What is JSON Schema? The Complete Guide

JSON Schema Definition

JSON Schema is a declarative specification language for describing the structure and validation constraints of JSON documents. A JSON Schema is itself a valid JSON document that uses reserved keywords like type, properties, required, and format.

Key JSON Schema Keywords (Draft-07)

• type — Value type: string, number, integer, boolean, array, object, null.
• properties — Defines the expected keys and their schemas for an object.
• required — Array of property names that must be present.
• format — Semantic format: "date-time", "email", "uuid", "uri".
• enum — Restricts the value to a specific set of allowed values.
• minimum / maximum — Numeric range constraints.
• minLength / maxLength — String length constraints.
• pattern — Regular expression constraint for strings.
• items — Schema for array elements.
• $ref — Reference to another schema definition.

Common Validators Supporting JSON Schema

Ajv (JavaScript), jsonschema (Python), Jackson (Java), Newtonsoft (C#), OpenAPI/Swagger (all languages).

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "title": "User",
  "required": ["id", "email", "name"],
  "properties": {
    "id": {
      "type": "integer",
      "minimum": 1
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "name": {
      "type": "string",
      "minLength": 2,
      "maxLength": 100
    },
    "role": {
      "type": "string",
      "enum": ["admin", "editor", "viewer"]
    },
    "createdAt": {
      "type": "string",
      "format": "date-time"
    }
  }
}