SOURCE

var initialValue = 0;
var sum = [{x: 1}, {x:2}, {x:3}].reduce((accumulator, currentValue) => {
    return accumulator + currentValue.x;
}, 0)

console.log(sum) // logs 6

var flattened = [[0, 1], [2, 3], [4, 5]].reduce((acc, cur) => acc.concat(cur),
 []
);
console.log(flattened)
var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

var countedNames = names.reduce(function (allNames, name) {
  if (name in allNames) {
    allNames[name]++;
  }
  else {
    allNames[name] = 1;
  }
  return allNames;
}, {});
console.log(countedNames)
var people = [
  { name: 'Alice', age: 21 },
  { name: 'Max', age: 20 },
  { name: 'Jane', age: 20 }
];

function groupBy(arr, props) {
  return arr.reduce(function (acc, obj) {
    var key = obj[props];
    if (!acc[key]) {
      acc[key] = [];
    }
    acc[key].push(obj);
    return acc;
  }, {});
}

var groupedPeople = groupBy(people, 'age');
console.log(groupedPeople)