JSTS to create polygons from spaghetti lines. Union on lines is the "planarize" step, and is extremely time-consuming for larg-ish numbers of lines (~300 it takes about 2 seconds per each new line to union).

lines2polysUnion.js

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

fs.readFile("/Users/ryan/Desktop/hv-lines.geojson", function (err, data) {
    var geojson = JSON.parse(data),
        reader = new jsts.io.GeoJSONReader(),
        writer = new jsts.io.GeoJSONWriter(),
        polygonizer = new jsts.operation.polygonize.Polygonizer(),

        geoms = _.map(geojson.features, function (feature) {
            return reader.read(feature.geometry);
        }),
        cleaned = null;

    geoms.forEach(function (geom, i, array) {
        console.time(i);
        if (i === 0) { cleaned = geom; }
        else { cleaned = cleaned.union(geom); }
        console.timeEnd(i);
    });

    if (cleaned !== null) {
        polygonizer.add(cleaned);

        var polygons = polygonizer.getPolygons(),
            output = {type: "FeatureCollection", features: []};

        polygons.array.forEach(function (poly) {
            var f = {type: "Feature", properties: {}, geometry: writer.write(poly)};
            output.features.push(f);
        });

        console.log(JSON.stringify(output));
    } else {
        console.log("Clean failed");
    }
});