a simple less middleware with source map, usage: app.use(lessMiddleware(path.join(__dirname))); -- only works for __dirname right now.

less-middleware-with-sourcemap.js

var less = require('less');
var fs = require('fs');
var path = require('path');
var SourceMapGenerator = require('source-map').SourceMapGenerator

function errfn(callback) {
    return function (err) {
        if (err) callback.call(this, err);
        else callback.apply(this, [].slice.apply(arguments).slice(1));
    }
}

function compileLess(filePath, callback) {
    fs.readFile(filePath, 'utf8', errfn(parse));

    function parse(str) {
        var parser = new (less.Parser)({
            paths: [path.dirname(filePath)],
            filename: filePath,
            dumpLineNumbers: 'comments'
        })
        parser.parse(str, errfn(gencss));
    }

    function gencss(tree) {
        try {
            var css = tree.toCSS();
            callback(null, css);
        } catch(parseError) {
            callback(parseError);
        }
    }
}

module.exports = function (root) {
    function removeRoot(filePath) {
        if (filePath.indexOf(root) === 0) {
            filePath = filePath.substring(root.length);
            if (filePath[0] != '/') filePath = '/' + filePath;
        }
        return filePath;
    }
    return function (req, res, next) {
        var isMap = false;
        if (req.url.match(/\.css\.map(\?|$)/i)) isMap = true;
        var fileName = req.url.replace(/\.css(\.map)?(\?|$)/i, '.less');
        if (!fileName.match(/\.less$/)) return next();
        var mapPath = fileName.replace(/\.less$/i, '.css.map');
        var cssName = fileName.replace(/\.less$/i, '.css');
        var filePath = path.join(root, fileName);
        compileLess(filePath, function (err, css) {
            var lines = css.split('\n');
            if (err) return next(err);
            if (isMap) {
                var smg = new SourceMapGenerator({ file: cssName });
                for (var i = 0; i < lines.length; i++) {
                    var match = lines[i].match(/\/\*\sline\s(\d+), ([^\*]+)\s\*\//);
                    if (match) {
                        smg.addMapping({
                            generated:{line:i+1,column:0},
                            original:{line:parseInt(match[1], 10),column:0},
                            source:removeRoot(match[2])
                        });
                    }
                }
                res.end(smg.toString());

            } else {
                res.end(css + '\n/*@ sourceMappingURL=' + mapPath + ' */');
            }
        })
    }
};