template, error) {
    m := minify.New()
    m.AddFunc("text/html", html.Minify)

    var tmpl *template.Template
    for _, filename := range filenames {
        name := filepath.Base(filename)
        if tmpl == nil {
            tmpl = template.New(name)
        } else {
            tmpl = tmpl.New(name)

        b, err := ioutil.ReadFile(filename)
        if err != nil {
            return nil, err

        mb, err := m.Bytes("text/html", b)
        if err != nil {
            return nil, err
    return tmpl, nil

Example usage:

templates := template.Must(compileTemplates("view.html", "home.html"))


Released under the [MIT license](LICENSE.md).

*Note that all licence references and agreements mentioned in the minify README section above are relevant to that project's source code only.