Check if input is number or letter javascript

You can use the isNaN function to determine if a value does not convert to a number. Example as below:

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

You could use the isNaN Function. It returns true if the data is not a number. That would be something like that:

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

Note: isNan considers 10.2 as a valid number.

Use Regular Expression to match for only letters. It's also good to have knowledge about, if you ever need to do something more complicated, like make sure it's a certain count of numbers.

function checkInp()
{
    var x=document.forms["myForm"]["age"].value;
    var regex=/^[a-zA-Z]+$/;
    if (!x.match(regex))
    {
        alert("Must input string");
        return false;
    }
}

Even better would be to deny anything but numbers:

function checkInp()
{
    var x=document.forms["myForm"]["age"].value;
    var regex=/^[0-9]+$/;
    if (x.match(regex))
    {
        alert("Must input numbers");
        return false;
    }
}