What Is JSON? A Five-Minute Guide

If you build apps, you'll see JSON everywhere: in API responses, in configuration files like package.json, in error messages, in database fields. It looks technical, but you can learn to read it in five minutes.

What it is

JSON (JavaScript Object Notation, usually pronounced "jay-son") is a text format for structured data. It's how programs write down information so another program — in any language — can read it back.

Here's a person, in JSON:

{
  "name": "Ana Silva",
  "age": 34,
  "email": "[email protected]",
  "isCustomer": true,
  "interests": ["photography", "hiking"],
  "address": {
    "city": "Lisbon",
    "country": "Portugal"
  },
  "phone": null
}

Even if you've never seen JSON, you can probably read that. That's the point: it's structured enough for computers and readable enough for people.

The building blocks

JSON has two ways to group things:

Objects — curly braces { } containing key: value pairs. Like a labelled form.

{ "city": "Lisbon", "country": "Portugal" }

Arrays — square brackets [ ] containing a list of values, in order.

["photography", "hiking", "cooking"]

And six kinds of values:

Type	Example
String (text)	`"Ana Silva"` — always double quotes
Number	`34`, `19.99` — no quotes
Boolean	`true` or `false` — no quotes
Null (nothing)	`null`
Object	`{ "city": "Lisbon" }`
Array	`["a", "b"]`

Objects and arrays can contain each other, as deeply as needed. An array of objects is very common — for example, a list of orders:

[
  { "id": 101, "total": 49.0, "status": "paid" },
  { "id": 102, "total": 12.5, "status": "refunded" }
]

The rules that trip people up

JSON is strict. These all make it invalid:

Single quotes. 'name' ❌ — must be "name" ✅
Keys without quotes. { name: "Ana" } ❌ — must be { "name": "Ana" } ✅
A trailing comma after the last item. ["a", "b",] ❌
Comments. // like this ❌ — standard JSON has no comments.
Missing commas between items, or unmatched brackets.

If you edit a JSON file by hand and something breaks with an error like Unexpected token or JSON.parse: expected ',' or '}', it's almost always one of these. Paste the JSON into an online validator, or ask your AI tool to find the problem.

(Some config files, like tsconfig.json, allow comments because the tool reading them is lenient. Most JSON doesn't.)

Where you'll meet JSON

API responses. When your app asks a service for data, the answer is usually JSON. (What Is an API?) You can see it yourself: in your browser's developer tools, Network tab, click a request and open Response.

Config files. package.json (explained here), tsconfig.json, .vscode/settings.json, Claude Code's settings.json, and many more.

Webhooks. When a service like Stripe notifies your app about an event, it sends JSON. (What Is a Webhook?)

Databases. PostgreSQL can store JSON in a column (jsonb) — handy for flexible data. And NoSQL databases like MongoDB store data in a JSON-like format.

Logs and errors. Many tools log in JSON so logs can be searched.

Reading JSON in code

In JavaScript, converting between JSON text and usable data is one line each way:

const data = JSON.parse('{"name": "Ana", "age": 34}')  // text → data
data.name  // "Ana"

const text = JSON.stringify({ name: "Ana", age: 34 })  // data → text

Other languages have the same: Python's json.loads and json.dumps, for example.

Tips

Pretty vs minified: JSON can be on one long line (small, for computers) or indented (readable, for people). Same data. Most editors can format it: in VS Code, "Format Document."
Big responses are easier to read in the browser. Most browsers display JSON URLs as a collapsible tree.
Don't put secrets in JSON config files that get committed to git. Use environment variables. (What Is an Environment Variable?)

The summary

JSON is text for structured data: objects { } of key-value pairs, arrays [ ] of values.
Values: strings (double quotes), numbers, true/false, null, objects, arrays.
It's strict: double quotes, quoted keys, no trailing commas, no comments.
You'll see it in APIs, config files, webhooks, and databases.

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