Flatten a two-dimensional and multi-dimensional array

flattenArray.js

/* Flatter Arrays */

'use strict';

// Case 1 (two-dimensional array)

let array1 = [1, 2, [4, 5, 6], 8, [9, 10], 11];
let newArray1 = array1.flat();
console.log(newArray1);

// Case 2 (multi-dimensional array, wrong)

let array2 = [1, 2, [4, 5, 6], 8, [9, 10, [12, 13, 14]], 15];
let newArray2 = array2.flat();
console.log(newArray2);

// Case 3 (multi-dimensional array, resolved)

let array3 = [1, 2, [4, 5, 6], 8, [9, 10, [12, 13, 14]], 15];

function flatten(input) {
    const stack = [...input];
    const res = [];

    while(stack.length) {
        // pop value from stack
        const next = stack.pop();

        if(Array.isArray(next)) {
            // push back array items
            stack.push(...next);
        } else {
            res.push(next);
        }
    }

    // reverse to restore input order
    return res.reverse();
}

let newArray3 = flatten(array3);
console.log(newArray3);