JSON

JSON is how programs hand structured data to each other. It is small, human-readable, and supported everywhere.

JSON (JavaScript Object Notation) is a text format for structured data. It grew out of JavaScript but is now language-neutral — every major language can read and write it, which is why APIs speak it.

What it looks like

{
  "id": 42,
  "name": "Ada",
  "active": true,
  "roles": ["admin", "editor"],
  "profile": {
    "city": "London",
    "age": null
  }
}

The whole type system

JSON has just six value types, and that is the point — it is deliberately tiny:

• string — always in double quotes: "hello"
• number — 42, -3.14 (no distinction between int and float)
• boolean — true or false
• null
• array — ordered list in [ ]
• object — key/value pairs in { }; keys are always strings

Parsing and producing it

// JavaScript
const obj = JSON.parse(text);        // text  -> value
const text = JSON.stringify(obj);    // value -> text
JSON.stringify(obj, null, 2);        // pretty-printed

# Python
import json
obj  = json.loads(text)
text = json.dumps(obj, indent=2)

Rules that trip people up

• Keys and strings must use double quotes, never single.
• No trailing commas.
• No comments — JSON has none.
• No undefined, no functions, no dates — dates are usually sent as ISO strings like "2026-11-08T12:00:00Z".