JavaScript is asynchronous because we can't stop the program to wait for server responses, etc.
Here I demonstrate the inital problem (with no async control); then I show how we used callbacks and the legibility issues they can cause (aka Pyramid of Doom); then we do promises to show how we can flatten this stuff out and use .then()s to build a process; finally I show async/await which makes it possible to write asynchronous code in the synchronous style that you're used to.
N.B. You will have to serve these files., rather than just open them, because es6 modules don't work with the file api (file:///). I'd suggest https://www.npmjs.com/package/http-server.