JSON Format: Syntax, Data Types, Rules, and Examples

JSON (JavaScript Object Notation) is a text format for exchanging structured data. A JSON document contains exactly one value: an object, array, string, number, boolean, or null. Objects and arrays can contain other JSON values to create nested structures.

Use the JSON Validator to check an example while learning the rules below.

A Complete JSON Example

{
  "id": 42,
  "name": "Ada Lovelace",
  "active": true,
  "roles": ["admin", "editor"],
  "profile": {
    "timezone": "Europe/London",
    "lastLogin": "2026-08-14T12:30:00Z"
  },
  "manager": null
}

This document’s top-level value is an object. It contains a number, string, boolean, array, nested object, and null.

The Six JSON Data Types

JSON type	Example	Common language equivalent
Object	{"name":"Ada"}	Map, dictionary, object
Array	[1,2,3]	List, slice, array
String	"hello"	String
Number	42, -1.5, 6.02e23	Integer or floating-point type
Boolean	true, false	Boolean
Null	null	Null, nil, None

JSON has no native date, binary, set, map, comment, or undefined type. Applications represent those concepts using agreed conventions.

JSON Object Syntax

An object is an unordered collection of name/value members surrounded by braces:

{
  "name": "Ada",
  "age": 36
}

Object rules:

• Property names are strings and require double quotes.
• A colon separates each name from its value.
• A comma separates members.
• A trailing comma is invalid.
• Names should be unique for interoperable data.

Although some parsers accept duplicate names, they do not agree on whether the first, last, or every value should be preserved.

JSON Array Syntax

An array is an ordered list of values surrounded by square brackets:

[
  {"id": 1, "name": "Ada"},
  {"id": 2, "name": "Grace"}
]

Array values can use different JSON types, although consistent item shapes are easier for applications and schemas to process.

An empty array is []; an empty object is {}.

JSON String Rules and Escaping

Strings use double quotes. Escape characters that would otherwise end the string or represent a control character:

{
  "quote": "She said \"hello\".",
  "path": "C:\\Users\\Ada",
  "multiline": "first line\nsecond line",
  "tabbed": "name\tvalue",
  "symbol": "\u2764"
}

Common escapes are \", \\, \/, \b, \f, \n, \r, \t, and \u followed by four hexadecimal digits. Raw newlines are not allowed inside a JSON string. See Multiline Strings in JSON for practical patterns.

JSON Number Rules

Valid JSON numbers include:

[0, -12, 3.14, 6.02e23, -2E-3]

Invalid forms include:

01
.5
1.
NaN
Infinity
0xFF

JSON does not define a maximum numeric size or precision, but receiving programming languages do. Large integers may lose precision in JavaScript and other IEEE-754-based environments. Use a string when an identifier or exact integer can exceed the consumer’s safe range.

Booleans and Null

The literals are lowercase:

{
  "enabled": true,
  "archived": false,
  "deletedAt": null
}

null is a JSON value. It is different from a missing property, an empty string, zero, and an empty object.

Whitespace and Formatting

Spaces, tabs, carriage returns, and line feeds can appear between JSON tokens. Minified and pretty-printed versions represent the same data:

{"name":"Ada","roles":["admin","editor"]}

{
  "name": "Ada",
  "roles": [
    "admin",
    "editor"
  ]
}

Use JSON Formatter to add indentation and JSON Minify to remove unnecessary whitespace.

What JSON Does Not Allow

These JavaScript conveniences are invalid in standard JSON:

• Comments.
• Single-quoted strings.
• Unquoted property names.
• Trailing commas.
• Functions, regular expressions, undefined, NaN, or infinity.
• Raw tab or newline characters inside strings.

Use JSONC to JSON when a configuration file intentionally contains comments.

Dates in JSON

JSON has no date type. ISO 8601 strings are the most interoperable convention:

{
  "createdAt": "2026-08-14T12:30:00Z",
  "birthday": "1815-12-10"
}

The producer and consumer must agree on whether a value is a date, local time, or instant. Read JSON Date Format for time-zone guidance.

JSON Files, APIs, and Streaming

• .json files normally contain one complete JSON value.
• HTTP APIs commonly use the application/json media type.
• A sequence of independent records can use JSON Lines/NDJSON, where each line is a complete JSON value.
• JSON text exchanged between independent systems should use UTF-8 for maximum interoperability.

Common Syntax Errors

{
  name: 'Ada',
  "active": True,
}

Problems:

1. name is not quoted.
2. The string uses single quotes.
3. True must be lowercase true.
4. The final property has a trailing comma.

Use the JSON Parse Error guide for error-specific fixes.

JSON Schema

Valid syntax does not guarantee valid application data. JSON Schema can require fields, restrict types and values, and document a format:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "required": ["id", "name"],
  "properties": {
    "id": {"type": "integer"},
    "name": {"type": "string", "minLength": 1}
  }
}

See JSON Schema Examples or validate a document with the Schema Validator.

Frequently Asked Questions

Must JSON start with { or [?

No. Modern JSON specifications allow any JSON value at the top level, including a string, number, boolean, or null. Some older systems still expect an object or array.

Does property order matter?

Applications should not assign meaning to object-property order. Array order is significant.

Is JSON the same as a JavaScript object?

No. JSON is text with a smaller grammar. JavaScript object literals can contain features that JSON does not allow.

References and Next Steps

• RFC 8259: The JSON Data Interchange Format
• JSON Lines and NDJSON
• Comments in JSON
• YAML vs JSON
• JSON vs XML