Check if input is number or letter javascript

Question

HTML and javascript forms are what I'm utilizing. 

Only if the user enters a LETTER and hits submit do I want an alert to appear.

I have the HTML code here:

<form name="myForm" action="" onsubmit="return checkInp()" method="post">
    First name: <input type="text" name="age">
<input type="submit" value="Submit">   

And the javascript code:

function checkInp()
{
var x=document.forms["myForm"]["age"].value;
if (x consists of any letters) // this is the code I need to change
{
alert("Must input numbers");
return false;
}
}

Answer 1

The isNaN function can be used to determine whether or not a value converts to a number. Here's an example:

function checkInp()
{
  var x=document.forms["myForm"]["age"].value;
  if (isNaN(x)) 
  {
    alert("Must input numbers");
    return false;
  }
}