JSON Syntax Overview

Learning Focus

JSON has exactly six value types and a handful of structural characters. Learn them once and you will never write invalid JSON again.

The Six Syntax Rules

Rule	What It Means
1. Key-value pairs	"key": value — keys must be double-quoted strings
2. Comma-separated	Each pair or element is separated by ,
3. Objects use {}	Unordered set of key-value pairs
4. Arrays use []	Ordered list of values
5. Strings use ""	Double quotes only — single quotes are invalid
6. Six value types	string, number, object, array, boolean, null

Structural Characters

{  }   Begin / end object
[  ]   Begin / end array
:      Key-value separator
,      Pair / element separator
"      String delimiter

Complete Valid Example

{
  "orderId": 9871,
  "status": "pending",
  "isPriority": false,
  "discount": null,
  "tags": ["express", "fragile"],
  "customer": {
    "name": "Alice Chen",
    "email": "alice@example.com",
    "address": {
      "city": "Singapore",
      "postalCode": "018989"
    }
  },
  "items": [
    { "sku": "A100", "qty": 2, "price": 29.99 },
    { "sku": "B200", "qty": 1, "price": 149.00 }
  ]
}

String Escape Sequences

Sequence	Character
"	Double quote
\	Backslash
\/	Forward slash
	Backspace
	Form feed
`
`	Newline
`
`	Carriage return
	Tab
\uXXXX	Unicode escape (4 hex digits)

Number Rules

{ "ok_integer": 42, "ok_negative": -17, "ok_float": 3.14 }

Invalid Formats

0xFF (hex), 0755 (octal), 007 (leading zero), +42 (plus), NaN, Infinity → all parse errors

Quick Validation

# jq — exit 0 = valid
echo '{"name":"Alice"}' | jq '.' > /dev/null && echo valid || echo INVALID

# Python
python3 -m json.tool data.json > /dev/null && echo valid

Concept Map

Concept Flow

JSON Document
  └── Root Value
        ├── Object  → Key-Value Pairs
        └── Array   → Ordered Elements
              └── string / number / object / array / boolean / null

Common Pitfalls

Error	Bad Example	Fixed
Single-quoted strings	{'key': 'val'}	{"key": "val"}
Trailing comma	{"a": 1,}	{"a": 1}
Unquoted key	{key: "val"}	{"key": "val"}
undefined value	{"x": undefined}	{"x": null}
Comment inside JSON	{"a": 1 // note}	remove comment

What's Next

• Previous: JSON vs XML vs YAML — Review format comparison.
• Next: JSON Data Types — Dive into each value type in detail.
• Section Overview — Return to the Introduction module index.