What are reactors / reactions in event-sourcing?

Question

I am new to CQRS and I read about projections and projectors, as well as reactors, but my resource does not cover it completely. Can someone define the reactors completely in the Event History or give me a link or book to read? my main questions are:

• What are reactors?
• What is the structure of the reactors?
• How do we sell reactors?

Answer 1

A projection is a function that takes a domain event and converts it into an action on a list, tree, graph, any ...

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    // ...
    mark.asDone();
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