REST API Basics: How REST APIs Work (For Frontend Developers)

Almost every app you build talks to a REST API, to load data, save a form, log a user in. As a frontend developer you don't need to build one to use it well, but you do need to understand how it works. Here's REST explained in plain terms, with the pieces you actually touch.

What is a REST API?

A REST API is a way for a client and a server to communicate over HTTP using a consistent set of rules. You request a resource by its URL (an endpoint), using an HTTP method that describes the action, and the server responds with data (usually JSON) and a status code. That's it, REST is a convention, not a technology.

The HTTP methods (verbs)

How a REST API is used comes down to matching a method to an action on a resource:

The URL names the resource (/users, /users/5); the method names what you're doing to it. This consistency is what makes REST predictable.

Status codes: what the server tells you

Every response includes a status code that tells you what happened. Learn the common ones and you can debug most API issues fast:

• •2xx success: 200 OK, 201 Created, 204 No Content.
• •4xx you made a mistake: 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found.
• •5xx the server broke: 500 Internal Server Error, 503 Service Unavailable.

Calling a REST API from JavaScript

You call a REST API with fetch. A GET is the simplest, request a URL, then parse the JSON response:

async function getUsers() {
  const res = await fetch("https://api.example.com/users");
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  return res.json();
}

To send data (POST), add a method, headers and a JSON body:

async function createUser(user) {
  const res = await fetch("https://api.example.com/users", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(user),
  });
  return res.json();
}

REST is stateless

A defining REST principle: the server keeps no memory of your previous requests. Each request must carry everything needed to handle it, including auth (usually a token in a header). Why REST is stateless matters: it makes APIs scalable and predictable, any server can handle any request.

fetch("/api/profile", {
  headers: { Authorization: `Bearer ${token}` },
});

Stateless means each request stands alone. That's why you send your auth token with every request, the server doesn't remember you from the last one.

The short version

A REST API lets your frontend talk to a server over HTTP: URLs name resources, methods (GET/POST/PUT/DELETE) name actions, and status codes tell you the result. Call them with fetch, send JSON, include your auth token every time, and remember REST is stateless, each request stands on its own.