What are the JSON Schema types?

The JSON Schema types are string, number, integer, boolean, null, object and array. You declare them with the type keyword, which can be a single string or an array of strings to allow multiple types JSON Schema type keyword Numeric types.

How the type keyword works

Use type as a single string to require exactly one kind of instance (for example, "type": "object"). Use an array of strings to allow more than one possibility (for example, ["string", "null"]). This is the standard way to permit nullable fields without changing your core type JSON Schema type keyword.

{
  "type": ["string", "null"]
}

Integer vs number

JSON Schema distinguishes integer (whole-number values) and number (includes integral and fractional values). Both numeric types share the same validators, including minimum, maximum, exclusiveMinimum, exclusiveMaximum and multipleOf Numeric types.

• An instance like 1.0 is considered an integer value by JSON Schema, so it satisfies type: "integer" Numeric types.
• Numbers expressed as strings (e.g., "42") do not satisfy numeric types Numeric types.

{
  "type": "integer",
  "minimum": 0,
  "multipleOf": 1
}

Common per-type validators you will actually use

• string: minLength, maxLength, pattern, format
• number/integer: minimum, maximum, exclusiveMinimum, exclusiveMaximum, multipleOf
• object: properties, required, additionalProperties, patternProperties
• array: items, minItems, maxItems, uniqueItems

These validators are defined for their respective types in the reference docs JSON Schema type keyword Numeric types.

How const relates to types

The const keyword restricts a value to exactly one value. That single value includes its type: validation compares using JSON Schema’s notion of instance equality. So const: 1 matches only the JSON number 1, not the string "1"; const: "US" matches only that exact string Constant values Draft 2020-12 core: instance equality.

{
  "type": ["string", "null"],
  "const": "US"
}

In this example, the instance must be the exact string "US". Although type also allows null, both constraints apply together, leaving only that exact value. This is not a keyword-precedence rule. Remove const if you truly need both "US" and null.

Solid examples

Single type

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "id": { "type": "integer", "minimum": 1 },
    "name": { "type": "string", "minLength": 1 }
  },
  "required": ["id", "name"],
  "additionalProperties": false
}

Multiple allowed types (nullable)

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "nickname": { "type": ["string", "null"], "maxLength": 30 }
  }
}

const fixes a value (and its type)

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "country": { "type": "string", "const": "United States of America" },
    "version": { "type": "number", "const": 1 }
  }
}

Allowing null does not make a property required. The nullable nickname example also accepts an absent nickname because it has no required list. Likewise, do not assume format always rejects invalid strings: assertion depends on the chosen draft, vocabulary and validator configuration. It is commonly annotation-only by default.

Practical tips and pitfalls

• Be explicit: pick integer when you will reject fractional input; pick number if decimals are valid Numeric types.
• Allow nulls intentionally using type arrays, not by overloading "" (empty string). For a quick refresher on null vs empty, see Is null an empty string?
• Choose object vs array based on whether you need named keys or ordered items. See a primer on {} vs []: The difference between {} and [] in JSON.
• If you add const, remember it narrows your allowance to one exact instance value. Remove or adjust it if you need a small set of options; use enum for a list of allowed values Enumerated and constant values.

Quick checklist: getting type and const right

• Confirm the exact type you expect for each property (string, number, integer, boolean, null, object, array).
• If null is allowed, use type with an array (e.g., ["string", "null"]).
• Use integer only for whole numbers; otherwise use number and set ranges or multipleOf as needed.
• Reserve const for a single exact value (and remember its type must match).
• Add per-type validators (minLength, items, properties, etc.) after you lock the type.

FAQ

Is integer a separate JSON data type? In the JSON data model there is a single number type, but JSON Schema’s validation vocabulary defines both number and integer for practical constraints. JSON Schema treats values like 1 and 1.0 as the same mathematical value; integer accepts such values when they are integral, and both numeric types share the same validators Numeric types Draft 2020-12 core: instance model.