how to show an alert box In javascript

Question

I've got 2 text-boxes wherein the user has to enter a number. If the user enters a letter, an alert box should appear, indicating that only numbers are to be entered.

How can that be done in JavaScript?

Updates:

This is the code I used for the alert box:

function checkIfNumeric(total) {
    if (isNaN(total)) {
        alert ("not a number!"); 
    }
}

Answer 1

If you want to check if the user has entered a number or not, you could use isNaN :

function foo() {
 var a = document.getElementById("inputField").value;
 if (isNaN(a)) 
 {
 alert("Must input numbers");
 }
 else
 {
 alert("its a number");
 }
}

where inputField is the ID of your input field. Of course you can use

 if (isNaN(a)) 
 {
 alert("Must input numbers");
 }
 else
 {
 alert("its a number");
 }

in any context, you don't have to place it in an apart function like I did here. JSFiddle: http://jsfiddle.net/m5cTr/1/