Node.js 事件发射器

If you worked with JavaScript in the browser, you know how much of the interaction of the user is handled through events: mouse clicks, keyboard button presses, reacting to mouse movements, and so on.

On the backend side, Node.js offers us the option to build a similar system using the events module.

This module, in particular, offers the EventEmitter class, which we'll use to handle our events.

You initialize that using

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This object exposes, among many others, the on and emit methods.

• emit is used to trigger an event
• on is used to add a callback function that's going to be executed when the event is triggered

For example, let's create a start event, and as a matter of providing a sample, we react to that by just logging to the console:

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When we run

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the event handler function is triggered, and we get the console log.

You can pass arguments to the event handler by passing them as additional arguments to emit():

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Multiple arguments:

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The EventEmitter object also exposes several other methods to interact with events, like

• once(): add a one-time listener
• removeListener() / off(): remove an event listener from an event
• removeAllListeners(): remove all listeners for an event

You can read more about these methods in the official documentation.