insert text at a given position

gistfile1.js

insertAtCursor: function(myField, myValue) {
    //IE support
    if (document.selection) {
      myField.focus();
      sel = document.selection.createRange();
      sel.text = myValue;
    }
    //MOZILLA/NETSCAPE support
    else if (myField.selectionStart || myField.selectionStart == '0') {
      var startPos = myField.selectionStart;
      var endPos = myField.selectionEnd;
      if (myValue == 'backspace') {
        myValue ='';
        if ( startPos > 0 )
          startPos -= 1;
      } else
      if (myValue == 'del') {
        myValue ='';
        endPos += 1;
      } else
      if (myValue == 'up') {
        myField.selectionStart = 0;
        myField.selectionEnd = myField.selectionStart;
        return false;
      } else
      if (myValue == 'down') {
        myField.selectionStart = myField.value.length;
        myField.selectionEnd = myField.selectionStart;
        return false;
      } else
      if (myValue == 'left') {
        myField.selectionStart = startPos - 1;
        myField.selectionEnd = myField.selectionStart;
        return false;
      } else
      if (myValue == 'right') {
        myField.selectionStart = startPos + 1;
        myField.selectionEnd = myField.selectionStart;
        return false;
      }
      
      myField.value = myField.value.substring(0, startPos)
      + myValue
      + myField.value.substring(endPos, myField.value.length);
      myField.selectionStart = startPos + myValue.length;
      myField.selectionEnd = myField.selectionStart;
      return true;
      
    } else {
      myField.value += myValue;
      return true;
    }
  }