JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Test Results</h1>
<ul>
</ul>

CSS

* { font-family: sans-serif; line-height: 150% }
li { margin-bottom: 20px }
li.success { color: green }
li.error { color: red }
li code { font-family: monospace; font-weight: bold; }

JavaScript

// ------------------------------------------
// FILL IN YOUR ANSWERS HERE AND CLICK RUN!
// TIME TAKEN: x MINUTES
// ------------------------------------------

function addOne(arr, base) {
	const sign = arr[0] < 0 ? -1 : 1
  const positive = arr.map((el)=>{
		const result = el< 0 ? -el : el;
    return result
  })
  const int = parseInt(positive.join(""), base)
  const total = int + sign
  const numbers = total.toString(base).split("");
  const result = numbers.map((el)=>{
	  let	intVersion = Number.parseInt(el,base)
  	if (intVersion < 10) {
    	return Number.parseInt(el,base) * sign
    } else {
    	return stringToHex(el, sign)
    }
  })
  return result
}

function stringToHex (hexString, sign) {
	if (sign > 0) {
  	return hexString.toUpperCase()
  } else {
  	return `-${hexString.toUpperCase()}`
  }
}

// --------------------------------------------
// TESTS BELOW - PLEASE ADD YOUR OWN TEST CASES
// --------------------------------------------

expect('addOne([5, 1, 2])', addOne([5, 1, 2]), [5, 1, 3]);
expect('addOne([0])', addOne([0]), [1]);
expect('addOne([9, 9])', addOne([9, 9]), [1, 0, 0]);
expect('addOne([1, 9, 9])', addOne([1, 9, 9]), [2, 0, 0]);
expect('addOne([-3, -7])', addOne([-3, -7]), [-3, -6]);
expect('addOne([-1, 0, 0])', addOne([-1, 0, 0]), [-9, -9]);
expect('addOne([1, 0, 1], 2)', addOne([1, 0, 1], 2), [1, 1, 0]);
expect('addOne([1, 1, 1], 2)', addOne([1, 1, 1], 2), [1, 0, 0, 0]);
expect('addOne([B, 3, 9], 16)', addOne(['B', 3, 9], 16), ['B', 3, 'A']);
expect('addOne([B, 3, F], 16)', addOne(['B', 3, 'F'], 16), ['B', 4, 0]);
expect('addOne([-3, -F], 16)', addOne([ -3, '-F'], 16), [-3, '-E']);
// ------------------------------------------

function compareArrays(arr1, arr2) {
    return $(arr1).not(arr2).length == 0 && $(arr2).not(arr1).length == 0;
}

function expect(name, a, b) {
  if (a !== b && !(a instanceof Array && b instanceof Array && compareArrays(a, b))) {
    $('ul').append('<li class="error">Testing <code>' + name + '</code>...