消息事件内的回调变异

Question

So I ran into some Javascript weirdness if i write a function like this my passed callback works fine

messageHandler(callback) {
    this.ws.onmessage = (data) => {
        callback(data.data);
    };
}

If you write like this

messageHandler(callback) {
    this.ws.on('message', (data) => {
        callback(data);
    });
}

It mutates the callback into a message event I can't seem to figure out why but I'd like to understand this behavior if anyone can explain. Thanks in Advance.

Answer 1

I think the question omits crucial information but based on the code examples, I think you're using https://github.com/websockets/ws implementation, correct?

If so, then .ws.on('message', ... refers to Event Emitter listener . The params passed to your handler is the message or stream or whatever data object the emitter emits.

.ws.onmessage refers to the browser's Websocket API - so the parameter passed there is a MessageEvent . As you can see from the spec, data is a property on MessageEvent class which explains why you have to refer with 1 level of indirection as data.data .

Perhaps it'd be less confusing if you call your parameter event instead of data in the first example:

this.ws.onmessage = (event) => {
    callback(event.data);
};

You can also view this as server side events vs. client side events - your first example is a client side event whereas the second example is a server side event. Client side events conform to Websockets API whereas server side events, in NodeJS world, typically use event emitters.