en.javascript.info/1-js/02-first-steps/12-while-for/6-repeat-until-correct/solution.md

let num;

do {
  num = prompt("Enter a number greater than 100?", 0);
} while (num <= 100 && num);

The loop do..while repeats while both checks are truthy:

1. The check for num <= 100 -- that is, the entered value is still not greater than 100.
2. The check && num is false when num is null or a empty strig. Then the while loop stops too.

P.S. If num is null then num <= 100 is true, so without the 2nd check the loop wouldn't stop if the user clicks CANCEL. Both checks are required.