A glimpse into the mind of a JavaScript framework author

 2 years ago
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Have you ever read the source code of a popular library and come across comments which explain what the code is doing? Sometimes the explanation given is straightforward but other times it leaves you scratching your head. I had one such moment recently and after much head scratching, it left me with a deeper appreciation of the effort that goes into open source tools and inspired me to write this post.

I came across the code in question whilst investigating how frontend frameworks attach event handlers to DOM elements. Over the last few months, I have been rebuilding different parts of the frontend stack as a way of improving my knowledge, and part of that has involved creating a UI framework based on the virtual DOM paradigm.

The code I was looking at was from Mithril . Internally, it registers events on DOM elements by creating an object and passing it as the second argument to the document.addEventListener method. The relevant source code is:

The third point: “ The object does not inherit from Object.prototype , to avoid any potential interference with that (e.g. setters). ” is what caught my attention. I reached out to Mithril core maintainer Isiah Meadows via the Mithril Gitter chat and his explanation is what we will go into next.

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