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A lot of the work we do with Javascript involves sending information back and forth from servers.
You are probably familiar with the concept of APIs, which send data to servers or websites in a particular format, to get a specific response back.
These are known as REST APIs. Although useful, they are not very good at constant streams of data.
If you try to do something real-time with REST APIs, you're going to have a bad time. Fortunately, if we want a real-time connection with a user, we have an alternative, known as WebSockets.