It is possible to send JavaScript code to be executed on client from a C++ module loaded on server.

Run JS code on client

The JS code you want to execute on client can be sent from server to client as a string using WApplication’s doJavaScript function (SDK/Headers/wt_inc/Wt/WApplication). Your module needs to link against Wt lib located in SDK (SDK/Libraries/wt_lib/).

1. Include XeUpdateLock.h from SDK (SDK/Headers/inc/) - it will also include Wt/WApplication

2. Take XeUpdateLock before getting WApplication object

3. Get WApplication object and call doJavaScript on WApplication object

Example how to print something to console:

XeUpdateLock lock;
if (lock)
{
    Wt::WApplication::instance()->doJavaScript("console.log('This message is printed to console');");
}

Note: If you do not take XeUpdateLock, Wt::WApplication::instance() will return a null pointer.

The JS code is evaluated on client by calling window.eval() function before run and an exception will be thrown if it is not valid what will cause a client disconnect.

Post a message to parent frame

You can use this mechanism and run JavaScript code that will post a message from iframe to its parent. There are two ways to send a message:

Via ‘xepstm_to_p’ event

Create a xepstm_to_p event with two members _m for message name and _mc for message content.

XeUpdateLock lock;
if (lock)
{
    std::string js = "{"
        "var event = document.createEvent('Event');"
        "event.initEvent('xepstm_to_p', true, true);"
        "event._m = messageName;"
        "event._mc = messageContent;"
        "window.dispatchEvent(event);"
        "}";
    Wt::WApplication::instance()->doJavaScript(js);
}

messageName and messageContent are arbitrary strings.

This will send a JSON message like this to parent frame:

{
    messageName: messageName,
    message: messageContent
}

Using postMessage

Alternatively you can send any message formatted as you want to the parent frame using window parent’s postMessage function. E.g. just send a string:

XeUpdateLock lock;
if (lock)
{
    std::string js = "if (window.parent) {"
                         "window.parent.postMessage("my message", '*');"
                     "}";
    Wt::WApplication::instance()->doJavaScript(js);
}