How to detect if the value entered in HTML CSS input is a number

Question

I wrote a code like this:

Age: <input type="text" id="age">

And JavaScript:

<script>
    function validateForm() {
        let age = document.getElementById("age").value;
        if (age) // is number?
    }
</script>

I want the input to give an error if the value entered is a letter, I will only accept numbers. How can I tell if the entered value is not a number?

Answer 1

Without using/depending on CSS you can apply the style to HTML elements using plain JavaScript as follows.

        function validateForm() {
            let age = document.getElementById("age").value;             
            if (!isNaN(age)) {
               document.getElementById("age").style.borderColor="red";
            }
        }

   Age: <input type="text" onchange ="validateForm()" id="age">