Language: TypeScript
Label: vulnerable
CWE: CWE-248 \u2014 Uncaught Exception
An exception is thrown from a function, but it is not caught.

Code:
    const callback = async (errorCode, errorContext) => {
      if (errorCode) {
        this.emit("connection_error", {
          req,
          code: errorCode,
          message: Server.errorMessages[errorCode],
          context: errorContext,
        });
        this.abortRequest(res, errorCode, errorContext);
        return;
      }

      const id = req._query.sid;
      let transport;

      if (id) {
        const client = this.clients[id];
        if (!client) {
          debug("upgrade attempt for closed client");
          res.close();
        } else if (client.upgrading) {
          debug("transport has already been trying to upgrade");
          res.close();
        } else if (client.upgraded) {
          debug("transport had already been upgraded");
          res.close();
        } else {
          debug("upgrading existing transport");
          transport = this.createTransport(req._query.transport, req);
          client.maybeUpgrade(transport);
        }
      } else {
        transport = await this.handshake(
          req._query.transport,
          req,
          (errorCode, errorContext) =>
            this.abortRequest(res, errorCode, errorContext)
        );
        if (!transport) {
          return;
        }
      }

      // calling writeStatus() triggers the flushing of any header added in a middleware
      req.res.writeStatus("101 Switching Protocols");

      res.upgrade(
        {
          transport,
        },
        req.getHeader("sec-websocket-key"),
        req.getHeader("sec-websocket-protocol"),
        req.getHeader("sec-websocket-extensions"),
        context
      );
    };