exercise.js

// dummy data
let orders = [{
    userId: 0
}, {
    userId: 1
}, {
    userId: 1
}];

let users = [{
    id: 0,
    locationId: 0
}, {
    id: 1,
    locationId: 1
}]

let locations = [{
    id: 0,
    state: 'NY'
}, {
    id: 1,
    state: 'CA'
}]

// map order.userId->order count
let userCounts = orders.reduce((acc, order) => {
    acc[order.userId] = acc[order.userId] + 1 || 1;
    return acc;
}, {});

// map user.locationId->user-order count
let locationCounts = users.reduce((acc, user) => {
  let orderCount = userCounts[user.id];
  acc[user.locationId] = acc[user.locationId] + orderCount || orderCount;
  return acc;
}, {});

// find location with the highest count
let highest = locations.reduce((acc, location) => {
  // check if this location's order count is higher
  // than our current best. if so, return it as the current best.
  let orderCount = locationCounts[location.id];
  if (orderCount > acc.count) {
    return {state: location.state, count: orderCount}
    
  // otherwise, just return the current best
  } else {
    return acc;
  }
}, {state: null, count: 0});

console.log(highest.state);