Institute of Information Science - University of Miskolc

JSON Schema is a tool to define and validate the structure of JSON data. It ensures that the data adheres to specific types, structures, and constraints, making it useful for APIs and data validation.

This schema defines a “Person” object with properties `name`, `age`, and `email`.

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "Person",
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "integer", "minimum": 0 },
    "email": { "type": "string", "format": "email" }
  },
  "required": ["name", "email"]
}

Explanation:

• name and email are required.
• age is optional but must be a non-negative integer if provided.
• email should be a valid email format.

{
  "name": "John Doe",
  "age": 28,
  "email": "john@example.com"
}

You can validate arrays by specifying the type of items and enforcing constraints like minItems or uniqueItems.

Array Schema Example

{
  "type": "array",
  "items": { "type": "string" },
  "minItems": 1,
  "uniqueItems": true
}

This schema defines an array where:

• Items must be strings.
• The array must have at least one item.
• All items must be unique.

Valid JSON Array:

[
  "apple",
  "banana",
  "cherry"
]

JSON Schema supports conditional logic using if, then, and else.

Schema for Conditional Validation

{
  "type": "object",
  "properties": {
    "isEmployed": { "type": "boolean" }
  },
  "if": {
    "properties": { "isEmployed": { "const": true } }
  },
  "then": {
    "properties": {
      "employer": { "type": "string" }
    },
    "required": ["employer"]
  },
  "else": {
    "properties": {
      "reasonUnemployed": { "type": "string" }
    },
    "required": ["reasonUnemployed"]
  }
}

Explanation:

• If isEmployed is true, employer must be provided.
• If isEmployed is false, reasonUnemployed must be provided.

Valid Example (Employed):

{
  "isEmployed": true,
  "employer": "TechCorp"
}

Valid Example (Unemployed):

{
  "isEmployed": false,
  "reasonUnemployed": "Student"
}

JSON Schema is versatile for defining the structure and validation rules for JSON data. It supports basic types, complex structures like arrays, and conditional logic, allowing developers to ensure data consistency and correctness. By using JSON Schema, you can make sure your data adheres to predefined formats, helping with integration and error prevention in APIs and applications.