`JSMin`

The JavaScript Minifier

Douglas Crockford
www.crockford.com

2003-12-04

JSMin is a filter which removes comments and unnecessary whitespace from JavaScript files. It typically reduces filesize by half, resulting in faster downloads. It also encourages a more expressive programming style because it eliminates the download cost of clean, literate self-documentation.

What `JSMin` Does

JSMin is a filter that omits or modifies some characters. This does not change the behavior of the program that it is minifying. The result may be harder to debug. It will definitely be harder to read.

JSMin first replaces carriage returns ('\r') with linefeeds ('\n'). It replaces all other control characters (including tab) with spaces. It replaces comments in the // form with linefeeds. It replaces comments in the /* */ form with spaces. All runs of spaces are replaced with a single space. All runs of linefeeds are replaced with a single linefeed.

It omits spaces except when a space is preceded and followed by a non-ASCII character or by an ASCII letter or digit, or by one of these characters:

\ $ _

It is more conservative in omitting linefeeds, because linefeeds are sometimes treated as semicolons. A linefeed is not omitted if it precedes a non-ASCII character or an ASCII letter or digit or one of these characters:

\ $ _ { [ ( + -

and if it follows a non-ASCII character or an ASCII letter or digit or one of these characters:

\ $ _ } ] ) + - " '

No other characters are omitted or modified.

JSMin knows to not modify quoted strings and regular expression literals.

JSMin does not obfuscate, but it does uglify.

Before:

// is.js

// (c) 2001 Douglas Crockford
// 2001 June 3


// is

// The -is- object is used to identify the browser.  Every browser edition
// identifies itself, but there is no standard way of doing it, and some of
// the identification is deceptive. This is because the authors of web
// browsers are liars. For example, Microsoft's IE browsers claim to be
// Mozilla 4. Netscape 6 claims to be version 5.

var is = {
    ie:      navigator.appName == 'Microsoft Internet Explorer',
    java:    navigator.javaEnabled(),
    ns:      navigator.appName == 'Netscape',
    ua:      navigator.userAgent.toLowerCase(),
    version: parseFloat(navigator.appVersion.substr(21)) ||
             parseFloat(navigator.appVersion),
    win:     navigator.platform == 'Win32'
}
is.mac = is.ua.indexOf('mac') >= 0;
if (is.ua.indexOf('opera') >= 0) {
    is.ie = is.ns = false;
    is.opera = true;
}
if (is.ua.indexOf('gecko') >= 0) {
    is.ie = is.ns = false;
    is.gecko = true;
}

After:

var is={ie:navigator.appName=='Microsoft Internet Explorer',java:navigator.javaEnabled(),ns:navigator.appName=='Netscape',ua:navigator.userAgent.toLowerCase(),version:parseFloat(navigator.appVersion.substr(21))||parseFloat(navigator.appVersion),win:navigator.platform=='Win32'}
is.mac=is.ua.indexOf('mac')>=0;if(is.ua.indexOf('opera')>=0){is.ie=is.ns=false;is.opera=true;}
if(is.ua.indexOf('gecko')>=0){is.ie=is.ns=false;is.gecko=true;}

Character Set

JSMin requires, but does not verify, that the character set encoding of the input program is either ASCII or UTF-8. It might not work correctly with other encodings.

Caution

Be sure to retain your original source file. JSMin is a one-way trip: Once done, it cannot be undone.

Do not put raw control characters inside a quoted string. That is an extremely bad practice. Use \xhh notation instead. JSMin will replace control characters with spaces or linefeeds.

Use parens with confusing sequences of + or -. For example, minification changes

a + ++b

into

a+++b

which is interpreted as

a++ + b

which is wrong. You can avoid this by using parens:

a + (++b)

JSLint checks for all of these problems. It is suggested that JSLint be used before using JSMin.

Command Line Options

Optional parameters will be listed at the beginning of the output as comments. This is a convenient way of replacing copyright messages and other documentation.

Example:

jsmin <fulljslint.js >jslint.js "(c)2002 Douglas Crockford"

Errors

JSMin can produce three error messages to stderr:

Unterminated comment.
Unterminated string constant.

Unterminated regular expression.

It ignores all other errors that may be present in your source program.

Get Minified

You can get a zip file containing an MS-DOS.exe file, or you can get the C source code and build it yourself. Now in C# and Java and JavaScript and Perl and PHP and Python and OCAML and Ruby.

JSMin