There are several ways you can accomplish this. The most supported way is through a technique called Comet or long-polling. Basically, the client sends a request to the server and the server doesn't send a response until some event happens. This gives the illusion that the server is pushing to the client.
There are other methods and technologies that actually allow pushing to the client instead of just simulating it (i.e. Web Sockets), but many browsers don't support them.
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