Jquery Text:Allow text box only for letters using jQuery?

I want to make a text box allow only letters (a-z) using jQuery, Allow text box only for letters using jQuery, textbox to allow only alphabets using JQuery

HTML

<input id="txtFirstName" value="">

JavaScript

$('#txtFirstName').on('keydown paste',function(e){
  if (e.shiftKey || e.ctrlKey || e.altKey) {
    e.preventDefault();
  } else {
    var key = e.keyCode;
    if (!((key == 8) || (key == 32) || (key == 46) || (key >= 35 && key <= 40) || (key >= 65 && key <= 90) || (key >= 48 && key <= 57))) {
      e.preventDefault();
    }
  }
});