JSON Schema Validator – A Complete Guide

What Is JSON Schema?

JSON Schema is a vocabulary for annotating and validating the structure of JSON. It is itself written in JSON and describes what keys are required, what types values must be, what patterns strings must match, and more.

Think of a JSON Schema as a contract: any JSON that passes validation is guaranteed to have the shape your code expects.

// A simple JSON Schema for a user object
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "required": ["name", "age", "email"],
  "properties": {
    "name":  { "type": "string", "minLength": 1 },
    "age":   { "type": "integer", "minimum": 0, "maximum": 150 },
    "email": { "type": "string", "format": "email" }
  },
  "additionalProperties": false
}

// This JSON PASSES the schema above:
{ "name": "Alice", "age": 30, "email": "[email protected]" }

// This JSON FAILS (age is a string, extra key "role" not allowed):
{ "name": "Bob", "age": "thirty", "email": "[email protected]", "role": "admin" }

JSON Schema Versions (Drafts)

JSON Schema has evolved through several drafts. The most important ones to know:

Essential JSON Schema Keywords

Validate with Ajv (JavaScript)

Ajv is the most popular JSON Schema validator for JavaScript. Install it: npm install ajv

import Ajv from "ajv";

const ajv = new Ajv();

const schema = {
  type: "object",
  required: ["name", "age"],
  properties: {
    name: { type: "string" },
    age:  { type: "integer", minimum: 0 },
  },
  additionalProperties: false,
};

const validate = ajv.compile(schema);

const data = { name: "Alice", age: 30 };
const valid = validate(data);

if (!valid) {
  console.log(validate.errors);
  // [{ instancePath: '/age', message: 'must be integer' }]
} else {
  console.log("Valid!");
}

Validate with jsonschema (Python)

Install: pip install jsonschema

import json
import jsonschema

schema = {
    "type": "object",
    "required": ["name", "age"],
    "properties": {
        "name": {"type": "string"},
        "age":  {"type": "integer", "minimum": 0},
    },
    "additionalProperties": False,
}

def validate_user(raw_json: str):
    data = json.loads(raw_json)          # Step 1: syntax check
    jsonschema.validate(data, schema)    # Step 2: schema check

try:
    validate_user('{"name": "Alice", "age": 30}')
    print("Valid!")
except json.JSONDecodeError as e:
    print(f"Syntax error: {e.msg}")
except jsonschema.ValidationError as e:
    print(f"Schema error: {e.message}")

Frequently Asked Questions