What is JSON Schema? — JSON Schema Explained

JSON Schema is a declarative language for annotating and validating JSON documents. It describes the expected structure, data types, required fields, and constraints of JSON data, enabling automated validation and documentation.

What Is It?

JSON Schema uses JSON itself to describe the structure of other JSON documents. You define a schema with keywords like type, properties, required, minimum, maximum, pattern, and enum. Tools and libraries validate JSON documents against the schema, reporting any violations. JSON Schema is language-independent and widely supported.

How It Works

You write a JSON Schema document that describes the expected structure. For example, { type: 'object', properties: { name: { type: 'string' }, age: { type: 'integer', minimum: 0 } }, required: ['name', 'age'] }. Any JSON document matching this schema must have a string name and non-negative integer age. Validators in every language check documents against the schema.

Key Characteristics

• Self-describing — schemas are written in JSON, no separate language needed
• Supports all JSON types — object, array, string, number, boolean, null
• String constraints — minLength, maxLength, pattern (regex), format (email, uri)
• Nested validation — deeply validate complex nested JSON structures
• Multiple drafts — Draft-07 most common, 2020-12 is latest

Common Use Cases

• API request/response validation — ensure input data is correct
• Configuration file validation — catch errors before runtime
• Automated testing — generate and validate test data
• Documentation — schemas serve as living documentation
• Form generation — generate forms from schemas (react-jsonschema-form)

Frequently Asked Questions