JSON Transform Rules

About this tool

JSON Transform Rules reshapes a JSON document with a small declarative rule list. Each rule names an output target path and where its value comes from: a JSONPath-style selector, a literal value, or a removal operation in merge mode. It is useful when you need to normalize an API response, build a smaller payload for another system, redact fields before sharing a sample, or debug how a mapping will behave before you put it in code.

The shorthand form is designed for quick work:

id = $.user.id
name = $.user.name
email = $.user.contact.email
source = "import"

For larger mappings, paste a JSON object of target -> selector pairs or a JSON array of rule objects with target, source, value, default, transform, separator, when, and op. Selectors support $, .key, ["key"], [0], [*], .*, and ..key. Target paths use dot and bracket notation, create missing objects and arrays, and can append with tags[].

Worked example — map a nested API response into a flatter payload:

Source JSON:

{
  "user": {"id": 7, "name": "Ada Lovelace", "email": "[email protected]"},
  "orders": [{"total": 19.5}, {"total": 30.5}]
}

Rules:

id = $.user.id
name = $.user.name
email = $.user.email
total = $..total
source = "import"

Output:

{
  "id": 7,
  "name": "Ada Lovelace",
  "email": "[email protected]",
  "total": [
    19.5,
    30.5
  ],
  "source": "import"
}

Use Each selector when one input array should become one output object per item. For example, set each to $.automobiles[*], then rules like title = $.model and year = $.year run against every automobile and return an array of mapped objects. Use Output → Rule match report when a mapping comes out empty; it shows which rules matched, wrote, skipped, or used defaults.

Limits and edge cases

• Source JSON is capped at 5 MB, rules at 200 KB, and one run at 500 rules. A selector can match at most 100,000 values, and each can fan out over at most 50,000 items.
• This is a JSONPath subset, not a full query language. Filters such as [?(@.price > 10)], slices, arithmetic expressions, and script callbacks are intentionally out of scope. Use when for simple truthy guards and transforms such as sum, count, first, join, upper, trim, keys, and values for common reshaping steps.
• mode = build starts with an empty object. mode = merge starts with a copy of the input, so -path shorthand or op = "remove" can redact fields.
• on_missing = skip omits missing targets, null writes null, and error stops on the first missing selector. A rule-level default overrides all three.
• array_mode = auto returns a scalar for one match and an array for many matches. Choose always for stable array shapes or first when only the first value should survive.
• Target array indexes are capped at 10,000 to catch accidental sparse-array writes like items[999999].

FAQ