Getting throw new ERR_INVALID_ARG_TYPE(name, ‘Function’, value);

I am using socket.io and getting this error:

throw new ERR_INVALID_ARG_TYPE(name, 'Function', value);
^

TypeError [ERR_INVALID_ARG_TYPE]: The "listener" argument must be of type function. Received undefined

I am making a controller for the callbacks on the socket, this is how it looks:

const { onDisconnect } = require('./app/controllers/socket');

/*
...
*/

io.on('connection', (socket) => {
    socket.on('disconnect', onDisconnect(socket));
});

I get the error right on the “onDisconect” function. However if I declare the function like this, it works:

io.on('connection', (socket) => {
    socket.on('disconnect', () => {
        console.log(`${socket.id} disconnected`);
    });
});

I have the “onDisconect” function on another script, which contains this:

const onDisconnect = (socket) => {
    console.log(`${socket.id} disconnected`);
}

module.exports = { onDisconnect };

Answers:

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Method 1

That’s because you’re calling onDisconnect(), which returns undefined, which is not a function. You have two options:

1. Simply change socket.on("disconnect", onDisconnect(socket)) to socket.on("disconnect", onDisconnect)
2. Return a function in onDisconnect:

function onDisconnect(socket) {
  return () => {
    console.log(`${socket.id} disconnected`);
  };
}

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