flatten.js

const complexArray = [[1, 2], 3, [5, [6, 7]]]

const flatten = (complexArray) => {
  const output = [];
  (function flatDepth(complexArray) {
    array.forEach((data) => {
      if (Array.isArray(data)) {
        flatDepth(data);
      } else {
        output.push(data);
      }
    });
  })(array);
  return output;
}

console.log(flatten(complexArray));