Edit in JSFiddle

function action(){
    var input = prompt('숫자를 입력해주세요', '');
    var number = Number(input); //string->number

            //조건문 ?    참값   : 거짓값
    var result = (number > 0) ? '자연수' : '자연수 아님';

    document.getElementById('output').innerHTML = result;
}