Map and reduce instead of entangled 'for..each' and 'if...else' blocks.

original.js

function extractData(messages) {
  let data = {};
  messages.forEach(message => {
    if (message.symbols) {
      let symbols = message.symbols;
      symbols.forEach(symbol => {
        if (data[symbol.symbol]) {
          data[symbol.symbol].count += 1;
        } else {
          data[symbol.symbol] = {
            count: 1,
            title: symbol.title
          };
        }
      });
    }
  });
  
  return arrayify(d);
}

refactor.js

function extractData(messages) {
  let data = messages
  .map(m => m.symbols)
  .reduce((result, curr) => result.concat(curr), [])
  .reduce((result, curr) => {
    let ticker = curr.symbol;
    result[ticker] = result[ticker] || {title: curr.title, count: 0};
    result[ticker].count += 1;
    return result;
  }, {});

  return arrayify(data);
}

looks really neat... :)