1 of 4
Arnauld
  • 192k
  • 20
  • 175
  • 635

#JavaScript (ES6), 51 bytes

f=(a,b)=>a|b&&~~Math.max(a%10,b%10)+10*f(a/10,b/10)

Try it online!

Arnauld
  • 192k
  • 20
  • 175
  • 635