Microsoft JScript runtime error: Object expected

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Javascript

Hi,

I have a button control as

<asp:Button runat="server" ID="btn" Text="OK" Width="100PX"  OnClientClick="return changeMessage();"/>

in aspx page.

In head section of page add a external js file using,

<script language="javascript" type="text/javascript" src="../Script/Map.js"></script>

Map.js content as below,

----------------------------------

//Function displays the google map
function initialize() {
    
   
    var mapOptions = {
        zoom: 6,
        center: new google.maps.LatLng(8.558058925477096, 76.87785568237302),
        mapTypeId: google.maps.MapTypeId.ROADMAP,
        navigationControl: true,
        navigationControlOptions:
    {
        style: google.maps.NavigationControlStyle.ZOOM_PAN
    },
        scaleControl: true,
        DoubleClickZoom: true,
        draggable: true,
        streetViewControl: true,
        draggableCursor: 'pointer'
    };

    map = new google.maps.Map(document.getElementById('map_canvas'), mapOptions);
    map.setZoom(16);
    
    //document.getElementById('messageDisplay').style.visibility = "hidden";
}



//Function handle the click event
function route() {
   
    alert("hai");
}

------------------------------------------------------------------------------

Then, in aspx page i call function 'route()' in Map.js inside a <script> tag as below

--------------------------

<script language="javascript" type="text/javascript">

    function changeMessage() {

        if (document.getElementById("MainContent_btn").value == "OK") {
            document.getElementById("MainContent_messageLabel").innerHTML = "Click START to start drawing route";
            document.getElementById("MainContent_routetxt").style.visibility = "hidden";
            document.getElementById("MainContent_btn").value = "START";
            return false;
        }

        //Calling route() in Map.js

        if (document.getElementById("MainContent_btn").value == "START") {

            route();
        } 
    }

</script> 

But it give error as ,
Microsoft JScript runtime error: Object expected.

what is wrong in code?..