Syntax

Rules

{ }	Object (unordered key-value pairs)
[ ]	Array (ordered list of values)
"key": value	Keys must be double-quoted strings
No trailing comma	Last item must not have a comma
No comments	JSON does not allow comments

Minimal Example

{
  "name": "Alice",
  "age": 30,
  "active": true
}

Data Types

Six Value Types

"string"	Double-quoted UTF-8 text
42 / 3.14	Number (integer or floating point)
true / false	Boolean
null	Null (absence of value)
{ }	Object
[ ]	Array

String Escape Sequences

\"	Double quote
\\	Backslash
\n \t	Newline, tab
\uXXXX	Unicode escape (hex)

Objects

Object Syntax

{
  "id": 1,
  "name": "Widget",
  "tags": ["new", "sale"]
}

Rules

Keys	Must be unique double-quoted strings
Values	Any valid JSON type
Order	Key order is not guaranteed
Nesting	Objects can contain objects

Arrays

Array Syntax

[1, "two", true, null, {"key": "val"}]

Mixed Type Array

{
  "matrix": [[1, 2], [3, 4]],
  "empty": []
}

Rules

Ordered	Elements maintain insertion order
Mixed types	Array items can be different types
Indexing	Zero-based (in most languages)

Nesting

Nested Structure

{
  "user": {
    "name": "Alice",
    "address": { "city": "Boston" },
    "scores": [95, 88, 72]
  }
}

Access Patterns

obj.user.name	Dot notation (JavaScript)
obj["user"]["name"]	Bracket notation
obj.user.scores[0]	Array index within nested object

Schema Validation

JSON Schema Example

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer", "minimum": 0 }
  },
  "required": ["name"]
}

Schema Keywords

type	string, number, integer, boolean, object, array, null
required	Array of required property names
properties	Defines expected object properties
enum	Restrict to a fixed set of values
minLength / maxLength	String length constraints
minimum / maximum	Number range constraints

jq Basics

Common Filters

.	Identity — pass input through
.key	Access object key
.key.nested	Access nested key
.[0]	First array element
.[]	Iterate all array elements
select(.age > 20)	Filter by condition
map(.name)	Transform each element
length	Array length or string length
keys	Object keys as array

jq Examples

echo '{"a":1}' | jq '.a'        # 1
echo '[1,2,3]' | jq 'map(. * 2)' # [2,4,6]
cat data.json | jq '.users[].name'
cat data.json | jq '.[] | select(.active)'

Common Patterns

API Response

{
  "status": 200,
  "data": [{"id": 1, "name": "Alice"}],
  "meta": {"total": 42, "page": 1}
}

Config File

{
  "host": "localhost",
  "port": 8080,
  "debug": false,
  "features": ["auth", "logging"]
}

Tips

Validate	Use jsonlint or python -m json.tool
Pretty print	jq . file.json or python -m json.tool
JSONL	One JSON object per line (newline-delimited)
JSON5 / JSONC	Extensions allowing comments and trailing commas

JSON QUICK REFERENCE