JSON data types explained

JSON has only six kinds of values. That small set is a big part of why JSON is easy to parse in every language, but it also means common types such as dates, binary data, very large integers and decimals have no native representation. This guide covers each JSON type, how it maps to popular languages, and how to represent the types JSON doesn't have.

The six types

json
{
  "string": "hello",
  "number": 42.5,
  "boolean": true,
  "null": null,
  "array": [1, "two", false],
  "object": { "nested": "value" }
}

Strings

Strings are sequences of Unicode characters in double quotes. Single quotes aren't allowed. Special characters are escaped with a backslash: \", \\, \n, \t, and é for any Unicode code point. See Escape JSON strings for the full list.

Numbers

JSON has one number type. It covers integers and decimals, negative values, and exponents (1.5e10). It does not allow:

• Leading zeros (007), a leading +, or a bare decimal point (.5)
• Hexadecimal (0x1F)
• NaN, Infinity or -Infinity

The JSON spec doesn't limit precision, but parsers do. JavaScript stores every number as a 64-bit float, so integers above 2^53 − 1 (9,007,199,254,740,991) lose precision:

javascript
JSON.parse('{"id": 9007199254740993}').id; // 9007199254740992

That's why APIs send large IDs, such as Twitter/X snowflake IDs, as strings.

Booleans

true and false, lowercase only. True, "true" and 1 are different values.

null

null means "no value". It's different from a missing key, and APIs should choose deliberately which one they use; see API JSON best practices.

Arrays

Ordered lists in square brackets. Items can be any type, and mixed types are allowed, although consistent item types are much easier for clients to handle. Order is meaningful.

Objects

Unordered collections of key/value pairs in curly braces. Keys must be strings in double quotes. The spec says names should be unique; with duplicates, most parsers silently keep the last value.

How JSON types map to languages

JSON	JavaScript	Python	Java (Jackson)	Go
string	string	str	String	string
number	number	int or float	int, long, double, BigDecimal	float64 (or typed field)
true / false	boolean	bool	boolean	bool
null	null	None	null	nil
array	Array	list	List	slice
object	Object	dict	Map or POJO	map or struct

Types JSON doesn't have

You need	Common convention
Date or time	ISO 8601 string: "2026-03-15T14:30:00Z"; see Dates and times in JSON
Money / decimals	Integer minor units (1999) or a decimal string ("19.99")
64-bit integers	String: "id": "1234567890123456789"
Binary data	Base64 string; see How Base64 encoding works
Enum	String from a fixed set: "status": "shipped"
Set	Array with unique items
Map with non-string keys	Array of [key, value] pairs, or stringified keys

Document these conventions in your API's schema so clients know how to read each field. JSON Schema basics shows how to express formats such as date-time and uuid.

Common type mistakes

• Quoting numbers inconsistently: "price": "10" in one response and "price": 10 in another breaks typed clients.
• Using strings for booleans: "active": "false" is truthy in JavaScript.
• Floating-point money: 0.1 + 0.2 isn't 0.3 in binary floating point.
• Assuming key order: objects are unordered, so never rely on the "first" key; see Sort JSON keys.

To see the types in a document at a glance, paste it into the JSON Formatter and switch to tree view.