Practical JSON Schema examples covering user objects, API responses, nested arrays, string patterns, enums, and validation rules. Use these patterns as a starting point for your own schemas.
Try JSON Schema Validator →JSON Schema is a vocabulary for annotating and validating JSON documents. It is specified by an IETF standard (currently Draft 2020-12) and lets you define the structure, data types, required fields, value constraints, and string patterns for any JSON document. Think of it as a contract: a schema describes what valid data must look like, and a validator checks whether actual data conforms to that contract.
JSON Schema is used across many scenarios:
The simplest possible JSON Schema validates that a value is a string:
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "string"
}Valid: "hello world" - Invalid: 42
A basic object schema with type and properties:
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "object",
"properties": {
"id": { "type": "integer" },
"name": { "type": "string" },
"email": { "type": "string" },
"age": { "type": "integer", "minimum": 0, "maximum": 120 }
}
}Use the required array to mark fields that must be present. Without it, all properties are optional:
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "object",
"properties": {
"username": { "type": "string", "minLength": 3, "maxLength": 30 },
"email": { "type": "string", "format": "email" },
"password": { "type": "string", "minLength": 8 },
"role": { "type": "string", "default": "user" }
},
"required": ["username", "email", "password"],
"additionalProperties": false
}additionalProperties: false rejects any keys not listed in properties. This is useful for strict API contracts where unexpected fields should cause a validation error.
JSON Schema supports all JSON primitive types. Here is a reference schema that exercises every type keyword:
{
"type": "object",
"properties": {
"count": { "type": "integer", "minimum": 1 },
"price": { "type": "number", "exclusiveMinimum": 0 },
"label": { "type": "string" },
"active": { "type": "boolean" },
"tags": { "type": "array" },
"metadata": { "type": "object" },
"deletedAt": { "type": ["string", "null"] }
}
}Note that integer and number are distinct: integer rejects decimals like 3.14, while number accepts both. The type array ["string", "null"] allows a value to be either a string or null — useful for nullable fields in APIs.
Use format for semantic validation and pattern for custom regex constraints:
{
"type": "object",
"properties": {
"email": {
"type": "string",
"format": "email"
},
"website": {
"type": "string",
"format": "uri"
},
"phoneUS": {
"type": "string",
"pattern": "^\\+1[2-9]\\d{9}$"
},
"hexColor": {
"type": "string",
"pattern": "^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$"
},
"postalCodeUK": {
"type": "string",
"pattern": "^[A-Z]{1,2}[0-9][0-9A-Z]? ?[0-9][A-Z]{2}$"
},
"createdAt": {
"type": "string",
"format": "date-time"
}
}
}The format keyword is an annotation by default. Whether it is enforced depends on your validator — Ajv enforces it when you pass { formats: require("ajv-formats") }. Common format values include: email, uri, date, date-time, time, uuid, ipv4, ipv6, hostname.
Use items to define the schema for array elements, and minItems / maxItems to constrain length:
{
"type": "object",
"properties": {
"tags": {
"type": "array",
"items": { "type": "string" },
"minItems": 1,
"maxItems": 10,
"uniqueItems": true
},
"scores": {
"type": "array",
"items": { "type": "number", "minimum": 0, "maximum": 100 }
},
"coordinates": {
"type": "array",
"items": { "type": "number" },
"minItems": 2,
"maxItems": 2
}
}
}uniqueItems: true ensures no duplicate values. The coordinates example shows a tuple-style array with exactly 2 numbers (latitude/longitude).
A real-world user profile schema with nested address and preferences objects:
{
"$schema": "https://json-schema.org/draft/2020-12/schema",
"type": "object",
"properties": {
"id": { "type": "string", "format": "uuid" },
"username": { "type": "string", "minLength": 3 },
"email": { "type": "string", "format": "email" },
"address": {
"type": "object",
"properties": {
"street": { "type": "string" },
"city": { "type": "string" },
"country": { "type": "string", "minLength": 2, "maxLength": 2 },
"zip": { "type": "string" }
},
"required": ["city", "country"]
},
"preferences": {
"type": "object",
"properties": {
"theme": { "type": "string", "enum": ["light", "dark", "system"] },
"notifications": { "type": "boolean" },
"language": { "type": "string", "default": "en" }
}
},
"roles": {
"type": "array",
"items": { "type": "string", "enum": ["admin", "editor", "viewer"] },
"uniqueItems": true
}
},
"required": ["id", "username", "email"]
}enum restricts a value to a specific set of allowed values. const pins a value to a single exact value:
{
"type": "object",
"properties": {
"status": {
"type": "string",
"enum": ["pending", "active", "suspended", "deleted"]
},
"currency": {
"type": "string",
"enum": ["USD", "EUR", "GBP", "JPY", "INR"]
},
"schemaVersion": {
"const": "2.0"
},
"priority": {
"type": "integer",
"enum": [1, 2, 3]
}
}
}enum can contain mixed types — strings, numbers, booleans, even null. const is equivalent to an enum with a single value and is used to pin a schema version or a fixed field value in a discriminated union.
JSON Schema is a vocabulary that allows you to annotate and validate JSON documents. It is defined by a series of IETF drafts (currently Draft 2020-12) and lets you describe the structure, data types, required fields, string patterns, value ranges, and more for any JSON document.
The type keyword specifies the data type of a value (string, number, integer, boolean, array, object, null). The properties keyword defines the expected keys of a JSON object and the schema each value must conform to. You typically use both together: type sets the object type, and properties defines its structure.
Use the required keyword at the object level, passing an array of property names that must be present. For example: { "type": "object", "required": ["email"] }. Note that required is separate from properties and applies to the parent object, not to individual property definitions.
For new projects, use Draft 2020-12 (the latest standard) or Draft 2019-09. Legacy projects often use Draft-07, which is still widely supported by validators like Ajv. The core keywords (type, properties, required, enum, items) are consistent across all drafts.
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