JS-Script for a report about the values of a simple field over all items of the currently selected collection in Zotero or your whole Zotero library

field-report.js

/*
  Change the constant 'field' to any simple Zotero field
  (see https://api.zotero.org/itemFields?pprint=1 for a list)
  and run the script as async funtion to see a report
  of the currently selected collection or your complete
  library. [License: CC0]
*/
const field = "language";

var collection = ZoteroPane.getSelectedCollection();
var observables = [];

if (collection) {
   var items = collection.getChildItems();
   observables = items.map(item => item.getField(field));
}
else {
   var s = new Zotero.Search();
   s.libraryID = ZoteroPane.getSelectedLibraryID();
   s.addCondition('noChildren', 'true')
   var itemIDs = await s.search();
   for (let id of itemIDs) {
      let item = Zotero.Items.get(id);
      observables[id] = item.getField(field);
   }
}

count = {}
for (let obs of observables) {
   if (obs in count) {
      count[obs]++;
   }
   else {
      count[obs] = 1;
   }
}

// sort the keys and create a new object in
// this order to be output
var sortedKeys = Object.keys(count).sort(function (a, b) {
    return a.toLowerCase().localeCompare(b.toLowerCase());
});
var countSorted = {};
for (let key of sortedKeys) {
   countSorted[key] = count[key];
}

return countSorted;