Question

My code is below, it outputs what the user is typing in the text box. It should output an error message if the user puts anything other than a number. I’m confused as to how to do this, though. Quite frankly, I’d settle with it being able to detect whether or not the first letter of input is a B, but I can’t quite figure that out either and the former option is preferred.

HTML

<label for="bannerID">Banner ID: B</label><input type="text" name="bannerID" id="bannerID" onkeyup="showBannerID()" value="" /><br /> 

<p id="bannerOutput"></p>

JavaScript

function showBannerID() {

var textInput = document.getElementById('bannerID').value;

if (textInput.length == 0) {
    document.getElementById('bannerOutput').innerHTML = "<strong class=\"error\">Field can't be empty!</strong>";
}
else if (textInput.charAt(0) == "B") {
    document.getElementById('bannerOutput').innerHTML = "<strong class=\"error\">Please omit the B! It's not necessary.</strong>
}
else {
    document.getElementById('bannerOutput').innerHTML = "Your Banner ID is: <strong>B" + textInput + "</strong>.";
}
}

Answer 1

You can use isNaN() (Not a Number) as a condition.

isNaN(123) would give false, isNaN(“hello”) would give true.