challenge.js

const arr1 = [
  ["name", "id", "age", "country"],
  ["Susan", "3", "20", "mali"],
  ["John", "1", "21", "chad"],
  ["Jose", "2", "23", "oman"],
  ["Alex", "4", "20", "fiji"],
];

const arr2 = [
  ["name", "id", "height"],
  ["Jose", "2", "50"],
  ["John", "1", "45"],
  ["Alex", "4", "43"],
  ["Susan", "3", "48"],
];

const arr3 = [
  ["name", "id", "parent"],
  ["Susan", "3", "yes"],
  ["John", "1", "yes"],
];

const arr4 = [
  ["name", "id", "isCool"],
  ["Alex", "4", true],
  ["John", "1", true],
  ["Susan", "3", true],
  ["Jose", "2", true]
];

// turns one of the arr1, arr2, etc., into an array of objects
function parseList(items) {
  const result = [];
  const headers = items.shift();

  items.forEach((cells) => {
    const item = {};

    cells.forEach((value, index) => {
      const key = headers[index];
      item[key] = value;
    });

    result.push(item);
  });

  return result;
}

// adds one of the results of parseList (an object) into the list array - checks if it exists first
// if it does, merges both objects, otherwise adds a new item to the list
function appendToList(list, object) {
  const index = list.findIndex(item => item.id === object.id);
  if (index === -1) {
    list.push(object);
  } else {
    list[index] = { ...list[index], ...object };
  }
  return list;
}

// takes all lists as parameters and builds the final result
function buildList(...items) {
  const objects = items.map(item => parseList(item)).flat();

  let result = [];
  objects.forEach(object => {
    result = appendToList(result, object);
  });

  result.sort((a, b) => a.id - b.id);
  return result;
}

buildList(arr1, arr2, arr3, arr4);

/*
[
  { "name": "John", "id": "1", "age": "21", "country": "chad", "height": "45", "parent": "yes", "isCool": true },
  { "name": "Jose", "id": "2", "age": "23", "country": "oman", "height": "50", "isCool": true },
  { "name": "Susan", "id": "3", "age": "20", "country": "mali", "height": "48", "parent": "yes", "isCool": true },
  { "name": "Alex", "id": "4", "age": "20", "country": "fiji", "height": "43", "isCool": true }
]
*/