How to compare two JSON files: structural diffs that ignore key order and whitespace

How this is calculated

JSON objects are unordered by definition, so {"a":1,"b":2} and {"b":2,"a":1} hold the same data. Arrays are ordered, so [1,2] and [2,1] are different, unless the array is really a set, like a list of tags. A good diff lets you pick. Arrays need care in another way too. Insert one item at the start of a list and a naive comparison sees every later index as changed. Lining the arrays up with a longest common subsequence fixes that, so the insert shows as one addition. Numbers need a rule as well. 1.0 and 1 are the same value written two ways, while 9007199254740993 and 9007199254740992 are different values that JavaScript happens to read as the same double. A diff that runs JSON.parse first can call those equal, which is exactly the wrong answer for IDs. Repeated keys are the last trap. When an object has the same key twice, parsers disagree on which copy wins, so an honest diff refuses to guess.

Verdict

Use a structural diff to review API changes, config edits and fixtures. Keep array order on for lists where position matters, like steps or rankings, and turn it off for sets like tags or permissions. Treat a repeated key as a bug to fix before comparing. When a diff says two documents match, it should mean every key and every exact value matches, not just that the formatted text looked similar.