Flatten an array without using flat method.

flatternArray.js

const flattenArray = nestedArr => {
  // Checking for the emptiness of array
  if (!nestedArr.length) return null;
  // Creating a new array for every recursive call
  const newArr = [];
  // If the current element is an array, we recursively call the function and then push the values present in the returned array.
  // Else we just push the current element on to the new Array.
  nestedArr.forEach(el =>
    Array.isArray(el) ? newArr.push(...flattenArray(el)) : newArr.push(el)
  );
  // Finally we return the array to continue the recursion, or exit from the final call stack.
  return newArr;
};