JSON Syntax Guide

Data types, rules, examples, and common gotchas

The Six Data Types

JSON has exactly six value types. No more, no less.

Type	Example	Notes
String	"hello"	Always double quotes. No single quotes.
Number	42, -7, 3.14, 1e10	No NaN or Infinity. No leading zeros.
Boolean	true, false	Lowercase only.
null	null	Lowercase only. Represents absence of value.
Object	{"key": "value"}	Keys must be strings in double quotes.
Array	[1, "two", true]	Ordered list, mixed types allowed.

Complete Example

{
  "name": "Alice",
  "age": 30,
  "active": true,
  "score": 98.6,
  "address": null,
  "tags": ["developer", "designer"],
  "location": {
    "city": "New York",
    "zip": "10001"
  }
}

Strings

Strings must use double quotes. Certain characters must be escaped with a backslash:

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

Numbers

✓ Valid numbers

42
-7
3.14
0.5
1.5e10
1.5E-3

✗ Invalid numbers

077          // No leading zeros
.5           // Must start with digit
NaN          // Not supported
Infinity     // Not supported
"42"         // That's a string

Objects

Objects are unordered key-value pairs. Keys must be strings in double quotes. Trailing commas are not allowed.

✓ Valid object

{"a": 1, "b": 2}

✗ Single-quoted keys

{'a': 1}       // Keys must use double quotes

✗ Trailing comma

{"a": 1, "b": 2,}   // No trailing commas

✗ Unquoted key

{a: 1}         // Keys must be quoted

Arrays

Ordered lists. Values can be any JSON type, including mixed types.

✓ Valid arrays

[1, 2, 3]
["a", "b", "c"]
[1, "two", true, null, {"x": 1}]

✗ Trailing comma

[1, 2, 3,]    // No trailing commas

Comments

Standard JSON does not support comments. This is one of the most common frustrations with the format.

✗ Comments are not valid JSON

{
  // This is a comment — INVALID
  "name": "Alice"  /* Also invalid */
}

For configuration files that benefit from comments, consider JSONC (JSON with Comments, used by VS Code) or JSON5, both of which are supersets of JSON that add comment support.

Common Gotchas

undefined is not valid JSON

JavaScript's undefined has no JSON representation. JSON.stringify({ x: undefined }) silently omits the key. Use null when you need to represent an absent value.

Duplicate keys

Duplicate keys in an object are technically allowed by the JSON spec but discouraged — behaviour when parsing is implementation-defined. Most parsers use the last value.

Number precision

JSON numbers can represent any number, but JavaScript (and many languages) parse them as IEEE 754 doubles. Integers larger than 2^53 - 1 lose precision. Use strings for large IDs.

// This loses precision in JavaScript!
{ "id": 9007199254740993 }

// Use a string instead
{ "id": "9007199254740993" }

Validate Your JSON

Use the JSON Formatter on io9.me to validate, format, and analyze any JSON — with clear error messages showing exactly where the problem is.