en.javascript.info/1-js/4-data-structures/2-number/3-repeat-until-number/_js.view/solution.js

function readNumber() {
  let num;

  do {
    num = prompt("Enter a number please?", 0);
  } while ( !isFinite(num) );

  if (num === null || num === '') return null;
  
  return +num;
}