Stop 23 · Interview PreparationTechnical · 6/8

REST APIs

In short

Calling fetch is easy. Interviews check whether you understand APIs as a contract: which method is safe to retry, how to paginate, and what to do when things fail.

Being fluent in discussing API design and consumption trade-offs — status codes, idempotency, versioning — beyond just knowing how to call `fetch`.

The jaadu carefully presses one big button once at a booking counter as a ticket slides out.

Think of it like this

The IRCTC booking counter

Checking seat availability ten times changes nothing. Pressing 'Book' twice could book two tickets. Good APIs are designed knowing which actions are safe to repeat and which are not.

Watch it happen

Which requests are safe to retry

Client

API

PUT /users/7

1/6The client updates user 7 with a PUT.

The big ideas

What you need to know

Idempotent methods

Doing it once or five times has the same effect. GET, PUT and DELETE are idempotent. POST usually isn't.

PUT vs PATCH

PUT replaces the whole resource. PATCH changes only the fields you send.

Pagination

Offset (?page=2&limit=20) is simple. Cursor (?after=abc) holds up better when new data keeps arriving.

Versioning and errors

Version with /v1/ or headers, and return consistent error shapes so the frontend can show useful messages.

How it works

Talking through an API you'd design

1
Name the resources
Nouns like /users, /posts, /posts/:id/comments.
2
Map the actions
Which method for read, create, update and delete.
3
Shape the data
Request and response bodies, including errors.
4
Handle scale
Pagination, filtering, caching headers and rate limits.

See it in code

Walk through the code

1async function api(path, options = {}, retries = 2) {2  const method = options.method ?? "GET";3  const safeToRetry = ["GET", "PUT", "DELETE"].includes(method);4 5  const res = await fetch(`/api/v1${path}`, options);6  if (res.status >= 500 && safeToRetry && retries > 0) {7    return api(path, options, retries - 1);8  }9  if (!res.ok) {10    const body = await res.json().catch(() => ({}));11    throw new Error(body.message ?? `Request failed: ${res.status}`);12  }13  return res.status === 204 ? null : res.json();14}