What is JSON?

The short answer

JSON stands for JavaScript Object Notation. It is a lightweight, text-based format for storing and exchanging data. Despite its name, JSON is completely language-independent — it is used by virtually every modern programming language including Python, Java, Go, Ruby, PHP, and of course JavaScript.

JSON was introduced by Douglas Crockford in the early 2000s as a simpler alternative to XML. Today it is the dominant data format for web APIs, configuration files, and data storage.

JSON syntax rules

JSON has six simple rules:

• Data is written as key/value pairs
• Keys must be double-quoted strings
• Data is separated by commas
• Objects use curly braces {}
• Arrays use square brackets []
• No trailing commas and no comments allowed

JSON data types

JSON supports exactly six data types:

A real JSON example

Here is a JSON object representing a user profile:

{
  "id": 1042,
  "name": "Ana García",
  "email": "ana@example.com",
  "age": 28,
  "isPremium": true,
  "address": {
    "city": "Madrid",
    "country": "Spain"
  },
  "tags": ["developer", "designer"],
  "avatar": null
}

Common use cases

• REST APIs — virtually all modern web APIs send and receive JSON
• Configuration files — package.json, tsconfig.json, .eslintrc
• Databases — MongoDB, PostgreSQL JSONB, Firebase all store JSON natively
• Data exchange — between microservices, between frontend and backend
• Local storage — browsers store data as JSON strings in localStorage

JSON vs XML

Before JSON, XML was the standard for data exchange. JSON replaced XML in most web contexts because it is easier to read, shorter to write, and faster to parse. A JSON object with three fields takes roughly half the characters of the equivalent XML.

XML is still used in some industries (finance, healthcare, enterprise) and for documents with complex metadata, but for web APIs and application data, JSON is the clear standard.

Common JSON mistakes

• Using single quotes instead of double quotes for strings or keys
• Adding a trailing comma after the last item in an object or array
• Adding comments — JSON does not support comments
• Using undefined as a value — it is not a valid JSON type
• Forgetting to quote keys — unlike JavaScript objects, all JSON keys must be quoted