What is JSON?

JSON is a text format for storing and transporting data. It's based on JavaScript object syntax, but it's language-independent—every major programming language can read and write it.

json

{
  "name": "Ada Lovelace",
  "born": 1815,
  "achievements": ["first programmer", "analytical engine"],
  "active": false
}

It won the data format wars of the 2000s, beating XML for most web applications. Today, it's the default choice for APIs, config files, and data storage.

Why JSON Won

Human readable - You can actually read it without a decoder ring
Lightweight - No closing tags like XML (</firstName>)
Universal - Every language has built-in JSON support
Flexible - Nested objects, arrays, any structure you need

Data Types

JSON supports six data types:

Type	Example	Notes
String	`"hello"`	Must use double quotes
Number	`42`, `3.14`, `-17`	No quotes, no NaN/Infinity
Boolean	`true`, `false`	Lowercase only
Null	`null`	Lowercase only
Array	`[1, 2, 3]`	Ordered list
Object	`{"key": "value"}`	Key-value pairs

That's it. No dates, no undefined, no functions, no comments.

Example: API Response

json

{
  "status": "success",
  "data": {
    "user": {
      "id": 12345,
      "email": "dev@example.com",
      "roles": ["admin", "user"],
      "settings": {
        "theme": "dark",
        "notifications": true
      }
    }
  },
  "meta": {
    "requestId": "abc-123",
    "timestamp": 1706634000
  }
}

Where You'll See This

REST APIs - Request and response bodies
Config files - package.json, tsconfig.json
Databases - MongoDB, PostgreSQL JSON columns
Local storage - Browser data persistence
Message queues - Kafka, RabbitMQ payloads

Common Gotchas

⚠️Trailing Commas

JSON does NOT allow trailing commas. This is invalid: {"name": "test",} — that comma before the closing brace will break parsing.

Double quotes only - 'single quotes' are invalid. Always use "double quotes".
No comments - JSON has no comment syntax. Use JSONC or JSON5 if you need comments.
No undefined - Use null instead.
Keys must be strings - {name: "test"} is invalid. Must be {"name": "test"}.
No trailing commas - Unlike JavaScript, the last item can't have a comma after it.

JSON vs Alternatives

Format	Best For	Drawback
JSON	APIs, general data	No comments, verbose for config
YAML	Config files	Whitespace-sensitive, complex spec
XML	Legacy systems, documents	Verbose, harder to parse
TOML	Simple config	Less common, limited nesting
CSV	Tabular data	No nested structures

In Code

javascript

// Parse JSON string to object
const data = JSON.parse('{"name": "test"}');

// Convert object to JSON string
const json = JSON.stringify({ name: "test" });

// Pretty print with indentation
const pretty = JSON.stringify(data, null, 2);

Try It

Format JSON

"JSON: Because XML developers saw the light... and it was 50% smaller."