reduce and flatten methods from ground up

Reduce-Flatten.js

const nums = ['1', '2', '3', '4', '5'];
const listOfPeeps = [
  {
    _id: '123512',
    name: 'Ryan',
    friends: ['Hermione', 'Ron', 'Harry', 'Draco'],
  },
  {
   
    name: 'Donnie', _id: '32434',
    friends: ['Batman', 'Superman', 'Thing', 'Human Torch', 'Cyborg'],
  }
]
const customReduce = (arr, callBack, memo = arr.shift()) => {
  for (let i = 0; i < arr.length; i++) {
    memo = callBack(memo, arr[i]);
  }
  return memo;
};


const sum = customReduce(listOfPeeps, (object, item) => {
  object[item._id] = item;
  return object;
}, {});

console.log(sum);

const flatten = (arr) => {
  let newArr = [];
  for ( let i = 0; i < arr.length; i++ ) {
    if (Array.isArray(arr[i])) {
      newArr = newArr.concat(flatten(arr[i]));
    } else {
      newArr.push(arr[i]);
    }
  }
  return newArr;
};

const nestedArr = [1, 2, 3, [4], [[5], [6, [7], [[[[[[[[[[[[[[[[[[[[[[[[[[[[[[3, [[[[[[[[[[[[[[[[[[[[[[[[[[[234]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]];

console.log(flatten(nestedArr));