let arr = [1, 2, [3, 4, 5, [6, 7, 8], 9], 10, [11, 12]];

flatten1 = arr => arr.flat(Infinity)

flatten2 = arr => arr.toString().split(',').map(item => Number(item))

flatten3 = arr => {

    while (arr.some(Array.isArray)) {

        arr = [].concat(...arr);

    }

    return arr

}

flatten4 = arr => {

    return arr.reduce((result, item) => {

        return result.concat(Array.isArray(item) ? flatten4(item) : item)

    }, [])

}