largerHexxeh.js

function reverse (string) {
  return string.split('').reverse().join('')
}

function largerHexxeh (kAsString) {
  var len = kAsString.length
  var k10 = parseInt(kAsString, 16)
  var firstHalf = kAsString.slice(0, Math.ceil(len / 2))
  var firstHalfInt = parseInt(firstHalf, 16)
  var firstHalfA = firstHalfInt.toString(16)
  var firstHalfB = (firstHalfInt + 1).toString(16)
  var candidates
  
  // we need to determine if this is input is larger than k
  // we are going to cheat here
  // 
  if (len % 2 === 0) {
    candidates = [
      firstHalfA + reverse(firstHalfA),
      firstHalfB + reverse(firstHalfB)
    ]
  } else {
    candidates = [
      firstHalfA.slice(0, -1) + reverse(firstHalfA),
      firstHalfB.slice(0, -1) + reverse(firstHalfB)
    ]
  }
  if (parseInt(candidates[0], 16) > k10) {
    return candidates[0]
  } else {
    return candidates[1]
  }
}

console.log(largerHexxeh('1c3'))
console.log(largerHexxeh('1331'))
console.log(largerHexxeh('d25a'))
console.log(largerHexxeh('ffff')) //this is edge case... need to handle this too

// output

// ~ ❯ node hello.js
// 1d1
// 1441
// d33d
// 100001