Deep Diff of Two Objects

Implement deepDiff(a, b), which compares two values and returns an array of change records describing how to get from a to b.

Each record is one of:

• { path, type: 'added', newValue }: the key or index exists only in b,
• { path, type: 'removed', oldValue }: it exists only in a,
• { path, type: 'changed', oldValue, newValue }: it exists in both with different values.

Rules:

• path is a string of keys joined by dots, with array indexes as numbers: "user.tags.1". A difference at the root has path "".
• Two plain objects are compared key by key, and two arrays index by index, recursively. Only the innermost differing values are reported.
• Everything else (primitives, dates, functions, an object compared with an array...) is compared with Object.is; if different, report one changed record for that path with the whole values.
• Order of records: first, for each key of a in order, its removed or changed records; then, for each key only in b, its added record.
• Equal values return []. A key whose value is undefined exists, so {a: undefined} versus {} is a removal.