Is JSON part of JavaScript?

JSON (JavaScript Object Notation) was inspired by JavaScript object literal syntax, but it is an independent, language-agnostic format defined by RFC 8259. Every major programming language has JSON support built in or available via a standard library: Python (json module), Java (Jackson, Gson), PHP (json_encode/json_decode), Go (encoding/json), Rust (serde_json), and so on.

What is the difference between JSON and a JavaScript object?

A JavaScript object is a runtime data structure. Keys can be unquoted, values can be functions, undefined, or Date objects, and you can use trailing commas. JSON is a text format for data interchange. Keys must be quoted strings, values can only be the 6 JSON types (no functions, no undefined, no Date), and trailing commas are forbidden. You convert between them with JSON.stringify() and JSON.parse().

Can JSON have comments?

No. Comments (// or /* */) are not part of the JSON specification. A parser will reject any document that contains them. If you need comments in a config file, consider JSONC (JSON with Comments, used by VS Code and TypeScript) or YAML, which supports # comments natively.

Is JSON always an object?

No. A JSON document can be any valid JSON value: an object {}, an array [], a string, a number, a boolean, or null. {"name":"Alice"} and [1,2,3] and "hello" are all valid JSON documents. In practice, most APIs return an object or an array at the top level, but the spec allows any value.

What is JSON?

JSON explained: data types, syntax rules, and why it became the universal data format for APIs and config files.

6 min read·Updated June 2026

JSON (JavaScript Object Notation) is a lightweight text format for storing and exchanging structured data. It was standardised in RFC 8259 and is now the default data format for REST APIs, configuration files, databases, and practically everything that passes data between systems.

JSON was derived from JavaScript object literal syntax in the early 2000s, but it is completely language-agnostic — every major programming language can read and write it natively.

The 6 JSON data types

JSON supports exactly six value types. Nothing else is valid.

string

"Hello""2024-01-31"""

Always double-quoted. Single quotes are invalid.

number

423.14-71.5e10

Integer or float. No NaN, no Infinity, no hex.

boolean

truefalse

Lowercase only. True and False are invalid.

null

null

Represents the intentional absence of a value.

array

[1, 2, 3]["a", "b"][]

Ordered list of any JSON values. Can be mixed types.

object

{"key": "val"}{}

Unordered key-value pairs. Keys must be strings.

Syntax rules

JSON has a small, strict set of rules. Break any one of them and the document is invalid.

❌Keys must be double-quoted strings

✅ valid{"name": "Alice"}

❌ invalid{name: "Alice"}

❌Strings use double quotes only

✅ valid"hello"

❌ invalid'hello'

❌No trailing commas

✅ valid{"a": 1, "b": 2}

❌ invalid{"a": 1, "b": 2,}

❌No comments

✅ valid{"rate": 0.15}

❌ invalid{"rate": 0.15} // VAT

❌Numbers have no leading zeros

✅ valid{"code": 7}

❌ invalid{"code": 007}

❌No undefined, NaN, or Infinity

✅ valid{"val": null}

❌ invalid{"val": undefined}

JSON vs XML

Before JSON, XML dominated data exchange. Here is the same data in both formats.

JSON 66 chars

{
  "name": "Alice",
  "age": 30,
  "admin": true,
  "tags": ["dev", "ops"]
}

XML 122 chars

<user>
  <name>Alice</name>
  <age>30</age>
  <admin>true</admin>
  <tags>
    <tag>dev</tag>
    <tag>ops</tag>
  </tags>
</user>

	JSON	XML
Verbosity	Compact	Verbose
Native types	number, boolean, null, array	Strings only (attributes aside)
Comments	Not supported	Supported
Schema validation	JSON Schema	XSD, DTD
Human-readable	Easy	Cluttered with tags
Parse speed	Fast	Slower
Still used for	APIs, config, storage	SOAP, SVG, Office docs