JSON Schema Validation: Beginner to Advanced

What Is JSON Schema?

JSON Schema is a vocabulary for describing the structure and constraints of JSON data. It lets you define what fields are required, what types they must be, and what values are valid — then validate any JSON document against that schema.

Basic Schema Structure

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "User",
  "type": "object",
  "required": ["id", "name", "email"],
  "properties": {
    "id": { "type": "integer", "minimum": 1 },
    "name": { "type": "string", "minLength": 1, "maxLength": 100 },
    "email": { "type": "string", "format": "email" },
    "age": { "type": "integer", "minimum": 0, "maximum": 150 },
    "roles": {
      "type": "array",
      "items": { "type": "string", "enum": ["admin", "user", "moderator"] },
      "uniqueItems": true
    }
  },
  "additionalProperties": false
}

Validation in Node.js (AJV)

const Ajv = require('ajv');
const addFormats = require('ajv-formats');

const ajv = new Ajv();
addFormats(ajv);

const schema = {
  type: 'object',
  required: ['name', 'email'],
  properties: {
    name: { type: 'string' },
    email: { type: 'string', format: 'email' }
  }
};

const validate = ajv.compile(schema);
const valid = validate({ name: 'Alice', email: 'alice@example.com' });
if (!valid) console.log(validate.errors);

Validation in Python (jsonschema)

from jsonschema import validate, ValidationError

schema = {
    'type': 'object',
    'required': ['name', 'email'],
    'properties': {
        'name': {'type': 'string'},
        'email': {'type': 'string', 'format': 'email'}
    }
}

try:
    validate({'name': 'Alice', 'email': 'alice@example.com'}, schema)
    print('Valid')
except ValidationError as e:
    print(f'Invalid: {e.message}')

Schema Composition

// $ref — reuse schema definitions
{
  "$defs": {
    "Address": {
      "type": "object",
      "required": ["street", "city"],
      "properties": {
        "street": { "type": "string" },
        "city": { "type": "string" }
      }
    }
  },
  "type": "object",
  "properties": {
    "home": { "$ref": "#/$defs/Address" },
    "work": { "$ref": "#/$defs/Address" }
  }
}

// allOf, anyOf, oneOf
{
  "oneOf": [
    { "$ref": "#/$defs/Cat" },
    { "$ref": "#/$defs/Dog" }
  ]
}

FAQ