flatten

flatten.js

/*

  assumption: result is initialized in outer scope before first call

*/

const flatten = (input, result) => {
  // check inputs
  if (!Array.isArray(input))
    throw new Error("Input to this function is required to be an array");

  input.forEach(element => {
    // if element is an array, recurse it
    if (Array.isArray(element)) flatten(element, result);
    // check the input, make sure it's a number
    else if (!Number.isInteger(+element))
      throw new Error(
        `Element in array is expect to be an integer, got ${element}`
      );
    // all good, push to the result
    else result.push(Number.parseInt(element));
  });
  // and done
};

module.exports = flatten;

flatten.test.js

// run with node flatten.test.js
// result should say "Test complete. All passed." when done

const assert = require("assert");
const flatten = require("../src/flatten");
console.log("Test running...");

// declare a variable to hold results of test (for logging)
let r;
let expected;

assert.throws(() => flatten(), "Empty input should throw error");

assert.throws(
  () => flatten(["a"]),
  "Array with non integer should throw error"
);

assert.throws(() => flatten([{}]), "Array with non integer should throw error");

assert.throws(
  () => flatten(["1", "a"]),
  "Array with non integer should throw error"
);

assert.throws(
  () => flatten(["1", true]),
  "Array with boolean should throw error"
);

assert.throws(
  () => flatten(["1", () => {}]),
  "Array with function should throw error"
);

assert.throws(
  () => flatten(["1", new Symbol()]),
  "Array with Symbol should throw error"
);

assert.throws(
  () => flatten(["1", BigInt(4)]),
  "Array with BigInt should throw error"
);

assert.throws(
  () => flatten(["1", new Symbol()]),
  "Array with Symbol should throw error"
);

assert.throws(() => flatten(["1", null]), "Array with null should throw error");

assert.throws(
  () => flatten(["1", undefined]),
  "Array with undefined should throw error"
);

r = [];
flatten([1], r);
expected = [1];
assert.deepStrictEqual(
  r,
  expected,
  `single item input array did not match expected output, received ${r}; expected ${expected}`
);

r = [];
flatten([1, [2]], r);
expected = [1, 2];
assert.deepStrictEqual(
  r,
  expected,
  `1 level nested input array did not match expected output, received ${r}; expected ${expected}`
);

r = [];
expected = [1, 2, 3];
flatten([1, [2, [3]]], r);
assert.deepStrictEqual(
  r,
  expected,
  `2 level nested input array did not match expected output, received ${r}; expected ${expected}`
);

r = [];
expected = [1, 2, 3, 4, 5];
flatten([1, [2, [3, [4]]], 5], r);
assert.deepStrictEqual(
  r,
  expected,
  `3 level nested input array did not match expected output, received ${r}; expected ${expected}`
);

r = [];
expected = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
flatten([1, [2, [3, [4, 5, [6], 7], 8], 9], 10], r);
assert.deepStrictEqual(
  r,
  expected,
  `3 level nested/mixed input array did not match expected output, received ${r}; expected ${expected}`
);

r = [];
expected = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1, 50000, 5, 6];
flatten([1, [2, [3, [4, 5, [6], 7], 8], 9], 10, 1, [[50000, [5], 6]]], r);
assert.deepStrictEqual(
  r,
  expected,
  `3 level nested/mixed input array did not match expected output, received ${r}; expected ${expected}`
);

console.log("Test complete. All passed.");