"On" events can block event loop ?

[pas8690]

Hi at all,
imagine that i have this piece of code ( with usage of express module ) :

app.use("/*", async (req, res, next) => {  
    //some code
    res.on("finish", () => {  
        //example JSON.parse(JSON.stringify(req.body))
    });  
    next();  
});

The question is: "The event loop can be blocked If i write a methods that do JSON.parse() and JSON.stringify() ( or other network calls) inside on("finish") block ? This block can be solved with usage of async methods ?
Thank you

[Trott]

Yes, JSON.parse() will block the event loop if it takes a long time. Event callbacks don't run in a separate event loop.

Using async does not change this. JSON.parse() will still block in that case.

One possible solution is to run JSON.parse() in a worker (worker_thread in Node.js, WebWorker in the browser).