JSON Data Types

Definition and Usage

Every JSON value is one of six types: string, number, object, array, boolean, or null. A JSON document is just nested values of those kinds.

There is no built-in date, binary, or undefined type. Dates are usually strings (often ISO-8601). JSON Schema can describe those conventions with the type and format keywords.

The Six Types

• string Text in double quotes — "Ada"
• number Integers or decimals — 42, 3.14 (no NaN / Infinity)
• object Curly braces with quoted keys — {"id": 1}
• array Ordered list in square brackets — [1, 2, 3]
• boolean Exactly true or false
• null The literal null — meaning “empty” / no value

JSON Schema type

Schemas name expected types with the type keyword — for example "type": "string", or several options as an array: "type": ["string", "null"].

Schema also adds integer (whole numbers) as a distinct check, even though JSON text still writes them as numbers. Use format (like date-time) when a string carries special meaning.

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer" },
    "active": { "type": "boolean" },
    "nickname": { "type": ["string", "null"] }
  }
}

Examples

Use View Output for a quick result, or Try It Yourself to edit and run in the browser.

const data = {
  name: "Ada",          // string
  age: 36,              // number
  active: true,         // boolean
  skills: ["math"],     // array
  address: { city: "London" }, // object
  nickname: null        // null
};

console.log(data.name);
console.log(data.age);
console.log(data.active);
console.log(data.skills[0]);
console.log(data.address.city);
console.log(data.nickname);

Ada
36
true
math
London
null

const data = {
  name: "Ada",
  age: 36,
  active: true,
  skills: ["math"],
  address: { city: "London" },
  nickname: null
};

console.log(typeof data.name);      // string
console.log(typeof data.age);       // number
console.log(typeof data.active);    // boolean
console.log(Array.isArray(data.skills)); // true
console.log(typeof data.address);   // object
console.log(data.nickname === null); // true

string
number
boolean
true
object
true

const user = {
  id: 1,
  name: "Ada",
  roles: ["admin", "editor"],
  profile: {
    city: "London",
    verified: true
  }
};

console.log(user.roles[0]);
console.log(user.profile.city);
console.log(user.profile.verified);

admin
London
true