What does the TypeScript interface output look like?

It generates a typed export interface with inferred types for each field — string, number, boolean, nested objects, and arrays. ISO date strings and URLs are annotated with inline comments.

How does the mock data generation work?

The tool generates 5 realistic mock objects by varying values based on the field name. Name fields use real-sounding names, email fields generate valid-format emails, date fields increment by 30 days, and numbers are varied within ±20% of the original.

What is the mock endpoint format?

It generates Express.js router code with a GET /api/items list endpoint and a GET /api/items/:id detail endpoint. Both include the generated mock data inline so the endpoint works immediately.

API Mock Generator — JSON to TypeScript Interface & Mock Data

Paste JSON object

About the API Mock Generator

When building frontend features before the backend is ready, or testing integrations, you need realistic mock data. Paste any JSON object and this tool generates three useful outputs instantly.

Three outputs in one

TypeScript Interface — a fully typed interface inferred from your JSON structure, with smart annotations for dates, URLs, and emails
Mock Data ×5 — five realistic variations of your object with sensible values for names, emails, dates, and numbers
Mock Endpoint — a ready-to-use Express.js router with list and detail endpoints pre-populated with mock data

Why mock APIs during development

Mock APIs let front-end developers work independently of back-end teams by simulating the expected response structure before the real endpoint exists. They are also used in testing to isolate units from external dependencies, in demos where real data would expose sensitive information, and in performance testing where hitting production endpoints would be inappropriate.

TypeScript interfaces — generated from your JSON schema for type-safe front-end code
Mock objects — realistic sample data matching your response structure for use in tests
Express endpoints — paste the generated handler directly into a mock server
Contract testing — use the interface to validate that real API responses match expected shape

Frequently Asked Questions

What does the TypeScript output look like?

A typed export interface with inferred types — string, number, boolean, nested objects, arrays. ISO dates and URLs get inline comments.

How does mock generation work?

5 objects generated by varying values based on field names — real names, valid email formats, incrementing dates, numbers within ±20%.

What format is the mock endpoint?

Express.js router with GET list and GET /:id detail endpoints, both pre-populated with mock data.

What is a TypeScript interface?

A TypeScript interface defines the shape of an object: which properties it has and what types they are. Interfaces are compile-time constructs — they are erased from the JavaScript output. They enable type checking, autocomplete, and refactoring support in editors without adding any runtime overhead.

How do I use the generated Express mock endpoint?

Copy the generated route handler and paste it into an Express application. Install express (npm install express), create an app.js file, add the route, and start the server with node app.js. The endpoint responds with the mock data on every request, simulating the real API.