JS Grouping

orders.js

var orders = [
    { orderId: 'I8001', customerId: '5142' },
    { orderId: 'I1006', customerId: '5142' },
    { orderId: 'I1009', customerId: '5142' },
    { orderId: 'I1001', customerId: '5145' },
    { orderId: 'I9999', customerId: '5146' }
],
    
result = orders.reduce(function (r, a) {
  r[a.customerId] = r[a.customerId] || [];
  r[a.customerId].push(a);
  
  return r;
}, Object.create(null));

console.log(result);