Validating JSON with JSON Schema: A Practical Guide

JSON Schema

JSON Schema is a vocabulary for validating JSON documents. It describes the structure and constraints of your data.

Basic Schema

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

Type Keywords

{ "type": "string" }
{ "type": "number" }
{ "type": "integer" }
{ "type": "boolean" }
{ "type": "null" }
{ "type": "array" }
{ "type": "object" }
{ "type": ["string", "null"] }  // Multiple types

String Constraints

{
  "type": "string",
  "minLength": 1,
  "maxLength": 100,
  "pattern": "^[A-Za-z]+$",
  "format": "email"
}

Number Constraints

{
  "type": "number",
  "minimum": 0,
  "maximum": 100,
  "exclusiveMinimum": 0,
  "multipleOf": 0.01
}

Array Constraints

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

Validation in JavaScript

import Ajv from 'ajv';
const ajv = new Ajv();
const schema = {
  type: 'object',
  properties: {
    name: { type: 'string' },
    age: { type: 'integer', minimum: 0 }
  },
  required: ['name']
};
const validate = ajv.compile(schema);
const valid = validate({ name: 'John', age: 30 });
// true
const invalid = validate({ age: -5 });
// false
console.log(validate.errors);
// [{ keyword: 'required', ... }, { keyword: 'minimum', ... }]