Check if input is number or letter javascript

Question

I'm using forms in HTML and javascript. I would like an alert to pop up only if the user inputs a LETTER and clicks submit.

So I have the HTML code:

<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

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;   } } 

Answer 2

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;     } }