JSON Diff Checker

What a JSON diff is

A JSON diff compares two JSON values and reports exactly how they differ: which keys or array elements were added, which were removed, and which kept their path but changed value. Unlike a plain text diff, which compares files line by line, a JSON diff understands the structure of the data. It parses both inputs into real objects first, so differences in formatting, indentation, and key order are ignored. Two files that are byte-for-byte different but semantically identical produce zero reported changes.

This matters whenever JSON is the wire format. Developers reach for a structural diff to spot what changed between two API responses, to review a configuration change before deploying it, to debug why two environments behave differently, or to confirm that a data migration preserved every field. The tool above does this entirely in your browser: paste two values, separate them with a line containing only three dashes, and read the annotated list of changes.

How the comparison works

The tool parses each side with JSON.parse, then walks both trees in parallel, building a dotted path as it descends. At every node it applies a simple rule:

walk(a, b, path):
  if a === b: no change
  if leaf values differ:  ~ path: old -> new
  key only in b:          + path: value      (added)
  key only in a:          - path: value      (removed)
  both objects/arrays:    recurse into each key / index

Object keys are compared by name and are order-independent, while array elements are compared by position (index 0 against index 0, and so on). A path like tags[2] points at the third element of the tags array; a path like user.city points at a nested object key. The walk is linear in the size of the data, so even multi-megabyte payloads diff in milliseconds.

Worked example

Compare these two objects (the built-in Sample button loads them):

{"name":"Alice","age":30,"city":"SF","tags":["a","b"]}
---
{"name":"Alice","age":31,"city":"NYC","tags":["a","b","c"]}

The tool walks both trees and reports three changes. name matches and is silent. age changed, so it prints ~ age: 30 -> 31. city changed, giving ~ city: "SF" -> "NYC". The tags array matches at indices 0 and 1 but has an extra element at index 2 in the second input, so it prints + tags[2]: "c". The status line reads "3 difference(s) found." Reordering the keys in either object, or reformatting the whitespace, would not change this result.

How to read the output

Common pitfalls

• Missing the separator. The two values must be split by a line containing only three dashes. Without it the tool cannot tell where the first JSON ends.
• Invalid JSON. Trailing commas, single quotes, or unquoted keys are not valid JSON and produce a parse error. Validate or format the input first.
• Expecting smart array matching. Arrays diff by index, not by content, so inserting an element near the front can cascade into many reported changes even though only one item was added.
• Treating number and string as equal. The value 5 and the string "5" are different types and will be flagged as a change.
• Forgetting to copy. The result lives only in the browser tab; click Copy before navigating away, as nothing is saved.

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Frequently asked questions