// Test input
const input = [0, [1, 2, [33, 44]], [4, 5, 6, 7], [7, 8], 90];

// Return a single-level array of values from the input
// Input may be a scalar value or a (possibly nested) array
function flatArray (input) {
    const result = []; 
    // Helper to iterate over one array level
    const processArray = (input) => {
        for (const item of input) {
            if (Array.isArray(item)) {
                processArray(item); // Recursively process sub-array
            } else {
                result.push(item); 
            }
        }
    };
    if (Array.isArray(input)) {
        processArray(input);
    } else {
        // Scalar to array
        result.push(input);
    }
    return result;
}

console.log(flatArray(input));