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Compared to regular Objects there's a number of advantages to using Maps: - They support proper iteration. - They have a simple way to check for the existence of data. - They have a simple/efficient way to check the number of elements. If this functionality was added today, I cannot imagine that we'd choose an Object for this data. Note also how in the scripting-implementation the `actions` were already converted into a Map, via the `createActionsMap` helper. In the Firefox PDF Viewer sending `Map`s to the scripting-implementation should be fine, since it uses the browser `Cu.cloneInto` functionality; see https://searchfox.org/firefox-main/source/toolkit/components/pdfjs/content/PdfSandbox.sys.mjs However with QuickJS, used by the GENERIC viewer, all data needs to be stringified and unfortunately `JSON.stringify()` doesn't support Maps. Hence we convert Maps to Objects, via a [`replacer` function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#replacer), since the existing `createActionsMap` usage will convert the actions-Objects back to Maps.