JSON Diff

A JSON diff shows the structural differences between two JSON documents: added keys (present in the second but not the first), removed keys (present in the first but not the second), and changed values (present in both but with different values). Unchanged keys are typically omitted or shown for context. A full diff can also detect array reorderings and type changes.

JSON Patch (RFC 6902) describes changes to a JSON document as a series of operations: add, remove, replace, move, copy, and test. Each operation is an object with an op type and a path (JSON Pointer). For example: {"op": "replace", "path": "/name", "value": "Alice"}. JSON Patch is used in HTTP PATCH requests and is the standard way to describe incremental JSON updates in APIs.

Array comparison is tricky because two semantically equivalent arrays may differ in element order. Simple diffing compares by index — array[0] in A vs array[0] in B. This can produce misleading diffs when an element was inserted at the beginning. More sophisticated algorithms use longest-common-subsequence (LCS) to identify moved elements. This tool compares arrays by index for simplicity.

JSON Merge Patch (RFC 7396) is a simpler alternative to JSON Patch. It represents changes as a partial JSON document: keys with values replace existing keys; keys with null values delete keys; keys absent from the patch leave the original unchanged. It is simpler to construct but less expressive — it cannot represent setting a value to null without deleting the key.

Common use cases: comparing API responses before and after a code change, debugging configuration drift between environments, reviewing what changed in a JSON-based database record, comparing test fixtures, and verifying that a data migration produced the expected output. JSON diff is also useful for understanding what changed when a third-party API updates its response format.