Installation

sudo npm install less-middleware

Options

Option	Description	Default
force	Always re-compile less files on each request.	false
once	Only check for need to recompile once after each server restart. Useful for reducing disk i/o on production.	false
debug	Output any debugging messages to the console.	false
src	Source directory containing the .less files. Required.
dest	Desitnation directory to output the compiled .css files.	<src>
paths	Specify search paths for @import directives	The dirname of <src>
prefix	Path which should be stripped from the public pathname.
compress	Compress the output being written to the *.css files. When set to 'auto' compression will only happen when the css file ends with .min.css or -min.css.	auto
yuicompress	More involved minification with YUI compression	false
optimization	Desired level of LESS optimization. Optionally 0, 1, or 2	0
dumpLineNumbers	Add line tracking to the compiled css. Optionally 0, 'comments', or 'mediaquery'	0
relativeUrls	Adjust urls to be relative to directory of files imported with @import. If false, urls will remain unchanged.	false
sourceMap	Enable sourcemap support. You can compile your less and then use developer tools to see where in your less file a particular piece of css comes from.	false
preprocessor	Specify a preprocessing function applied to LESS source code before parsing. The function will receive the LESS source code and the Connect request object as parameters, and must return the modified source code.
treeFunctions	Object with custom functions added to less.tree.functions.

Examples

Connect

var lessMiddleware = require('less-middleware');

var server = connect.createServer(
    lessMiddleware({
        src: __dirname + '/public',
        compress: true
    }),
    connect.staticProvider(__dirname + '/public')
);

Express

var lessMiddleware = require('less-middleware');

var app = express();

app.configure(function () {
    // Other configuration here...

    app.use(lessMiddleware({
        src: __dirname + '/public',
        compress: true
    }));

    app.use(express.static(__dirname + '/public'));
});

Express - Different src and dest

When using a different src and dest you can use the prefix option to make the directory structure cleaner.

Requests for static assets (like stylesheets) made to the express server use a pathname to look up the file. So if the request is for http://localhost/stylesheets/styles.css the pathname will be /stylesheets/styles.css.

The less middleware uses that path to determine where to look for less files. In the original example it looks for the less file at /public/stylesheets/styles.less and compiles it to /public/stylesheets/styles.css.

If you are using a different src and dest options it causes for more complex directories structures unless you use the prefix option. For example, without the prefix, and with a src of /src/less and a dest of /public it would look for the less file at /src/less/stylesheets/styles.less and compile it to /public/stylesheets/styles.css. To make it cleaner you can use the prefix option:

var lessMiddleware = require('less-middleware');

var app = express();

app.configure(function () {
    // Other configuration here...

    app.use(lessMiddleware({
        dest: __dirname + '/public/stylesheets',
        src: __dirname + '/src/less',
        prefix: '/stylesheets',
        compress: true
    }));

    app.use(express.static(__dirname + '/public'));
});

Using the prefix it changes the pathname from /stylesheets/styles.css to /styles.css. With that prefix removed from the pathname it makes things cleaner. With the prefix removed it would look for the less file at /src/less/styles.less and compile it to /public/stylesheets/styles.css.

A new alternative way to achieve the same thing as using prefix but with greater flexibility is to supply a shared root value. From this shared root, you would specify a URI style path to the appropriate source and destination directories:

var lessMiddleware = require('less-middleware')
  , path = require('path')
  , pubDir = path.join(__dirname, 'public')
  , app = express();
  
app.configure(function() {
    app.use(lessMiddleware({
        dest: '/css', // should be the URI to your css directory from the location bar in your browser
        src: '/less', // or '../less' if the less directory is outside of /public
        root: pubDir,
        compress: true
    }));
    
    app.use(express.static(pubDir));
});

This will allow any file under the /less directory, including subdirectories, to be compiled into an identical directory structure under /css.

Express - Using a temp directory for dest

Since less middleware relies on static content to be served by express.static, using temp directories just requires that you inform express about where generated files are built:

var lessMiddleware = require('less-middleware'),
    os = require('os');

var app = express();

app.configure(function () {
    // Other configuration here...

    var tmpDir = os.tmpDir();
    app.use(lessMiddleware({
        src: __dirname + '/public/stylesheets',
        dest: tmpDir,
        compress: true
    }));

    app.use(express.static(__dirname + '/public'));
    app.use(express.static(tmpDir));
});

Using a temp directory is useful for read-only file systems, such as a Heroku deployment. By using a temp directory the css files can still be written and served.

Importing less

By default the directory in which the compiled files live in is already set as an import directive:

// file1.less

@import 'file2.less';

body {
  color: @bodyColor;
}

// file2.less

@bodyColor: #333333;

However, you can use the paths option if you need to specify other directories in which to search for importable less files.

var less = require('less-middleware'),
    path = require('path');

    lessMiddleware({
        src: path.join(__dirname, 'public'),
        paths: [path.join(__dirname, 'module', 'less')]
    });

// public/base.css

@import 'colors';

body {
  color: @bodyColor;
}

// module/less/colors.less

@bodyColor: #333333;

Using bootstrap

Here's an example on how to use Twitter's bootstrap within an Express.js set-up:

// package.json
{
  "name": "my-module",
  // ...
  "dependencies": {
    "less-middleware": "*",
    "bootstrap": "git+https://github.com/twitter/bootstrap.git#v2.2.2",
    "express": "3.0"
  }
}

// app.js
var express  = require('express')
  , path     = require('path')
  , app      = express()
  , less     = require('less-middleware');

app.configure(function(){
  // ...
  var bootstrapPath = path.join(__dirname, 'node_modules', 'bootstrap');
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use('/img', express['static'](path.join(bootstrapPath, 'img')));
  app.use(app.router);
  app.use(less({
    src    : path.join(__dirname, 'assets', 'less'),
    paths  : [path.join(bootstrapPath, 'less')],
    dest   : path.join(__dirname, 'public', 'stylesheets'),
    prefix : '/stylesheets'
  }));
  app.use(express['static'](path.join(__dirname, 'public')));
  // ...
});

// ...

// assets/less/base.less

@import 'bootstrap';
@import 'responsive';

@bodyBackground: #FAF7EC;
@navbarBrandColor: #989CAE;

.brand {
  font-family: @monoFontFamily;
}

Preprocessing

var lessMiddleware = require('less-middleware');

var app = express();

app.configure(function () {
    // Other configuration here...

    app.use(lessMiddleware({
        src: __dirname + '/public',
        preprocessor: function(src, req) {
            if (req.param("namespace")) {
                src = req.param("namespace") + " { " + src + " }";
            }
            
            return src;
        },
        compress: true
    }));

    app.use(express.static(__dirname + '/public'));
});

Troubleshooting

My less never recompiles, even when I use {force: true}!

Make sure you're declaring less-middleware before your static middleware, if you're using the same directory, e.g. (with express):

app.use(require('less-middleware')({ src: __dirname + '/public', debug: true }));
app.use(express.static(path.join(__dirname, 'public')));

not

app.use(express.static(path.join(__dirname, 'public')));
app.use(require('less-middleware')({ src: __dirname + '/public', debug: true }));

IIS

If you are hosting your app on IIS you will have to modify your web.config file in order to allow NodeJS to serve your CSS static files. IIS will cache your CSS files, bypassing NodeJS static file serving, which in turn does not allow the middleware to recompile your LESS files.