Fathom’s exceptions hang off an exceptions object in the top-level Fathom module. To import them, do something like this:

const {
  exceptions: { NoWindowError },
} = require('fathom-web');

This will result in a top-level NoWindowError symbol.

class CycleError()

A rule() depends on another rule which itself depends on the first rule again, either directly or indirectly.

class NoWindowError()

An examined element was not contained in a browser window object, but something needed it to be.