I spent several hours trying to understand why Babel wasn't transpiling const statements in Jeremy Wagner's yall.js library.

It was because the actual filename is yall.mjs, and I hadn't configured Babel to handle that extension.

The correct test configuration ended up being as follows:

test: /\.(m?j|t)s$/,

Or, if you don't need TypeScript:

test: /\.m?js$/,

Suffer me not to ponder the ills and advantages of the .mjs extension. That's for smarter people.

But since some folks do use that extension, it's probably worth updating your Babel config and related documentation/tutorials.