onkeydown event

The onkeydown event activates when a keyboard key is pressed down while on an element. The event can be specified as a JavaScript expression or the name of a JavaScript function.

If an expression is used, the JavaScript keyword this refers to the element on which the key was pressed down.

If the name of a function is used, the function will receive 2 parameters when called:

The browser event object; this object can be used to retrieve the JavaScript key code for the key pressed down using the keyCode property; the preventEvent() API can be used to disallow the key
A reference to the element on which the key was pressed down

Example:

The following will restrict keyboard entry to letters A, B, C, D, and F on a grade field.

onkeydown: gradeKeys

function gradeKeys(event, element) {
  var key = event.keyCode;
  switch (key) {
    case 65: // allow A
    case 66: // allow B
    case 67: // allow C
    case 68: // allow D
    case 70: // allow F
    case 8:  // allow backspace
    case 46: // allow delete
    case 37: // allow left arrow
    case 39: // allow right arrow
    case 40: // allow down arrow
    case 38: // allow up arrow
    case 13: // allow enter
      break;
    default: // disallow all other keys
      preventEvent(event);
  }
}