Introduction to APIs

An API (Application Programming Interface) is a contract that lets two pieces of software talk to each other. A web API lets a client (browser, mobile app) request data and actions from a server over HTTP.

How a web request works

Client                          Server
  │   ── HTTP Request ──────────▶  │
  │   GET /api/users/42            │
  │                                │ (looks up user)
  │   ◀────── HTTP Response ──────  │
  │   200 OK  { "name": "Priya" }  │

The client sends a request (method + URL + optional headers/body); the server returns a response (status code + headers + body, usually JSON).

JSON — the data format of the web

Most APIs exchange JSON (JavaScript Object Notation): lightweight, human-readable, and natively parsed by JavaScript.

{
  "id": 42,
  "name": "Priya Sharma",
  "skills": ["JavaScript", "React"],
  "active": true
}

Making a request in JavaScript

The browser's fetch API is the standard way to call an API:

async function getUser(id) {
  const response = await fetch(`https://api.example.com/users/${id}`);

  if (!response.ok) {
    throw new Error(`HTTP ${response.status}`);
  }

  const user = await response.json();
  return user;
}

getUser(42).then((user) => console.log(user.name));