JSON Cheat Sheet

Syntax and Values

Use the complete JSON value model and structural grammar.

JSON Values

Represent primitive and structured values.

json

{"name":"Ana","active":true,"score":98,"note":null}

📌 JSON has four primitive and two structured types

📌 Literal names must be lowercase

🔍 Any JSON value can be the root text

⚠️ undefined and NaN are not JSON values

valuestypessyntax

Whitespace and Delimiters

Format JSON without changing its data.

json

{"id":1,"name":"Ana"}

📌 Only space, tab, LF, and CR are whitespace

💡 Indentation improves human readability

⚠️ Trailing commas are invalid JSON

🔍 Whitespace is allowed around structural tokens

whitespaceformattinggrammar

Objects

Store unordered name and value pairs.

Object Members

Create objects with quoted, unique property names.

json

{"name":"Ana","age":30}

📌 Property names must be double-quoted strings

⚠️ Duplicate names have unpredictable behavior

🔍 Object member order is not semantically meaningful

🎯 Use stable, descriptive property names

objectspropertiesnesting

Arrays

Store ordered sequences of JSON values.

Arrays and Nesting

Create ordered lists with mixed valid values.

json

["red","green","blue"]

📌 Array order is significant

🔍 Array elements may have different JSON types

💡 Keep API collections structurally consistent

⚠️ Deep nesting increases validation complexity

arraysordernesting

Strings and Unicode

Encode Unicode text and escaped characters correctly.

String Escapes

Escape quotes, slashes, controls, and Unicode.

json

{"message":"She said \"hello\"."}

📌 JSON strings always use double quotes

📌 Control characters must be escaped

💡 UTF-8 can carry Unicode characters directly

🔍 Unicode escapes use four hexadecimal digits

stringsunicodeescaping

UTF-8 and Transport

Exchange interoperable JSON over networks.

json

Content-Type: application/json

📌 Network JSON must use UTF-8 for interoperability

⚠️ Generators must not add a byte order mark

🔍 application/json has no charset parameter

🎯 Set the media type explicitly in APIs

utf-8httpmedia-type

Numbers and Literals

Use interoperable numbers, booleans, and null.

JSON Numbers

Represent integers, fractions, and exponents.

json

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

📌 Leading zeros are not allowed except for zero

⚠️ Infinity and NaN are invalid JSON

🔍 Number precision depends on the parser

🎯 Send large identifiers as strings when precision matters

numbersprecisionexponents

Boolean and Null

Represent logical states and missing values.

json

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

📌 true, false, and null are lowercase

🔍 null is a value, not a missing property

🎯 Define null semantics in API contracts

⚠️ Do not quote booleans unless they are text

booleannullliterals

JavaScript

Parse and serialize JSON safely in JavaScript.

JSON.parse

Convert JSON text to a JavaScript value.

javascript

const data = JSON.parse('{"name":"Ana"}')

📌 JSON.parse does not execute code

💡 A reviver can transform parsed values

⚠️ Catch SyntaxError for untrusted input

🔍 Dates remain strings unless converted

javascriptparsereviver

JSON.stringify

Convert a JavaScript value to JSON text.

javascript

const text = JSON.stringify({ name: 'Ana', active: true })

📌 undefined object properties are omitted

⚠️ BigInt throws unless transformed

💡 The third argument controls indentation

🔍 replacer filters or transforms values

javascriptstringifyreplacer

Python

Decode and encode JSON with the standard library.

loads and dumps

Convert between JSON text and Python values.

python

import json

data = json.loads('{"name":"Ana"}')
text = json.dumps(data)

📌 loads reads text and dumps produces text

💡 ensure_ascii=False preserves readable Unicode

🔍 parse_float can preserve decimal precision

⚠️ default handlers must return serializable values

pythonloadsdumps

JSON Files

Read and write UTF-8 JSON files.

python

with open('data.json', encoding='utf-8') as file:
    data = json.load(file)

📌 load and dump operate on file objects

💡 Specify UTF-8 explicitly

🎯 Add a final newline for text-file tooling

⚠️ Validate data before overwriting files

pythonfilesencoding

JSON Schema

Describe and validate JSON documents with Draft 2020-12.

Object Schema

Validate object properties and required fields.

json

{"$schema":"https://json-schema.org/draft/2020-12/schema","type":"object"}

📌 $schema declares the schema dialect

🔍 properties alone does not require fields

📌 required lists mandatory property names

⚠️ additionalProperties changes extensibility

json-schemaobjectsvalidation

Array and Composition Schema

Validate collections and combine reusable rules.

json

{
  "type":"array",
  "items":{"type":"string"},
  "uniqueItems":true
}

📌 items applies a schema to array elements

💡 $defs and $ref reuse schema fragments

🔍 allOf requires every subschema to match

⚠️ oneOf requires exactly one match

json-schemaarrayscomposition

Pointer and Patch

Address values and describe partial document changes.

JSON Pointer

Address a value using RFC 6901 path tokens.

json

/users/0/name

📌 Pointer tokens are separated by /

🔍 Array indexes are decimal tokens

📌 ~1 escapes / and ~0 escapes ~

⚠️ A missing target makes operations fail

json-pointerpathsrfc-6901

JSON Patch

Describe ordered changes with RFC 6902 operations.

json

[ {"op":"replace","path":"/name","value":"Ana"} ]

📌 Patch operations run in array order

💡 test supports optimistic concurrency

🔍 - appends to an array

⚠️ Apply patches only after authorization

json-patchrfc-6902api

Validation and Security

Reject invalid input and avoid interoperability traps.

Common Invalid JSON

Recognize syntax accepted by JavaScript but not JSON.

json

Invalid:
{'name':'Ana'}
{"active":True}
{"items":[1,2,]}

📌 JSON is stricter than JavaScript object syntax

⚠️ Comments are not part of RFC 8259

💡 Use a parser instead of visual inspection

🎯 Return useful validation errors without leaking data

validationinvalid-jsonpitfalls

Security and Limits

Treat parsed JSON as untrusted structured input.

json

const data = JSON.parse(untrustedText)
validate(data)

⚠️ Never parse JSON with eval

📌 Parsing does not validate business rules

🔍 Parsers may impose size and depth limits

🎯 Validate shape before using values

securitylimitsvalidation

JSON

Table of Contents

Syntax and Values

Objects

Arrays

Strings and Unicode

Numbers and Literals

JavaScript

Python

JSON Schema

Pointer and Patch

Validation and Security