JSTS to create polygons from a network of arbitrary, or "spaghetti" lines. This uses jsts.noding to "clean" or "planarize" the line network, which is crazy fast compared to the "Union" alternative here: https://gist.github.com/rclark/6123614

lines2polysNoder.js

var jsts = require("jsts"),
    u = require("underscore"),
    fs = require("fs");

fs.readFile("/Users/ryan/Desktop/hv-lines.geojson", function (err, data) {
    var geojson = JSON.parse(data),
        writer = new jsts.io.GeoJSONWriter(),
        polygonizer = new jsts.operation.polygonize.Polygonizer(),
        noder = new jsts.noding.MCIndexNoder(),
        intersector = new jsts.algorithm.RobustLineIntersector(),
        intersectionFinder = new jsts.noding.IntersectionFinderAdder(intersector),
        factory = new jsts.geom.GeometryFactory(),
        jsUtil = require("javascript.util"),
        util = jsUtil.util,
        segments = new util.ArrayList(),
        cleaned = null,
        polygons = null,
        output = {type: "FeatureCollection", features: []},
        geoms = [],
        i = null;
    
    // List the lines as arrays of jsts.geom.Coordinates
    geojson.features.forEach(function (feature) {
        function line2coords(lineCoords) {
            return u.map(lineCoords, function (coord) {
                return new jsts.geom.Coordinate(coord[0], coord[1]);
            });
        }
        
        if (feature.geometry.type === "MultiLineString") {
            feature.geometry.coordinates.forEach(function (lineCoords) {
                geoms.push(line2coords(lineCoords));
            });
        } else if (feature.geometry.type === "LineString") {
            geoms.push(line2coords(feature.geometry.coordinates));
        }
    });
 
    // List the lines as jsts.noding.NodedSegmentStrings    
    geoms.forEach(function (coordArray) {
        segments.add(new jsts.noding.NodedSegmentString(coordArray));
    });
    
    // Run the Noder
    noder.setSegmentIntersector(intersectionFinder);
    noder.computeNodes(segments);
    cleaned = noder.getNodedSubstrings();
    
    // Extract LineStrings from the noder
    i = cleaned.iterator();
    while (i.hasNext()) {
        var ns = i.next(),
            coords = ns.getCoordinates(),
            ls = factory.createLineString(coords);
        polygonizer.add(ls);
    }
    
    // Output polygons  
    polygons = polygonizer.getPolygons();
    
    polygons.array.forEach(function (poly) {
        var f = {type: "Feature", properties: {}, geometry: writer.write(poly)};
        output.features.push(f);
    });

    process.stdout.write(JSON.stringify(output));
});

Hmm for some reason if I have shapes like this, this method returns only one polygon (outer ring)
http://bl.ocks.org/d/fd0203d849db8475632d8e55f2d48d10

If the "outer ring" is a rectangle the method works perfectly.
http://bl.ocks.org/d/cfb90664622a583921b76332789d22ff