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source property (event)

Browser support:
Returns a reference to the window object that contains the document that caused the onmessage event.
Note: The source property is supported in Internet Explorer from version 8, in Firefox from version 3 and in Opera from version 9.5.
An onmessage event occurs when the postMessage method sends a message to the current window. For details, see the pages for the postMessage method and the onmessage event.


You can find the related objects in the Supported by objects section below.
This property is read-only.

Possible values:

String that retrieves a reference to the caller window object.
Default: this property has no default value.

Example HTML code 1:

This example illustrates the use of the source property:
    <script type="text/javascript">
        function Init () {
            if (window.addEventListener) {  // all browsers except IE before version 9
                window.addEventListener ("message", OnMessage, false);
            else {
                if (window.attachEvent) {   // IE before version 9
                    window.attachEvent("onmessage", OnMessage);

        function GetState () {
            var frame = document.getElementById ("myFrame");
                // send the 'getstate' message to the frame window
            var message = "getstate";
            if (frame.contentWindow.postMessage) {
                frame.contentWindow.postMessage (message, "*");
            else {
                alert ("Your browser does not support the postMessage method!");

        function OnMessage (event) {
            var message =;
            var arr = message.split (",");
            if (arr[0] == "true") {
                alert ("The check box is checked.");
            else {
                alert ("The check box is not checked.");

            var selIndex = Number (arr[1]);
            alert ("The " + (selIndex + 1) + ". option is selected.");
<body onload="Init ();">
    <iframe id="myFrame" src="message.htm" width="500" height="200px"></iframe>
    <br /><br />
    <button onclick="GetState ()">Get the state of controls in the frame</button>
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