JSON array set diff

About this tool

Two JSON arrays, four questions: what is in both, what is in either, what is in A but not B, and what is on exactly one side. This tool answers all four — union, intersection, difference (A − B) and symmetric difference — over arrays you paste in, and reports the counts alongside the result so you can sanity-check the answer at a glance.

Elements are compared as JSON values, not as text. Object key order never matters, and 1, 1.0 and 1e0 are the same number, so two exports that serialise differently still line up. When the records differ in ways you do not care about — a changed name, a fresh updated_at — set Match on field to id (or a dot-path like meta.sku) and elements are paired on that field only, the way lodash's differenceBy family works.

It suits the everyday list jobs: finding the user ids present in an export but missing from the database, deduplicating a merged tag list, checking which SKUs two feeds share, or diffing two API result sets by primary key. Input can be a JSON array or NDJSON (one JSON value per line), which is what most exports paste as. Everything runs locally in your browser through WebAssembly — neither array is uploaded anywhere.

Worked example

Array A

[{"id":1,"name":"Ada"},{"id":2,"name":"Bo"},{"id":3,"name":"Cy"}]

Array B

[{"id":2,"name":"CHANGED"}]

With Operation set to Difference (A − B) and Match on field set to id, the result is:

{
  "operation": "difference",
  "matched_by": "id",
  "counts": {
    "a": 3,
    "b": 1,
    "a_unique": 3,
    "b_unique": 1,
    "only_in_a": 2,
    "only_in_b": 0,
    "in_both": 1,
    "union": 3,
    "result": 2
  },
  "result": [
    {"id": 1, "name": "Ada"},
    {"id": 3, "name": "Cy"}
  ]
}

Ada and Cy are missing from B; Bo matched on id even though every other field changed. The counts block always describes the two sets (distinct match keys), so it stays meaningful whether or not repeats are collapsed in the result. Switch Output to Result array only to get the bare [{"id":1,…},{"id":3,…}] you can paste straight into the next step, and set Indent to 0 to minify it onto one line.

Limits and edge cases

• Each array is capped at 50,000 elements; anything larger is rejected with a clear message.
• Input must be a top-level JSON array, or NDJSON with two or more non-blank lines. A lone {"id":1} is treated as a mis-paste rather than a one-record export.
• Difference is directional: it is A − B. Swap the two boxes for B − A, or use symmetric difference to get both sides at once.
• When Match on field is set, every element of both arrays must carry that field — a missing one is an error naming the array and index, not a silent skip. Leave it blank to compare whole values.
• Result elements are always taken from array A wherever an operation could draw from either side (union and symmetric difference append B's leftovers afterwards), and the first occurrence of a repeated key wins.
• With Collapse repeats on (the default) the result is a true set. Turn it off to keep every matching occurrence, including duplicates within one array.
• Numbers are compared through 64-bit floats, so integers beyond 2^53 (about 9·10^15) can compare equal despite differing in their last digits. Match on a string key if you carry ids that large.
• Ignore string case folds case inside the matched value only; it does not change the elements that come back, which are returned exactly as pasted.
• Element order inside a compared value is significant: [1,2] and [2,1] are different elements, even though the arrays A and B themselves are treated as unordered sets.
• Indent accepts 0–8 spaces; larger values are clamped to 8.

FAQ