getEndIndex (Selection.getEndIndex method)

// define the function which converts the selected text in an instance,
// and convert the string to upper or lower case.
function convertCase(target, menuItem) {
    var beginIndex:Number = Selection.getBeginIndex();
    var endIndex:Number = Selection.getEndIndex();
    var tempString:String;
    // make sure that text is actually selected.
    if (beginIndex>-1 && endIndex>-1) {
    // set the temporary string to the text before the selected text.
    tempString = target.text.slice(0, beginIndex);
    switch (menuItem.caption) {
    case 'Uppercase...' :
        // if the user selects the "Uppercase..." context menu item,
        // convert the selected text to upper case.
        tempString += target.text.substring(beginIndex, endIndex).toUpperCase();
        break;
    case 'Lowercase...' :
        tempString += target.text.substring(beginIndex, endIndex).toLowerCase();
        break;
    }
    // append the text after the selected text to the temporary string.
    tempString += target.text.slice(endIndex);
    // set the text in the target text field to the contents of the temporary string.
    target.text = tempString;
    }
}