JSON Schema Validation: A Complete Guide

Table of Contents

• What is JSON Schema?
• Basic Structure
• Data Types
• Validation Keywords
• Nested Objects and Arrays
• Using JSON Schema in Node.js

What is JSON Schema?

JSON Schema is a vocabulary that allows you to annotate and validate JSON documents. It acts as a contract between your API and its consumers, ensuring data always matches the expected shape.

Basic Structure

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "required": ["name", "email"],
  "properties": {
    "name": { "type": "string", "minLength": 1 },
    "email": { "type": "string", "format": "email" },
    "age": { "type": "integer", "minimum": 0 }
  }
}

Data Types

Validation Keywords

• minLength / maxLength — string length constraints
• minimum / maximum — number range
• pattern — regex for strings
• enum — allowed values
• required — mandatory keys

Nested Objects and Arrays

{
  "type": "object",
  "properties": {
    "tags": { "type": "array", "items": { "type": "string" } },
    "address": { "type": "object", "properties": { "city": { "type": "string" } } }
  }
}

Using JSON Schema in Node.js

import Ajv from "ajv";
import addFormats from "ajv-formats";

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

const schema = {
  type: "object",
  required: ["email"],
  properties: { email: { type: "string", format: "email" } }
};
const validate = ajv.compile(schema);
const valid = validate({ email: "test@example.com" });
if (!valid) console.log(validate.errors);