SOURCE

var arr1 = [1, 2, [3, 4, [5, 334]]];


Array.prototype.myFlat = function (newArr = []) {
    this.forEach((item) => {
        Array.isArray(item) ? item.myFlat(newArr) : newArr.push(item);
    })
    return newArr
}

console.log(arr1.myFlat())