Building a Small Web API Project

Put Express, JSON, and an in-memory store together into one working mini API

Big idea: A web API is a program that listens for requests and sends back data (usually JSON). In this topic you will build a tiny "books" API step by step, using everything you have learned: Express to handle requests, JSON as the data format, and an in-memory array as a simple store.

By the end you will be able to:

start an Express server and make it listen for requests,
keep data in an in-memory array and send it back as JSON,
build GET and POST endpoints, read route params and the request body,
send correct status codes like 200, 201, and 404.

1. What "an API" really means

An API (Application Programming Interface) is a way for programs to talk to each other. A web API does this over HTTP: a client (a browser, a phone app, or a tool) sends a request, and your server sends back a response with some data.

The client asks for data; the Express server reads its in-memory store and answers with JSON.

Every request has a method (like GET to read or POST to create) and a path (like /books). Your job is to write a small function for each method+path pair. That function is called a route handler.

2. Starting the Express server

First we bring in Express, create an app, and tell it to listen on a port. The function you pass to app.listen runs once, when the server is ready.

const express = require('express');
const app = express();
const PORT = 3000;

app.listen(PORT, () => {
  console.log('API running on http://localhost:' + PORT);
});

Run it with node server.js. The console prints:

API running on http://localhost:3000

The server now stays running, waiting for requests. It will not print anything else until a client connects.

Building a Small Web API Project