reduce.js

const nums = [1,2,3,4,5];

// your own
const reduce = (arr, fn, initValue) => {
  let result = initValue;
  arr.forEach( item => result = fn(result, item) );
  return result;
}
console.log(reduce(nums, (total, item) => total + item , 0))

// simple sum
const sum = nums.reduce((total, num) => total + num);
console.log(sum);

// avarage
const values = [10.2,15.4,20.5];
const avg = values.reduce( (total, val, i, {length: len}) => i === len - 1 ? (val + total) / len : val + total )
console.log(avg);

// flat array
const arr2 = [1, 2, 3, [11, 12, 13], 4, 5, 6, [14, 15, 16]];
const flat = arr2.reduce((result, item) => result.concat(item), []);
console.log(flat);

// transform
const users = [{id: 1, name: 'nir'}, {id: 2, name: 'ran'},{id: 3, name: 'or'},{id: 4, name: 'sahy'} ];
const map = users.reduce( (result, user) => ({...result, [user.id]: user }), {} );
console.log(map);

//  count
const data = ['angular','react','react','ember','angular','vue','vue','angular','react','react'];
const results = data.reduce( (result, entry) => {  
  result[entry] ? result[entry] += 1 : result[entry] = 1;      
  return result;
}, {} );
console.log(results)//log 

// pizza price
const discount = (price) => price / 2;
const addTax   = (price) => price * 0.2;
const giftWrap = (price) => price + 1;
const delivery = (price) => price + 12;

const total = [discount, addTax, giftWrap, delivery];
const pay = total.reduce( (price, fn) => price = fn(price), 10 );

console.log(pay);