This website hosts an extraction from the Siesta's testing tool codebase - a single widget, which provides a deep diff for the 2 pieces of JSON data.
The JSON diff is performed by walking both data pieces in-depth, and generating 2 "diff trees", containing the information from both left and right sides, so that the representation of the trees aligns horizontally. This provides a good readability of the diff.
In general, this tool supports deep diff for the arbitrary JavaScript objects, not just JSON. This includes circular references, Map, Set, Date instances etc. However, right now it's not possible to enter such data.
I hope this tool will be useful. If you have found a bug, or have any feedback, please file an issue on github.
Enjoy ☺