theorem.js

const assert = require("assert");

/**
 * Write some code, that will flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].
 */

function flattenArray(array) {
  if (!Array.isArray(array)) {
    throw new Error("Array expected");
  }

  return array.reduce(
    (accumulator, element) =>
      accumulator.concat(Array.isArray(element) ? flattenArray(element) : element),
    []
  );
}

assert.deepStrictEqual(flattenArray([[1,2,[3]],4]), [1,2,3,4], "Should flatten any nested array");
assert.deepStrictEqual(flattenArray([1,2,3,4]), [1,2,3,4], "Should not flatten if not needed");
assert.deepStrictEqual(flattenArray([]), [], "Should flatten empty array");
assert.deepStrictEqual(flattenArray([[]]), [], "Should flatten nested empty arrays");

const errorValues = [
  null, undefined, {}, 1, "string", Date()
]
errorValues.forEach(errorValue => {
  assert.throws(() => flattenArray(errorValue), 'Should throw an error when not an array is provided')
})