Detect numbers or letters with jquery/javascript?

Use event.key and modern JS!

No number codes anymore. You can check key directly.

const key = event.key.toLowerCase();

if (key.length !== 1) {
  return;
}

const isLetter = (key >= 'a' && key <= 'z');
const isNumber = (key >= '0' && key <= '9');

if (isLetter || isNumber) {
  // Do something
}

You could also use a simple regex. ^$ ensures 1 char, i ignores case

/^[a-z0-9]$/i.test(event.key)

or individually:

const isLetter = /^[a-z]$/i.test(event.key)
const isNumber = /^[0-9]$/i.test(event.key)

First, if you're doing this, make sure it's in the keypress event, which is the only event for which you can reliably obtain information about the character the user has typed. Then I'd use the approach Andy E suggested:

document.onkeypress = function(evt) {
   evt = evt || window.event;
   var charCode = evt.which || evt.keyCode;
   var charStr = String.fromCharCode(charCode);
   if (/[a-z0-9]/i.test(charStr)) {
       alert("Letter or number typed");
   }
};

If you want to check for backspace, I'd use the keydown event instead and check for a keyCode of 8 because several browsers (including Chrome) do not fire a keypress event for the backspace key.

If you want to check a range of letters you can use greater than and less than:

if (event.keyCode >= 48 && event.keyCode <= 57) {
  alert('input was 0-9');
}
if (event.keyCode >= 65 && event.keyCode <= 90) {
  alert('input was a-z');
}

For a more dynamic check, use a regular expression:

const input = String.fromCharCode(event.keyCode);

if (/[a-zA-Z0-9-_ ]/.test(input)) {
  alert('input was a letter, number, hyphen, underscore or space');
}

See the MDC documentation for the keyCode property, which explains the difference between that and the which property and which events they apply to.