APIs aren’t just about fetching data.
They’re about trust. Mutual understanding. Respect. And sometimes… brutal betrayal.

That trust is encoded in the API contract — the agreed structure of requests and responses between frontend and backend.

Today we’re decoding:

What an API contract actually is
How to survive without one (but maybe don’t)
Why types are your BFF
How to make this pact less painful

So… What’s an API Contract?

An API contract is the shared agreement between frontend and backend on:

What requests can be made
What structure the responses will have
What status codes are used
And what to expect on errors

It’s like a prenup for your components and your data.

Why It Matters

If frontend expects:

{ "name": "Faiaz", "avatar": "url", "isOnline": true }

…but backend sends:

{ "full_name": "Faiaz", "profile_img": "url" }

You don’t just get a broken UI — you get a broken soul.

The mismatch between expectations and reality is a silent killer of productivity and sanity.

Real-World Consequences of a Bad Contract

Issue	Pain
Fields missing/renamed	Breaks the UI
Wrong types	`undefined is not a function`
No docs or samples	Guesswork hell
Backend changes silently	Production bugs outta nowhere
Status code roulette	Can’t even show proper error UX

How to Make the Pact Stronger

1. Use OpenAPI/Swagger (aka. actual docs)

Machine-readable, dev-readable. Best of both worlds.

You’ll know what endpoints exist, their params, response shapes, and error formats — like a menu with pictures.

2. Define Types (TS, Zod, Yup)

If the response lies, your app knows.

type User = {
  id: string;
  name: string;
  avatar: string;
  isOnline: boolean;
}

Then use validators like Zod to make sure the backend ain't gaslighting you.

const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
  avatar: z.string().url(),
  isOnline: z.boolean(),
});

3. Create Shared Types Between FE & BE

Sounds like magic? Use tools like:

tRPC
TypeScript SDK generators
Or shared @types package in monorepos

So backend changes = frontend gets updated types automatically.

4. Start Mocking Early

Don’t wait for backend to finish — use:

Mock Service Worker (MSW)
JSON Server
Swagger-generated mock servers

Helps FE and BE work in parallel like real adults.

Common Frontend Struggles (and Fixes)

Problem	Fix
Response shape changed	Use type validation (Zod, Yup)
Can’t understand backend response	Ask for Swagger or example payload
Error response structure varies	Standardize error model with backend
Unclear field types (null vs string?)	Use `Partial<T>` or defensive parsing
API updates silently	Use semantic versioning + changelogs

Team Agreements That Save Lives

Agree on error shapes ({ code, message })
Document every new endpoint (even WIP ones)
Set up a shared Postman collection or Swagger URL
Never change response shape in prod without notice
Use versioned endpoints (/v1/users) when making breaking changes

Final Thoughts

APIs are the bridge between frontend dreams and backend logic.
But without a solid contract, it’s just cross-origin chaos.

So the next time someone says:

"Just hit the endpoint and see what comes back."

You reply:

"Cool, and should I also roll the dice while I’m at it?"

Be smart. Type your responses. Mock early. Validate often.
Make the contract sacred.

📚 This is Awaiting Response(),
Where we don’t just fetch data — we establish healthy boundaries.

Next up:
Debounce, Throttle, Cancel – Optimizing API Calls Like a Pro

Let’s save your UI from making 300 network requests per keystroke 😎

API Contracts – The Blood Pact Between FE & BE