how to mask first Five digit number on keyup using *

I modified your code to mask first 5 characters. Also this is bullet proof if someone paste a copied number.

https://codepen.io/anon/pen/PLvRWw

// Replace first 5 numbers with astericks
if (displayVal.length < 6){
  displayVal = displayVal.replace(/[0-9]/g, '*'); 
}
else{
  displayVal = '*'.repeat(5) + val.slice(5);
}