cities.js

var fs = require('fs');

fs.writeFileSync('cities.json', JSON.stringify({
    type: 'FeatureCollection',
    features: fs.readFileSync('cities.txt', 'utf8').split(/\n/).slice(1).map(function(l) {
        return {
            type: 'Feature',
            properties: {
                group: l.split(/\t/)[0],
                name: l.split(/\t/)[7],
                id: l.split(/\t/)[2],
                slug: l.split(/\t/)[6]
            },
            geometry: {
                type: 'Polygon',
                coordinates: [
                    // top left
                    [+l.split(/\t/)[3], +l.split(/\t/)[2]],
                    // top right
                    [+l.split(/\t/)[5], +l.split(/\t/)[2]],
                    // bottom right
                    [+l.split(/\t/)[5], +l.split(/\t/)[4]],
                    // bottom left
                    [+l.split(/\t/)[5], +l.split(/\t/)[2]],
                    // top left
                    [+l.split(/\t/)[3], +l.split(/\t/)[2]]
                ]
            }
        };
    })
}));