javascript - 将自定义方法分配给 WebSocket 时如何避免竞争条件？

Question

当我查看有关 WebSocket 的教程/文档时，我发现类似 this 的代码:

var ws = new WebSocket("ws://localhost:8765/dlt");
ws.onopen = () => {
  // do some very important stuff after connection has been established
  console.log("onopen");
}

但是这里的竞争条件呢？在 JavaScript 中是否以某种方式避免了？

例如，此代码(仅分配 onopen after 连接已打开)将失败:

var ws = new WebSocket("ws://localhost:8765/dlt");
setTimeout(() => {
  ws.onopen = () => {
    // do some very important stuff after connection has been established
    console.log("onopen"); ///  <== won't be called
  }
}, 100);

我可以确定在连接建立之前分配已经完成吗？

(我尝试使用自定义的 onopen() 方法扩展 WebSocket 但这似乎不起作用)

class MyWebSocket extends WebSocket {
  onopen() {
    console.log("onopen()");
    /// do some very important stuff after connection has been established
  }
}

Answer 1

您应该阅读有关 javascript 事件循环的文章:https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop#Event_loop

如果您查看有关Run-to-completion 的部分，您会得到以下有用的解释:

Each message is processed completely before any other message is processed. This offers some nice properties when reasoning about your program, including the fact that whenever a function runs, it cannot be pre-empted and will run entirely before any other code runs (and can modify data the function manipulates). This differs from C, for instance, where if a function runs in a thread, it may be stopped at any point by the runtime system to run some other code in another thread.

因此，在您的示例中，对 ws.onopen 的分配必须在 websocket 执行本质上异步的任何操作之前完成。通过将您的分配放在 setTimeout 中，您将它移出当前运行的上下文，因此它可能不会在 websocket 需要之前执行。