Three ways to use reduce in javascript

export const eg1 = [1, 45, 2, 3, 34, 4];
export const eg2 = [
  {
    gender: "male",
    name: "Ed"
  },
  {
    gender: "male",
    name: "Bob"
  },
  {
    gender: "female",
    name: "Sarah"
  }
];
export const eg3 = [
  1,45,2,3,34,4,1,2,
  3,4,5,6,3,2,9
];

const one = eg1.reduce(
  (prev, current) => prev + current,0
);
console.log(one);
//outputs 189

const two = eg2.reduce(
  (obj, item) => ({
    ...obj,
    [item['gender']]: [
      /** check if exists
       * if it doesn't start with blank array
       */
      ...obj[item['gender']] || [],
      item
    ]
  }),
  {} //default value
);
console.log(two)
//outputs {male: Array[2], female: Array[1]}

const three = eg3.reduce(
  (tally, item) => ({
    ...tally,
    [item]: (tally[item] || 0) + 1
  }),
  {}
);
console.log(three);
//outputs Object {1: 2, 2: 3, 3: 3, 4: 2, 5: 1…}