let numbers = [1, 2, 3, 4, 5];

numbers.reduce(function(prev, curr, idx, arr) {
    return curr;
}, "Hello");

let people = [
  {
    name: 'Joe Schmoe',
    yearsExperience: 1,
    department: 'IT'
  },
  {
    name: 'Sally Sallerson',
    yearsExperience: 15,
    department: 'Engineering'
  },
  {
    name: 'Bill Billson',
    yearsExperience: 5,
    department: 'Engineering'
  },
  {
    name: 'Jane Janet',
    yearsExperience: 11,
    department: 'Management'
  },
  {
    name: 'Bob Hope',
    yearsExperience: 9,
    department: 'IT'
  }
];

function classifyExperience(employee) {
  let years = employee.yearsExperience;

  if (years <= 1)  return 'NEWBIE';
  if (years <= 5)  return 'AMATEUR';
  if (years <= 10) return 'PRO';

  return 'EXPERT';
}

// - - -
console.clear();


console.log(
  "All experience sum",
  people.reduce((sum, current) => sum + current.yearsExperience, 0)
);

let departmentExperienceSums = people.reduce((groupedByObject, employee) => {
  let department = employee.department;
  if (!groupedByObject[department])
    groupedByObject[department] = 0;
  groupedByObject[department] += employee.yearsExperience;
  return groupedByObject;
}, {});
console.log("Sum each department's collective experience", departmentExperienceSums);






let groupedByExperience = people.reduce((accumulator, current) => {
    let xpClass = classifyExperience(current);
    if (!accumulator[xpClass]) 
      accumulator[xpClass] = [];
    accumulator[xpClass].push(current.name)
    return accumulator;
  }, {});
console.log("Group employees by experience", groupedByExperience);






let numEmployeesByDept = people.reduce((accumulator, current) => {
    if (!accumulator[current.department])
      accumulator[current.department] = 0;
    accumulator[current.department]++;
    return accumulator;
  }, {});
console.log("Count the number of employees in each department", numEmployeesByDept);