Flatten Array

 Problem: Implement a function flatten that returns a newly-created array with all sub-array elements concatenated recursively into a single level.

Solution: 

const results = [];

function flatten(array) {

if (Array.isArray(array)) {

array.forEach((element) => {

if (Array.isArray(element)) {

flatten(element);

} else {

results.push(element);

}

});

}

return results;

}

console.log(flatten([1, 2, 3, [5, 6, [8, 9]], 23])); // [1, 2, 3, 5, 6, 8, 9, 23]