Generate mock JSON from a schema
Paste a tiny schema using string, number, boolean, email, name, date, uuid, or [type] arrays — and get N realistic JSON instances. Schemas are JSON objects, so they're easy to share.
Click Generate to produce instances
How the schema works
Strings name types (e.g. "uuid", "name"). Arrays with one element mean "array of that type". Objects are nested literally.
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Write a schema
Plain JSON. Each leaf is a type name string.
Inputs are treated as UTF-8 throughout the pipeline and validated client-side as you type — nothing dispatches until the value is well-formed.
schemainpututf-8client-sidevalidatedschema · textjson · set{ "id": "uuid" }loaded · parsedclient-side - 02
Nest as needed
Objects nest naturally.
["string"]means "array of strings".Each toggle re-computes through a memoised hook, so flipping options never resets your input and the UI stays snappy on large inputs.
nestoptionslivememoisedtogglesnest · memoiseddeep · live{ "tags": ["string"] }options set · memoisedlive - 03
Pick count
1 to 500 instances per batch.
Randomness — where used — comes from
crypto.getRandomValues, neverMath.random. Heavy work sits inuseMemo.countcomputeinstantlocaldeterministiccount · instant5 · computed5 instances
computed · instantinstant - 04
Generate & copy
Output is a JSON array — paste into Postman, fetch mocks, or seeders.
Copy writes through
navigator.clipboardand downloads use aBlob— zero network traffic. Same input plus same options always yield the same bytes.exportcopydownloadzero-networkreproducibleok · copiedready · copied[ {…}, {…} ]shipped · ready to shipsuccess
Pro tips for everyday utilities
Reach for crypto.getRandomValues
Cryptographically random for IDs, tokens, salts. Math.random is for animation and games, not security.
Mock data with seeds
Faker-style libraries can be seeded so tests are reproducible. Embed the seed in the fixture file.
Reference cards beat memory
Cheatsheets for Vim, Git, Docker, Linux signals are faster than man pages when you’re mid-flow.
Browser APIs vary
Touch events, gamepads, and clipboard differ across vendors. Feature-detect before use.
Persist in the right place
localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data.
When to mock JSON
API tests, fetch mocks, seed data.
API tests
Generate request payloads matching real schemas. Random values come from crypto.getRandomValues — cryptographically secure for tokens, salts, and session IDs. Math.random is deterministic enough for animations but predictable enough to be a security bug.
CSPRNG-backedFetch mocks
Wire fake responses into frontend dev. Pair with hashids, ULID, or UUID v7 when you need both random and sortable — useful for primary keys, public-facing IDs, and event log ordering without a sequence column.
reproducibleDatabase seeding
Bulk JSON ready to import as documents. Output is deterministic given the same seed, so fixtures and tests stay reproducible across CI runs and contributor machines. Embed the seed in the fixture file for full replay.
snappyGraphQL fixtures
Match resolver shapes for snapshot tests. Heavy work runs through useMemo so the UI stays snappy on large inputs. Same input + same options yields identical bytes — the kind of stability snapshot tests rely on.
feature-detectedPerformance testing
Generate large arrays to load-test parsers. Storage tools persist to the right backend: localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data. Pick the right shelf and quota worries disappear.
deterministicDocumentation
Realistic JSON samples for API docs. Browser feature detection: clipboard, fullscreen, gamepad, and Web Bluetooth APIs gracefully fall back when unavailable. No surprise white screens on Safari or older Firefox.
production-readyFrequently asked questions
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