recursiveFlatten.js

function flatten(arr, ret = []) {
    for (const entry of arr) {
        if (Array.isArray(entry)) {
            flatten(entry, ret)
        } else {
            ret.push(entry)
        }
    }
    return ret;
}

const arr1 = [0, 1, 2, [3, 4]]; 
const arr2 = [1, 2, [3, 4, 5, [6, 7]]]; 

console.log(flatten(arr1)); // [ 0, 1, 2, 3, 4 ]
console.log(flatten(arr2)); // [1, 2, 3, 4, 5, 6, 7]