如何在 JavaScript 中组合函数调用？

Question

I'm currently developing a WebSocket. For that purpose I've created a JavaScript class. In this class I've 2 methods: subscribe() and bind() . The first method subscribes to a channel and the next one should listen to it.

But when I call my methods this way:

let socket = new CustomWebSocket();

socket.subscribe("my-channel").bind("my-event", function (response) {
    console.log(response);
});

I'm getting an error message in my console:

Uncaught TypeError: socket.subscribe(...).bind is not a function

This is how I wrote my functions inside my CustomWebSocket class:

subscribe(channelName) {
    let self = this;

    let subMsg = {
        type: "subscribe",
        channel: channelName
    };

    self.ws.send(JSON.stringify(subMsg));

    return channelName;
}

bind(eventName, callback) {
    let self = this;

    self.ws.onmessage = function (event) {
        let eventData = JSON.parse(event.data);
        if (eventData.type === "channelMessage") {
            callback(eventData.payload);
        }
    }
}

What did I wrong? It thought it can work this way...

Answer 1

You're returning channelName; from subscribe . So you're effectively trying to call bind on channelName .

To be able to call bind on the return value of subscribe , you need to return self or this from subscribe .