What does it mean "event-driven" in JavaScript and Node.js?

What does it mean "event-driven" in JavaScript and Node.js?

How does event-driven applies to Node.js? Node.js is an environment for running JavaScript outside of the browse. What does it mean "event-driven" in JavaScript and Node.js? How does event-driven applies to JavaScript in the browser? Just starting out with JavaScript and "event-driven" is all over the place?

Just starting out with JavaScript and "event-driven" is all over the place? Worry not and keep reading to learn more!

Event-driven and publish-subscribe

Event-driven architectures build on a common pattern in software development known as publish-subscribe or observer pattern.

In an event-driven architecture there are at least two actors: the subject and the observer.

The subject is like an FM radio, it broadcasts a message to any observer interested in listening what the subject has to say.

There could be just one or one hundred observers, it does not matter as long as the subject has some message to broadcast.

Keep in mind that event-driven, publish-subscribe, and observer pattern are not the same thing in practice, but ideally they use the same approach: an entity broadcasts a message while other entities listen for it.

The publish-subscribe pattern is old as me. It's been theorized around 1987, while the observer pattern appeared in the quintessential book "Design patterns" by the gang of four in 1994.

How does event-driven applies to JavaScript in the browser?

JavaScript runs in your browser thanks to an engine.

Most popular JavaScript engines are V8, used by Google Chrome and Node.js, SpiderMonkey for Firefox, and JavaScriptCore, used by Safari/WebKit.

JavaScript engines enhance the language by providing a rich environment, offering also an event-driven platform for JavaScript.

In practice, JavaScript in the browser can interact with HTML elements, which are event emitters, that is, subjects able to emit events.

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