domains.js

/**
  Usage: `node domains.js --file <inputfile.csv>
**/
var fs = require('fs');
var argh = require('argh').argv;
var _ = require('underscore');

var input = argh.file;

fs.readFile(input, function (err, filedata) {

    var rows = filedata.toString().split('\n');

    var domains = _.map(rows, function (e) {
        return e.toLowerCase().split('@')[1];
    });

    domains = _.countBy(domains, _.identity);

    domains = _.map(domains, function (val, key) {
        return { domain: key, count: val};
    });

    domains = _.sortBy(domains, 'count');

    domains = _.last(domains, 30);

    domains = _.pluck(domains, 'domain');

    console.log(domains);

});