JSON Cheat Sheet

Everything you need to know about JSON syntax — data types, structure rules, escaping, and common patterns — in one quick reference.

Last updated: May 21, 2026

Data Types

Type	Example	Notes
string	"Hello, world!"	Must use double quotes. Escape special chars with \
number	42 or 3.14 or -7 or 1e10	Integers and floats. No Infinity, NaN, or hex literals
boolean	true or false	Lowercase only — True and False are invalid
null	null	Lowercase only — Null and NULL are invalid
array	["a", 1, true, null]	Ordered list of any types. Trailing commas forbidden
object	{"key": "value"}	Unordered key-value pairs. Keys must be strings

String Escape Sequences

Sequence	Character
\"	Double quote (")
\\	Backslash (\)
\/	Forward slash (/) — optional
\n	Newline
\r	Carriage return
\t	Tab
\b	Backspace
\f	Form feed
\uXXXX	Unicode code point (e.g. \u0041 = "A")

Object Syntax

{
  "name": "Alice",           // string value
  "age": 30,                 // number value — NO trailing comma on last item
  "active": true,            // boolean value
  "score": null,             // null value
  "tags": ["admin", "user"], // array value
  "address": {               // nested object
    "city": "London",
    "postcode": "EC1A"
  }
}

• Keys must be strings in double quotes

• Key-value pairs separated by colons :

• Pairs separated by commas — no trailing comma after the last pair

• Objects can be nested to any depth

Array Syntax

[
  "string",
  42,
  true,
  null,
  [1, 2, 3],
  {"nested": "object"}
]

• Arrays hold ordered, comma-separated values

• Values can be of any JSON type including arrays and objects

• No trailing comma after the last item

• Zero-indexed when accessed programmatically

Common Patterns

Pattern	JSON example
API response	{"data": {...}, "error": null, "status": 200}
List of objects	[{"id":1,"name":"A"}, {"id":2,"name":"B"}]
Nested config	{"server":{"host":"localhost","port":3000}}
Optional field	{"value": null} or omit the key entirely
Empty object	{}
Empty array	[]
Root array	[1, 2, 3] — JSON root can be any type

Rules & Gotchas

No commentsJSON has no comment syntax. // and /* */ will cause a parse error.

No trailing commasA comma after the last item in an object or array is invalid JSON.

Double quotes onlyString keys and values must use double quotes. Single quotes are not valid.

No undefinedundefined is not a JSON value. Use null instead.

No functionsFunctions, methods, and RegExp literals are not valid JSON values.

No Infinity / NaNThese JavaScript values have no JSON equivalent — use null or a string.

No hex / octalNumber literals must be decimal. 0xFF and 077 are not valid JSON.

Unicode validJSON strings can contain any Unicode character. Use \uXXXX for control chars.

Number Formats

Format	Valid?	Example
Integer	Valid	42
Negative	Valid	-7
Float	Valid	3.14
Scientific notation	Valid	1.5e10 or 1.5E-3
Leading zero (integer)	Invalid	042 — parse error
Leading dot	Invalid	.5 — use 0.5
Hex literal	Invalid	0xFF — not supported
Infinity / NaN	Invalid	Use null instead

Client-Side Processing

Input Data Stays on Device

Instant Local Execution

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What is JSON Cheat Sheet?

JSON (JavaScript Object Notation) is the universal data interchange format used by REST APIs, configuration files, and databases. It is simple but strict: no comments, no trailing commas, only double-quoted strings, and only six data types.

This cheat sheet covers all JSON syntax rules, data types with examples, nesting patterns, string escaping sequences, and common mistakes that cause parse errors.

How to Use JSON Cheat Sheet

Browse the sections or use Ctrl+F to find a specific topic. Each entry shows the syntax with a concrete example. For validation and formatting, use the JSON Validator and JSON Formatter tools.

Common Use Cases

Developers building or consuming REST APIs who need a quick syntax reminder.
Students learning JSON for the first time who want a structured reference.
Developers debugging JSON parse errors who want to check escaping rules.
DevOps engineers writing JSON configuration files who need a quick lookup.

Example Input and Output

A JSON object demonstrating all six data types.

JSON structure

{
  "name": "Alice",
  "age": 30,
  "active": true,
  "score": null,
  "tags": ["admin", "user"],
  "address": { "city": "London" }
}

Data types shown

string, number, boolean, null, array, object

Tip: use a validator

When a JSON parse error is hard to find, paste into the JSON Validator tool — it reports the exact line and column of the syntax error.

Frequently Asked Questions

Can JSON have comments?

No. Standard JSON does not support comments. If you need comments in configuration files, consider JSONC (JSON with Comments), JSON5, or YAML instead. Parsers that accept JSON will throw on // or /* */ comments.

Why must strings use double quotes?

The JSON specification (RFC 8259) mandates double quotes for strings. Single-quoted strings are invalid JSON — this is a frequent source of parse errors when copying values from JavaScript code.

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Clean JSON by removing null values, empty strings, empty arrays, and empty objects. Configurable per value type with a removal count stat.