WebSocket.extensions
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since 2015年7月.
**WebSocket.extensions**是只读属性，返回服务器已选择的扩展值。目前，链接可以协定的扩展值只有空字符串或者一个扩展列表。
The WebSocket.extensions read-only property returns the extensions selected by the server. This is currently only the empty string or a list of extensions as negotiated by the connection.