What is JSON Schema?

JSON Schema defines the expected structure of JSON documents — required properties, data types, value constraints, array item schemas, and nested object structures. It serves multiple purposes: API request validation (reject malformed input), documentation (clear contract for API consumers), code generation (auto-generate types from schema), and testing (verify API responses). Validators exist for every language (PHP: justinrainbow/json-schema, Python: jsonschema, JavaScript: ajv). JSON Schema supports references ($ref) for reusable definitions, conditional validation (if/then/else), and composition (allOf, anyOf, oneOf). It is used in OpenAPI/Swagger specifications for API documentation.