Array.prototype.join()

const elements = ["Fire", "Air", "Water"];

console.log(elements.join());
// Expected output: "Fire,Air,Water"

console.log(elements.join(""));
// Expected output: "FireAirWater"

console.log(elements.join("-"));
// Expected output: "Fire-Air-Water"

join()
join(separator)

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

console.log(matrix.join()); // 1,2,3,4,5,6,7,8,9
console.log(matrix.join(";")); // 1,2,3;4,5,6;7,8,9

const arr = [];
arr.push(1, [3, arr, 4], 2);
console.log(arr.join(";")); // 1;3,,4;2

const a = ["", "", ""];
a.join(); // ',,'
a.join(", "); // ', , '
a.join(" + "); // ' +  + '
a.join(""); // ''

console.log([1, , 3].join()); // '1,,3'
console.log([1, undefined, 3].join()); // '1,,3'

const arrayLike = {
  length: 3,
  0: 2,
  1: 3,
  2: 4,
  3: 5, // length 3  join() .
};
console.log(Array.prototype.join.call(arrayLike));
// 2,3,4
console.log(Array.prototype.join.call(arrayLike, "."));
// 2.3.4