Guide · 5 min read

What is JSON?

JSON (JavaScript Object Notation) is a lightweight, text-based data format used to store and exchange structured data. It is the universal language of APIs — almost every web service sends and receives data as JSON. This guide explains what JSON is, how it works, and why it matters.

JavaScript Object

Valid JSON

// JavaScript object (NOT JSON)
const user = {
  name: 'Alice',
  age: 30,
  active: true
}

JSON in Plain English

JSON is a way to represent data as text so it can be sent between systems — between a server and a browser, between two microservices, or stored in a file. It was derived from JavaScript object syntax but is completely language-independent: Python, Go, Java, Ruby, and virtually every other language has built-in JSON support.

JSON was defined in RFC 8259 and is the most widely used data interchange format in software today, having largely replaced XML for web APIs.

JSON Syntax Rules

Data is in key/value pairs

"name": "Alice"

Keys must be strings in double quotes

"key": value

Values can be string, number, boolean, null, object, or array

"active": true

Objects are wrapped in curly braces { }

{"name": "Alice"}

Arrays are wrapped in square brackets [ ]

["red", "green", "blue"]

Items are separated by commas — no trailing comma allowed

{"a": 1, "b": 2}

No comments are allowed

// this would break JSON

JSON Data Types

JSON supports exactly 6 data types:

TypeExampleNote

String"name": "Alice"Must use double quotes

Number"age": 30Integer or float, no quotes

Boolean"active": truetrue or false, lowercase

Null"nickname": nullRepresents no value

Object"address": { "city": "NYC" }Nested key-value pairs

Array"tags": ["web", "api"]Ordered list of values

JSON vs XML — Why JSON Won

Before JSON, XML was the dominant data exchange format for web APIs (SOAP, RSS, etc.). JSON replaced it for most use cases because:

Simpler syntax

JSON: No opening/closing tags

XML: Every value needs a tag pair

Smaller file size

JSON: 20–30% smaller than equivalent XML

XML: Verbose tag structure adds overhead

Native to JavaScript

JSON: JSON.parse() and JSON.stringify() built-in

XML: Requires DOM parsing library

Easier to read

JSON: Key-value structure is intuitive

XML: Deeply nested tags are hard to scan

XML is still used in specific domains — SOAP APIs, RSS/Atom feeds, SVG, and some enterprise systems. But for modern REST APIs, JSON is the standard.

A Real-World JSON Example

Here is what a typical REST API response looks like — a user object with nested address and an array of roles:

{
  "id": 1042,
  "name": "Alice Johnson",
  "email": "[email protected]",
  "age": 30,
  "active": true,
  "address": {
    "street": "123 Main St",
    "city": "New York",
    "country": "US"
  },
  "roles": ["admin", "editor"],
  "lastLogin": null
}

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