Node.js: Build the Back End and APIs

Every app you use has a hidden half. When you log in, save a note or check an order, a program on another computer does the work. That program is the back end. This course builds one from an empty folder. If you hold one simple picture in your head first, every later lesson has a place to fit.

Think of a restaurant. You do not walk into the kitchen. You tell a waiter what you want, and the waiter brings back the food. The web works the same way.

• The client is whatever asks for something: a web page, a phone app or a script. It is the customer.
• The request is the order. It says what the client wants, such as the list of tasks.
• The server is a program that waits for requests and answers them. It is the kitchen. A back end is a server plus the rules and stored data behind it.
• The response is the answer the server sends back.

An API (application programming interface) is the menu. It lists what a client may ask for and what comes back. In this course the API is a set of web addresses, such as /tasks, that other programs can call.

Data usually travels as JSON. This is a plain-text format for structured data. It looks like a JavaScript object with double quotes around the names, for example {"title":"Buy milk","done":false}. Almost every language can read JSON, so a web page, a phone app and a Python script can all use one back end.

Node.js runs JavaScript outside the browser. So the language you already know can also build the kitchen. Node is free.

You will build one project, the Tasks API. It has accounts, login, and tasks that belong to each person. Every lesson adds one small piece, and by the end it is ready to run on the internet. A code block that starts with Save as is a complete file. Save it exactly as shown.

Get ready:

1. Download the LTS version of Node from nodejs.org. LTS means long-term support. Use Node 22.13 or newer. The code here was run on Node 24, and the Node documentation lists each feature we use as available from 22.13.
2. Open a terminal: Terminal on a Mac or Linux, or PowerShell on Windows.
3. Type node --version and press Enter. You should see v22.13 or a higher number.
4. Type npm --version. The tool npm comes with Node. You will use it to run scripts, but you will not install any packages in this course.

Before a real server, here is the whole idea in a few lines. The function answer plays the server. It takes a request object and returns a response object.

Save as scratch/hello.js:

Run it with node scratch/hello.js. You get:

The first request asked for /hello, which exists, so the status is 200, which means OK. The second asked for /nope. The server has no such page, so it answered 404, which means not found. A real server does exactly this, but over a network.

• Typing node alone. This opens a prompt where you type code. Press Ctrl+C twice to leave it, then run node followed by a file name.
• Running a command from the wrong folder. Node then says it cannot find the file. Use cd tasks-api first, then try again.
• Retyping code by hand and missing a character. Copy whole files instead, and keep the first comment line, because it tells you the file name.

node --version shows v22.13 or higher, and node scratch/hello.js prints the two lines shown above.