JSON Schema Validation: A Complete Guide with Examples

What is JSON Schema?

JSON Schema is a vocabulary for validating the structure and content of JSON documents. It lets you define required fields, value types, min/max constraints, string patterns, and more — then validate any JSON against those rules.

Basic Schema Example

{
  "type": "object",
  "required": ["name", "age"],
  "properties": {
    "name": { "type": "string", "minLength": 1 },
    "age": { "type": "integer", "minimum": 0, "maximum": 150 },
    "email": { "type": "string", "format": "email" }
  },
  "additionalProperties": false
}

Type Keywords

Array Validation

{
  "type": "array",
  "items": { "type": "string" },
  "minItems": 1,
  "maxItems": 10,
  "uniqueItems": true
}

Validate with AJV in JavaScript

import Ajv from 'ajv';

const ajv = new Ajv();
const schema = {
  type: 'object',
  required: ['name'],
  properties: { name: { type: 'string' } }
};

const validate = ajv.compile(schema);
const valid = validate({ name: 'Alice' });
if (!valid) console.log(validate.errors);

Test your JSON against a schema with the free JSON Schema Validator at jsondecode.com.