Ship frontend code before the backend is ready.
Stop waiting for API endpoints. Generate realistic mock JSON, simulate network latency, and format messy payloads instantly.
No signup required. No trackers.
The mock data problem
Lorem Ipsum doesn't crash your UI. Real data does.
Building against perfect static files creates false confidence. When the real API goes live, strings are longer than expected, arrays are null instead of empty, and numbers come back as strings. We built JSONMock to simulate the messy reality of production APIs.
Simulate Latency
Because production APIs never respond in 2ms. Test your loading states.
Edge Case Generation
Inject nulls, undefined keys, and massive arrays to stress test your frontend.
Engineering tools. No fluff.
Everything runs locally in your browser when possible. Fast, secure, and built for your daily workflow.
Mock API Generator
Define a schema and generate up to 10,000 rows of realistic JSON data instantly.
Launch GeneratorJSON Formatter
Paste minified JSON to format and validate it. Pinpoints syntax errors immediately.
Format JSONJWT Decoder
Decode JSON Web Tokens locally. Inspect claims without sending sensitive tokens to a server.
Decode TokenSchema Validator
Validate your JSON payloads against standard JSON Schema definitions before deploying.
Validate SchemaJSON to CSV
Flatten complex nested JSON arrays into tabular CSV format for Excel or database imports.
Convert FormatUUID Generator
Generate bulk v4 UUIDs instantly for database seeds or mock data foreign keys.
Generate UUIDs{
"transaction_id": "{{uuid}}",
"amount": "{{finance.amount}}",
"currency": "USD",
"customer": {
"name": "{{name.full}}",
"email": "{{internet.email}}"
},
"status": "{{helpers.arrayElement(['success', 'pending', 'failed'])}}"
}Deterministic, realistic generation.
Stop using "test user" for everything. Our generator builds highly realistic data payloads utilizing over 150 specific data types across finance, commerce, identity, and location domains.
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Relational Data
Generate foreign keys that actually match between datasets.
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Time Series Consistency
Ensure 'updated_at' is always chronologically after 'created_at'.
The cost of bad test data
| Testing Method | Bug Catch Rate | Setup Time |
|---|---|---|
| Static JSON Files | ~15% | Low |
| Production Database Clone | ~90% | Very High (Privacy risks) |
| Dynamic Mock API (JSONMock) | ~85% | Low |
40%
Of frontend bugs are related to unexpected null or missing payload keys that weren't caught in static mocking.
0s
Time spent configuring CORS or setting up a Node/Express mock server locally.
Engineering guides & best practices.
View all guidesARCHITECTURE
REST vs GraphQL
When to choose which, and how it impacts your mocking strategy.
REFERENCE
API Status Codes
A practical guide to HTTP status codes and when to actually use them.
DESIGN
Pagination Strategies
Offset vs Cursor based pagination compared with examples.
SECURITY
API Authentication
JWT, Sessions, and API keys. Implementation details for frontend devs.
Stateful interactions. Finally.
Static files are fine until you need to test a POST request followed by a GET. JSONMock supports ephemeral stateful sessions. Create an item, and it actually appears in your subsequent collection queries.
Session Concept
By passing a unique x-mock-session-id header, we provision an isolated, temporary in-memory datastore for your test suite. It auto-destroys after 10 minutes of inactivity. No database teardown scripts required.
Test async architecture without the architecture.
Modern applications rely on webhooks from Stripe, Twilio, or Shopify. Testing these locally usually involves ngrok and complex setup. JSONMock can dispatch synthetic webhooks to your local server on demand.
{
"id": "evt_3N8Wk1LkdI... ",
"object": "event",
"api_version": "2022-11-15",
"created": 1683228965,
"data": {
"object": {
"id": "pi_3N8Wk1LkdI...",
"amount": 2000,
"amount_received": 2000,
"currency": "usd",
"status": "succeeded"
}
},
"type": "payment_intent.succeeded"
}Instant Dispatch
Trigger webhooks instantly via our CLI or UI. No waiting for third-party sandboxes.
Valid Signatures
Generated webhooks include proper cryptographic signatures so you can test your middleware validation logic.
Terminal native.
Sometimes opening a browser is too slow. JSONMock offers a lightweight CLI for generating fixtures directly into your codebase. Perfect for CI/CD pipelines or local setup scripts.
Because production isn't always on a Gigabit connection.
Stop testing loading spinners against 2ms local endpoints. Define custom latency distributions to catch race conditions before they hit users.
PROFILE: CONSISTENT
Standard deviation of 20ms. Ideal for testing standard UI transitions.
PROFILE: SPIKY
Introduces random severe latency spikes. Finds places where timeouts aren't handled.
PROFILE: 3G MOBILE
Simulates poor cellular connections. Tests pessimistic UI updates.
Not just REST.
GraphQL schemas are complex. Writing mocks for deeply nested queries is painful. Upload your schema definition file (SDL) and JSONMock will instantly auto-generate resolvers that return realistic data matching your types.
Read our GraphQL vs REST mock guideZero trust. Zero data storage.
Developer tools shouldn't be a vector for data breaches. That's why the majority of our tools—including the JSON formatter, schema validator, and JWT decoder—run entirely locally in your browser using WebAssembly and standard JavaScript APIs.
We never transmit your payloads, private keys, or schemas to our servers. When using the hosted Mock API generator, the schemas are stored momentarily in memory and discarded the moment the session expires. We do not use third-party analytics trackers, cookies, or session recorders on our tools.
Test auth flows without bothering the security team.
Need to test JWT expiration logic? OAuth redirects? Role-based access control? JSONMock can generate endpoints that enforce mock authentication rules.
- Returns 401 Unauthorized for missing headers
- Returns 403 Forbidden based on decoded mock JWT roles
- Simulate token refresh endpoints
Protected Endpoint
Requires valid Bearer token
Authorization: Bearer expired_token_123
{ "error": "Token expired" }
Ready to write less boilerplate?
JSONMock is entirely free for developers. Start using our local-first formatting tools or spin up a mock API endpoint right now.