How can I implement process synchronization in Electron?

I am new to electron and about to build a mini application.In electron,I have no idea about this situation: main process need to send message to render process A when main process got signal from another render process. I tried:

ipc.on('signalA', function() {
    ipc.sendSync('signalB');

});

but it turned out to say sendSync is not a function.

via X.Wang