Repeat until the input is correct

Tutorial
The JavaScript language
JavaScript Fundamentals
Loops: while and for

Repeat until the input is correct

importance: 5

Write a loop which prompts for a number greater than 100. If the visitor enters another number – ask them to input again.

The loop must ask for a number until either the visitor enters a number greater than 100 or cancels the input/enters an empty line.

Here we can assume that the visitor only inputs numbers. There’s no need to implement a special handling for a non-numeric input in this task.

Run the 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:

The check for num <= 100 – that is, the entered value is still not greater than 100.
The check && num is false when num is null or an empty string. 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.