Devise a function that accepts an arbitrarily-nested array with elements of arbitrary types, and returns a flattened version of it. Do not solve the task using a built-in function that can accomplish the whole task on its own.

Example:
flattenList(["This is a string", 1, 2, [3], [4, [5, 6]], [[7]], 8, "[10, 11]"]) // ["This is a string", 1, 2, 3, 4, 5, 6, 7, 8, "[10, 11]"]