I'm curious about the origin of this.on event handler. Is it part of a particular library or framework?

While casually perusing through the application.js file in the express source code, I stumbled upon this interesting piece of code.

I'm curious about the origin of this '.on' event. Is it part of vanilla JavaScript or is it a feature provided by some library for handling events?

 this.on('mount', function onmount(parent) {
        // inherit trust proxy
        if (this.settings[trustProxyDefaultSymbol] === true
          && typeof parent.settings['trust proxy fn'] === 'function') {
          delete this.settings['trust proxy'];
          delete this.settings['trust proxy fn'];
        }

Link to Code

javascriptexpress

Answer №1

app, the application prototype (found in /lib/application.js and utilized in /lib/express.js), is endowed with the functionalities of an EventEmitter, a core Node.js type.

Within the current iteration of the codebase, this process occurs within /lib/express.js through the following line:

mixin(app, EventEmitter.prototype, false);

The function mixin originates from the merge-descriptors module.

Answer №2

According to the Node JS documentation:

The EventEmitter class is responsible for handling events emitted by objects in Node JS. By using the eventEmitter.on() function, one can attach multiple functions to listen for specific named events triggered by an object.

In this context, the instance of express app represented by this, as noted in this link, inherits all methods from EventEmitter, including the on() method.

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