Traditionally, a web page has to send a request to the server to receive new data; that is, the page requests data from the server. With server-sent events, it's possible for a server to send new data to a web page at any time, by pushing messages to the web page. These incoming messages can be treated as Events + data inside the web page.
The server-sent event API is contained in the EventSource interface; to open a connection to the server to begin receiving events from it, create a new EventSource object specifying the URI of a script that generates the events. For example: