```js run demo
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.