How to Create a Mock API for Free in Under 60 Seconds
As a developer, you’ve been there: the backend API isn't ready, but the deadline for your new feature is just around the corner. You can't build your UI without data, and creating placeholder data manually is tedious, error-prone, and a time sink.
What if you could generate a live, functioning REST API in less than a minute—for free?
This guide shows you how to go from a simple text description to a live API endpoint you can use in any application, with zero server setup.
Quick links:
What is a Mock API?
A mock API (or dummy API) is a simulated HTTP endpoint that returns predefined JSON—mimicking a real backend. It lets frontend devs, QA, and backend teams work in parallel without waiting for the real service.
Why use a mock API?
- Accelerate development: Unblock your UI work immediately.
- Improve testing: Reproduce edge cases and error states reliably.
- Simplify prototyping: Demo features for stakeholders without backend infra.
- Isolate dependencies: Build data workflows independently.
Important: Mock endpoints are public—never include secrets or PII.
The 3‑Step Workflow: From Prompt to Live API
We’ll use our AI JSON & Mock API Generator to handle everything end‑to‑end.
Step 1 — Describe your data in plain English
Go to the generator: /ai-json-generator
Describe the JSON you want to create”, provide a prompt like:
An array of 2 users. Each user object should have an integer id, a full name, a username, an email, and an address object containing a street, city, and zipcode. Also include an array of 3 tags for their interests.
Step 2 — Edit and validate your JSON
The generated JSON appears in “2. Edit or validate your JSON”.
- Edit any values or fields
- Add/remove nested structures
- Paste your own JSON if you already have it
You’ll see a live status: “Valid JSON” or “Invalid JSON”. Use the “Format JSON” action to pretty‑print and fix spacing fast. For heavy edits, you can also use:
Step 3 — Publish your API and get a live endpoint
When your JSON looks good:
- In the input below the editor, choose a unique endpoint name (e.g., my-test-users).
- Click “Create Your Own API”.
The tool instantly publishes a live, shareable URL:
https://formatjsononline.com/api/json/my-test-users
Guidelines:
- Endpoint may include letters, numbers, dashes, or underscores (3–60 chars).
- If taken, try users-v1, users_demo, etc.
- Versioning tip: Use -v1, -v2 suffixes as schemas evolve.
Test your new endpoint
Use the API Tester, or try these quick checks.
cURL:
curl -s https://formatjsononline.com/api/json/my-test-users | jq .JavaScript (fetch):
const url = "https://formatjsononline.com/api/json/my-test-users";
async function fetchUsers() {
const res = await fetch(url, { headers: { Accept: "application/json" } });
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const users = await res.json();
console.log(users);
}
fetchUsers();Python (requests):
import requests
url = "https://formatjsononline.com/api/json/my-test-users"
r = requests.get(url, headers={"Accept": "application/json"})
r.raise_for_status()
print(r.json())Use it in your app
Point your frontend at the mock URL while building UI and state. Swap to real endpoints later—same component structure. For E2E tests, publish predictable JSON and assert UI behavior.
Pro tip: Keep older versions live during migrations to avoid breaking consumers.
Updating your mock
Need changes?
- Open /ai-json-generator
- Generate/paste the new JSON and validate
- Publish under a new endpoint (e.g., products-v2) to maintain backward compatibility
Example: Auth‑style response
{
"user": {
"id": 42,
"name": "Ava Patel",
"email": "ava@example.com",
"roles": ["admin", "editor"]
},
"token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
"expiresIn": 3600
}Great for onboarding flows, role‑based UI, or protected route demos.
Common pitfalls and quick fixes
- CORS in the browser? Public endpoints are designed for client use; include Accept: application/json and avoid unusual headers.
- Large payloads? Trim arrays for prototypes; paginate or chunk for demos.
- Old data due to caching? In dev, append a cache‑buster (e.g., ?t=1699999999).
Frequently asked questions
Is this really free?
Yes. There’s a fair‑use daily limit to keep it fast for everyone.
Can I create complex, nested data?
Absolutely—nested objects, arrays of objects, mixed types. The more detail in your prompt, the better the result.
How long will my endpoint stay active?
Endpoints are intended for development and testing. While generally persisted long‑term, permanence isn’t guaranteed on the free tier.
Can I edit AI‑generated data before publishing?
Yes—edit inline with live validation.
Are POST/PUT supported?
Mock endpoints are read‑only (GET). For interactive testing, pair with /api-tester or a local mock server.
Is my data private?
Endpoints are public. Don’t publish secrets or PII.