Nunjucks does a lot of things, like loading templates, calling filters, and more. All of this is synchronous by default (which isn't a problem for loading templates, since they are loaded once and cached forever). This limits what you can do in filters and template loaders, since you can't use any async functions.
Asynchronous templates can be paused in the middle of rendering and resumed later.This hasn't been a problem for a long time, and for most people never will be a problem. You don't want to mix too much logic with your templates, so you usually do all the complicated async work in a controller and pass the data to the template.
However, I can imagine sites that are heavily template-driven and developers wanting to wrap up some sort of behavior that depends on an async operation into a filter or custom tag. Nunjucks is built to allow people to add logic to their templates as needed, and works great for large content-heavy projects where not everybody is familiar with the backend.
Regardless, I think asynchronous control is an interesting feature that doesn't adversely effect existing templates, so I decided to dig into it. Here's what I came up with.
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