reduce.js

var users = [
    { name: "John", age: 30 },
    { name: "Jane", age: 28 },
    { name: "Bill", age: 65 },
    { name: "Emily", age: 17 },
    { name: "Jack", age: 30 }
]

var reducedUsers = _.reduce(users, function (result, user) {
    if(user.age >= 18 && user.age <= 59) {
        (result[user.age] || (result[user.age] = [])).push(user);
    }
  
    return result;
}, {});

// reducedUsers -> { 
//     28: [{ name: "Jane", age: 28 }], 
//     30: [{ name: "John", age: 30 }, { name: "Jack", age: 30 }] 
// }