def parseMessageJson(messageList, username=None):
    """
    messageList: list - Should be a bunch of dictionaries within a list.
    username: string - optional - Used to find message only from that user. Username is case sensitive.
        If username is not given, everyone's message will be used

    Returns a big string that contains all of the user's message.
    """
    retString = ""
    for d in messageList:
        if username != "All":
            if username != d['author']['username']:  # If the message doesn't belong to the specified user.
                continue
        retString += " [BEGIN] " + d["content"] + " [END]"

    return retString