non-mutating flatten()

flatten.ts

/**
 * Return the flattened version of an arbitrarily deep nested array as a new
 * array, keeping the original unchanged. It uses a simple recursive algorithm.
 *
 * Positives: clean, intuitive, doesn't mutate the argument
 * Negatives: 1) probably uses more memory than alternative implementations.
 *            2) recursive, so in theory more limited by system resources than
 *               an alternative non-recursive algorithm would be. Unlikely to
 *               be a problem in most use cases but if you had an array nested
 *               say 1 million levels deep, probably wouldn't want to use this
 *               method.
 *
 * @param unflattened
 */

export default function flatten(unflattened: any[]) {
  try {
    return unflattened.reduce(
      (flattened: any[], currElement: any) =>
        flattened.concat(
          Array.isArray(currElement) ? flatten(currElement) : currElement,
        ),
      [],
    );
  } catch (error) {
    // because Typescript !== foolproof
    return console.error(`Error! ${error.message}`);
  }
}

function test(arg: any) {
  console.log(
    `flatten(${JSON.stringify(arg)}) = ${JSON.stringify(flatten(arg))}`,
  );
}

export function tests() {
  test([1, [2, 3, [4, 5, 6, [[7], 8], 9, 10]]]);
  test([1, 2, 3, 4]);
  test(2);
  test(('hello' as any) as number[]);
}