array.sort()

calling sort() by itself simply sorts the array in lexicographical (aka alphabetical) order

reduce

For reduce(function(previousValue, currentValue)..., most of time, previousValue and currentValue are the same type. Actually previousValue can be list, object which is different from currentValue too.

// map + filter
  entries() {
    return this.hashMap.keys.reduce((acc, key) => {
      if(key !== undefined) {
        acc.push(key.content);
      }
      return acc
    }, []);
  }

// Grouping objects by a property
let people = [
  { name: 'Alice', age: 21 },
  { name: 'Max', age: 20 },
  { name: 'Jane', age: 20 }
];

function groupBy(objectArray, property) {
  return objectArray.reduce(function (acc, obj) {
    let key = obj[property]
    if (!acc[key]) {
      acc[key] = []
    }
    acc[key].push(obj)
    return acc
  }, {})
}