Updated June 29, 2026

JSON Schema keywords

JSON Schema validates data using keywords. These are the ones you reach for most, grouped by what they apply to. Combine them to describe required fields, value types, and constraints.

Types and general

Keyword	Applies to	What it does
`type`	Any	Restricts the value's type: object, array, string, number, integer, boolean, or null.
`enum`	Any	Limits the value to a fixed list of allowed values.
`const`	Any	Requires the value to equal one exact value.
`default`	Any	Suggests a default; annotation only, not enforced.

Objects

Keyword	Applies to	What it does
`properties`	object	Defines a schema for each named property.
`required`	object	Lists properties that must be present.
`additionalProperties`	object	Allows or forbids properties not listed in properties.
`minProperties / maxProperties`	object	Limits the number of properties.

Arrays

Keyword	Applies to	What it does
`items`	array	Schema that every array element must match.
`minItems / maxItems`	array	Limits the array length.
`uniqueItems`	array	Requires all elements to be unique when true.

Numbers and strings

Keyword	Applies to	What it does
`minimum / maximum`	number	Inclusive lower and upper bounds.
`multipleOf`	number	Requires the value to be a multiple of a given number.
`minLength / maxLength`	string	Limits the string length.
`pattern`	string	Requires the string to match a regular expression.
`format`	string	Hints at a format such as email, date-time, or uri.

Snippet: required fields with types

A common object schema with required, typed properties.

{
  "type": "object",
  "required": ["id", "name"],
  "properties": {
    "id": { "type": "integer" },
    "name": { "type": "string", "minLength": 1 }
  },
  "additionalProperties": false
}

Snippet: email and enum

A string with an email format and a field limited to set values.

{
  "type": "object",
  "properties": {
    "email": { "type": "string", "format": "email" },
    "role": { "type": "string", "enum": ["admin", "editor", "viewer"] }
  }
}

Snippet: array of objects

An array whose items each follow a schema.

{
  "type": "array",
  "items": {
    "type": "object",
    "required": ["sku"],
    "properties": {
      "sku": { "type": "string" },
      "qty": { "type": "integer", "minimum": 1 }
    }
  }
}

Combining schemasallOf, anyOf, and oneOf combine subschemas, and not negates one. They let you express unions and conditional rules.

References

JSON Schema keywords reference — json-schema.org

Questions

Does 'required' make a property exist?

required lists which properties must be present for the data to be valid; it does not create them. A document missing a required property fails validation.

What is the difference between anyOf and oneOf?

anyOf passes if the data matches at least one subschema; oneOf passes only if it matches exactly one. Use oneOf for mutually exclusive options.