What is JSON Schema?

JSON Schema is a vocabulary for annotating and validating JSON documents. It lets you describe the structure of your data (which fields are required, what types they must be, what constraints apply) and then validate any JSON document against those rules.

Supported keywords (Draft-07 subset)

Keyword	Description
`type`	string, number, integer, boolean, null, array, object
`enum`	Allowed values list
`const`	Single allowed value
`required`	Required object keys
`properties`	Per-key sub-schemas
`additionalProperties`	Allow / deny / schema for extra keys
`minimum / maximum`	Numeric range (inclusive)
`exclusiveMinimum / exclusiveMaximum`	Numeric range (exclusive)
`minLength / maxLength`	String length
`pattern`	String regex pattern
`format`	date-time, date, email, uri, ipv4, uuid
`items`	Array item schema
`minItems / maxItems`	Array length
`uniqueItems`	No duplicate array items
`allOf / anyOf / oneOf / not`	Combining schemas
`$ref`	Reference to `#/definitions/…`

Common validation patterns

Pattern	Schema snippet
Non-empty string	`{"type":"string","minLength":1}`
Positive integer	`{"type":"integer","minimum":1}`
ISO 8601 date	`{"type":"string","format":"date"}`
Email address	`{"type":"string","format":"email"}`
Required object field	`{"required":["id","name"]}`
Nullable field	`{"anyOf":[{"type":"string"},{"type":"null"}]}`
Enum values	`{"enum":["draft","published","archived"]}`

Draft version history

Draft 04 (2013): first widely adopted version; introduced $schema, $ref, allOf
Draft 07 (2018): added if/then/else, readOnly, writeOnly, more format values
Draft 2019-09: introduced $anchor, deprecated definitions in favor of $defs
Draft 2020-12: current standard; improved $ref, added prefixItems for tuples and unevaluatedProperties

When to use JSON Schema

Validating API request and response payloads at runtime
Enforcing configuration file structure across teams
Documenting data contracts between microservices
Generating forms and UI dynamically from schema definitions
Enabling IDE autocomplete in JSON/YAML config files (via $schema property)