I am trying to:

1. send "" or null when "select all" is selected OR
2. send comma separated values/text when "select all" is selcted

My code:

<select id="selCrashType" class="form-control select" multiple onchange="RefreshCrashMap()">
                                                    <option value="Head on" selected>Head on</option>
                                                    <option value="Intersection">Intersection</option>
                                                    <option value="Other">Other</option>
                                                    <option value="Rear end">Rear end</option>
                                                    <option value="Run off road">Run off road</option>
                                                    <option value="Vulnerable">Vulnerable</option>
                                                </select>

$('#selCrashType option:selected').text()

above code returns

Head onIntersectionOtherRear endRun off roadVulnerable