const num1 = 5678;
const num2 = 1234;

const pivot = (start, end) => Math.floor((start + end) / 2);

const splitNumByPivot = num => {
  const stringyNum = num.toString()
  const len = stringyNum.length
  
  return [+stringyNum.slice(0, pivot(0, len)), +stringyNum.slice(pivot(0, len))]
}

const multiplyKaratsuba = (num1, num2) => {
  const [a, b] = splitNumByPivot(num1)
  const [c, d] = splitNumByPivot(num2)
  
  const ac = a * c //672
  const bd = b * d //2652  
  
  const step3 = (a + b) * (c + d) //6164  
  
  const step4 = step3 - ac - bd //2840
  
  return ac * Math.pow(10, 4) + step4 * Math.pow(10, 2) + bd
}

console.log(multiplyKaratsuba(num1, num2)) //7006652
console.log(num1 * num2) //7006652