​
Messages

During execution (or when the state of the queue changes), the PromptExecutor sends messages back to the client through the send_sync method of PromptServer. These messages are received by a socket event listener defined in api.js (at time of writing around line 90, or search for this.socket.addEventListener), which creates a CustomEvent object for any known message type, and dispatches it to any registered listeners. An extension can register to receive events (normally done in the setup() function) following the standard Javascript idiom: 
api.addEventListener(message_type, messageHandler);
If the message_type is not one of the built in ones, it will be added to the list of known message types automatically. The message messageHandler will be called with a CustomEvent object, which extends the event raised by the socket to add a .detail property, which is a dictionary of the data sent by the server. So usage is generally along the lines of: 
function messageHandler(event) {
    if (event.detail.node == aNodeIdThatIsInteresting) {
        // do something with event.detail.other_things
    }
}

​
Built in message types

During execution (or when the state of the queue changes), the PromptExecutor sends the following messages back to the client through the send_sync method of PromptServer. An extension can register as a listener for any of these.
eventwhendata
execution_startWhen a prompt is about to runprompt_id
execution_errorWhen an error occurs during executionprompt_id, plus additional information
execution_interruptedWhen execution is stopped by a node raising InterruptProcessingExceptionprompt_id, node_id, node_type and executed (a list of executed nodes)
execution_cachedAt the start of executionprompt_id, nodes (a list of nodes which are being skipped because their cached outputs can be used)
execution_successWhen all nodes from the prompt have been successfully executedprompt_id, timestamp
executingWhen a new node is about to be executednode (node id or None to indicate completion), prompt_id
executedWhen a node returns a ui elementnode (node id), prompt_id, output
progressDuring execution of a node that implements the required hooknode (node id), prompt_id, value, max
statusWhen the state of the queue changesexec_info, a dictionary holding queue_remaining, the number of entries in the queue

​
Using executed

Despite the name, an executed message is not sent whenever a node completes execution (unlike executing), but only when the node returns a ui update. To do this, the main function needs to return a dictionary instead of a tuple: 
# at the end of my main method
        return { "ui":a_new_dictionary, "result": the_tuple_of_output_values }
a_new_dictionary will then be sent as the value of output in an executed message. The result key can be omitted if the node has no outputs (see, for instance, the code for SaveImage in nodes.py)

​
Custom message types

As indicated above, on the client side, a custom message type can be added simply by registering as a listener for a unique message type name. 
api.addEventListener("my.custom.message", messageHandler);
On the server, the code is equally simple: 
from server import PromptServer
# then, in your main execution function (normally)
        PromptServer.instance.send_sync("my.custom.message", a_dictionary)

​
Getting node_id

Most of the built-in messages include the current node id in the value of node. It’s likely that you will want to do the same. The node_id is available on the server side through a hidden input, which is obtained with the hidden key in the INPUT_TYPES dictionary: 
    @classmethod    
    def INPUT_TYPES(s):
        return {"required" : { }, # whatever your required inputs are 
                "hidden": { "node_id": "UNIQUE_ID" } } # Add the hidden key

    def my_main_function(self, required_inputs, node_id):
        # do some things
        PromptServer.instance.send_sync("my.custom.message", {"node": node_id, "other_things": etc})