window.returnValue [ = vValue ]
Sets or retrieves the value returned from the modal dialog window
vReturnValue = window.showModalDialog(sURL [, vArguments] [, sFeatures])
    
        
            | sURL | Required. String that specifies the URL of the document to load and display. | 
    
Return Value
Variant. Returns the value of the returnValue property as set by the window of the document specified in sURL .
也就是说window.returnValue可以设置模态对话框的返回值。