popup.js

$(document).ready(function () {
    $("#number").keydown(function (e) {
        // Allow: backspace, delete, tab, escape, enter and .
        if ($.inArray(e.keyCode, [46, 8, 9, 27, 13, 110, 190]) !== -1 ||
            // Allow: Ctrl+A
            (e.keyCode == 65 && e.ctrlKey === true) ||
            // Allow: Ctrl+C
            (e.keyCode == 67 && e.ctrlKey === true) ||
            // Allow: Ctrl+X
            (e.keyCode == 88 && e.ctrlKey === true) ||
            // Allow: home, end, left, right
            (e.keyCode >= 35 && e.keyCode <= 39)) {
            // let it happen, don't do anything
            return;
        }
        // Ensure that it is a number and stop the keypress
        if ((e.shiftKey || (e.keyCode < 48 || e.keyCode > 57)) && (e.keyCode < 96 || e.keyCode > 105)) {
            e.preventDefault();
        }
    });

    $("#number").on('input', function () {
        var number = parseInt($("#number").val(), 10);
        if (isPrime(number) == true) {
            $("#result").text("is prime");
            $("#result").addClass("green");
            if ($("#result").hasClass("red"))
                $("#result").removeClass("red");
        } else {
            $("#result").text("isn't prime");
            $("#result").addClass("red");
            if ($("#result").hasClass("green"))
                $("#result").removeClass("green");
        }
    });
});

function isPrime(value) {
    var limit = value / 2;
    for (var i = 2; i <= limit; i++) {
        if (value % i == 0)
            return false;
    }
    return true;
}