More information at #612 by @Munter

I attempted to use an async function in .eleventy.js because I wanted to try something depending on disk access, and usually do those things async. It turned out everything I tried to configure didn't work. The reason for this was that the config resolution doesn't handle async or promises, and just went ahead and merged my returned promise with the default config.

Right now this isn’t possible, this issue suggests that it should be:

module.exports = async function(eleventyConfig) {};

The problem here is that the config is used in constructors all over the code base (which are not async friendly).