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JSON

Write, parse, validate, and exchange interoperable JSON using current standards.

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jsonapiserializationjson-schemadata
First page of the JSON PDF cheat sheet

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JSON PDF cheat sheet

A two-page JSON reference for printing or offline use.

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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