Flattening multidimensional arrays in JavaScript

Description: Write a function named flatten that accepts an array and merge them in a single array

flatten([1, 2, 3]) // [1, 2, 3]

flatten([1, 2, [1, 2], [1, 3], 6]) // [1, 2, 1, 2, 1, 3, 6]

flatten([1, [2, [3, [4]], 5], 6]) // [1, 2, 3, 4, 5, 6]

flat()

One layer:

const arr = [0, 1, 2, [3, 4]];
console.log(arr.flat()); 

// flat() = flat(1)
// [0, 1, 2, 3, 4]

Two layers:

const arr = [1, 2, [3, 4, [5, 6]]];
console.log(arr.flat(2));

// [ 1, 2, 3, 4, 5, 6 ]

Many layers:

const arr = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);

// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

concat()

One layer works only

const arr = [1, 2, [3, 4]];
const newArray = [].concat(...arr);
console.log(newArray);

// output: [ 1, 2, 3, 4 ]

isArray()

function flatten(arr) {
  let result = [];
  for (let i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      let flatArr = flatten(arr[i]);
      for (let j = 0; j < flatArr.length; j++) {
        result.push(flatArr[j]);
      }
    } else {
      result.push(arr[i]);
    }
  }
  return result;
}

console.log(flatten([1, 2, [1, 2], [1, 3], 6]));

Reference: Array.prototype.flat()