Diff two objects into added, removed and updated

Sending the entire object on every save loses concurrent edits made by someone else and makes the audit log useless. Sending only what changed needs a diff.

Implement diffObjects(before, after). Compare only the top-level own keys of the two objects and return an object with exactly three keys: added, removed and updated, each a plain object, each empty when nothing falls into it. added maps each key present in after but not in before to its new value. removed maps each key present in before but not in after to its old value. updated maps each key present in both whose values differ to an object { from, to } holding the old and new values. Values are compared structurally to any depth: nested objects and arrays with the same contents count as unchanged, an array is never equal to a plain object, and NaN counts as equal to itself. A nested change is reported as one entry for the whole top-level key, with from and to being the whole old and new values. Neither input is modified.

diffObjects({ a: 1, b: 2 }, { a: 1, b: 3, c: 4 });
// {
//   added:   { c: 4 },
//   removed: {},
//   updated: { b: { from: 2, to: 3 } },
// }

diffObjects({ a: { b: [1, 2] } }, { a: { b: [1, 2] } });
// { added: {}, removed: {}, updated: {} } — deep-equal values are unchanged

diffObjects({ a: { b: 1 } }, { a: { b: 2 } }).updated;
// { a: { from: { b: 1 }, to: { b: 2 } } } — nested changes report the whole value