flatten-array-recursive+iterative.js

function flattenArrayRecursive(arr) {
    if (!Array.isArray(arr)) return [arr];
    const res = [];
    for (const el of arr) {
        res.push(...flattenArrayRecursive(el));
    }
    return res;
}

function flattenArrayIterative(els) {
    const res = [];
    
    // modified queue, intead of push, unshift, to maintain origianl els order
    const queue = [els];
    while (queue.length) {
        const el = queue.shift();
        if (Array.isArray(el)) queue.unshift(...el);
        else res.push(el);
    }
    
    return res;
}

console.log(flattenArrayRecursive([1]));
console.log(flattenArrayIterative([1]));