JSON Schema Validation Guide: Data Validation & Doc Generation

JSONJSON and structured data

JSON Schema: A Contract for Your Data

JSON Schema is a structure-definition language for JSON data — similar to what TypeScript is to JavaScript, but designed for runtime validation and cross-language scenarios.

Use Case Description
Data validation Validate API requests/responses
Documentation generation Auto-generate API docs
IDE hints Smart autocomplete in VS Code
Mock data Auto-generate test data
Config validation package.json, tsconfig.json, etc.

1. Basic Types

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "name": { "type": "string", "minLength": 1, "maxLength": 100 },
    "age": { "type": "integer", "minimum": 0, "maximum": 150 },
    "email": { "type": "string", "format": "email" },
    "isActive": { "type": "boolean" },
    "score": { "type": "number", "minimum": 0, "maximum": 100 },
    "tags": {
      "type": "array",
      "items": { "type": "string" },
      "minItems": 1,
      "maxItems": 10,
      "uniqueItems": true
    }
  },
  "required": ["name", "email"],
  "additionalProperties": false
}

Built-in Formats

format Matching Rule Example
email RFC 5322 user@example.com
uri URI https://example.com
date ISO 8601 date 2026-06-02
time ISO 8601 time 14:30:00
date-time ISO 8601 date-time 2026-06-02T14:30:00Z
ipv4 IPv4 address 192.168.1.1
ipv6 IPv6 address ::1
uuid UUID 550e8400-e29b-41d4-a716-446655440000
hostname Hostname example.com

2. Advanced Patterns

Nested Objects

{
  "type": "object",
  "properties": {
    "user": {
      "type": "object",
      "properties": {
        "profile": {
          "type": "object",
          "properties": {
            "avatar": { "type": "string", "format": "uri" },
            "bio": { "type": "string", "maxLength": 200 }
          }
        }
      }
    }
  }
}

Enums and Union Types

{
  "status": {
    "type": "string",
    "enum": ["pending", "active", "completed", "cancelled"]
  },
  "id": {
    "oneOf": [
      { "type": "string", "format": "uuid" },
      { "type": "integer", "minimum": 1 }
    ]
  }
}

Conditional Validation

{
  "type": "object",
  "properties": {
    "paymentMethod": { "type": "string", "enum": ["credit_card", "bank_transfer"] },
    "cardNumber": { "type": "string" },
    "bankAccount": { "type": "string" }
  },
  "if": {
    "properties": { "paymentMethod": { "const": "credit_card" } }
  },
  "then": {
    "required": ["cardNumber"],
    "properties": { "cardNumber": { "pattern": "^\\d{16}$" } }
  },
  "else": {
    "required": ["bankAccount"]
  }
}

Regex Patterns

{
  "phone": { "type": "string", "pattern": "^1[3-9]\\d{9}$" },
  "postalCode": { "type": "string", "pattern": "^\\d{6}$" },
  "slug": { "type": "string", "pattern": "^[a-z0-9]+(?:-[a-z0-9]+)*$" }
}

3. $ref and Reusable Definitions

{
  "$defs": {
    "address": {
      "type": "object",
      "properties": {
        "street": { "type": "string" },
        "city": { "type": "string" },
        "zipCode": { "type": "string", "pattern": "^\\d{6}$" }
      },
      "required": ["street", "city", "zipCode"]
    }
  },
  "type": "object",
  "properties": {
    "homeAddress": { "$ref": "#/$defs/address" },
    "workAddress": { "$ref": "#/$defs/address" }
  }
}

4. Runtime Validation

Ajv (The Fastest Validator)

import Ajv from 'ajv';

const ajv = new Ajv({ allErrors: true, strict: false });

const schema = {
  type: 'object',
  properties: {
    name: { type: 'string', minLength: 1 },
    age: { type: 'integer', minimum: 0 },
  },
  required: ['name', 'age'],
};

const validate = ajv.compile(schema);

const valid = validate({ name: '张三', age: 25 });
if (!valid) {
  console.log(validate.errors);
  // [{ keyword: 'minimum', dataPath: '/age', message: 'must be >= 0' }]
}

Zod (TypeScript-First)

import { z } from 'zod';

const UserSchema = z.object({
  name: z.string().min(1).max(100),
  age: z.number().int().min(0).max(150),
  email: z.string().email(),
  tags: z.array(z.string()).min(1).max(10),
});

// Automatic type inference
type User = z.infer<typeof UserSchema>;

// Validation
const result = UserSchema.safeParse(input);
if (result.success) {
  console.log(result.data); // Type-safe
} else {
  console.log(result.error.issues);
}

5. IDE Autocomplete

VS Code Configuration

// .vscode/settings.json
{
  "json.schemas": [
    {
      "fileMatch": ["/config/*.json"],
      "url": "./schemas/config-schema.json"
    },
    {
      "fileMatch": ["package.json"],
      "url": "https://json.schemastore.org/package"
    }
  ]
}

Referencing a Schema in JSON Files

{
  "$schema": "./schemas/config-schema.json",
  "name": "my-project"
  // IDE autocomplete and error hints
}

Schema Store

Schema Store provides 400+ common JSON Schemas:

  • package.json
  • tsconfig.json
  • .eslintrc.json
  • docker-compose.json
  • GitHub Actions workflows

6. Generating a Schema from JSON

Using the Tool

  1. Open the JSON Formatter
  2. Paste your JSON data
  3. Click "Generate Schema"
  4. Types, enums, and ranges are inferred automatically

Command-Line Tools

# Using json-schema-generator
npx json-schema-generator sample.json -o schema.json

# Using quicktype
npx quicktype sample.json -o schema.json --lang schema

7. Mock Data Generation

import { faker } from '@faker-js/faker';

// Generate mock data based on a schema
function generateMock(schema) {
  if (schema.type === 'string') {
    if (schema.format === 'email') return faker.internet.email();
    if (schema.format === 'uri') return faker.internet.url();
    if (schema.enum) return faker.helpers.arrayElement(schema.enum);
    return faker.lorem.word();
  }
  if (schema.type === 'integer') return faker.number.int(schema);
  if (schema.type === 'boolean') return faker.datatype.boolean();
  if (schema.type === 'array') {
    return Array.from({ length: 3 }, () => generateMock(schema.items));
  }
  if (schema.type === 'object') {
    const result = {};
    for (const [key, propSchema] of Object.entries(schema.properties)) {
      result[key] = generateMock(propSchema);
    }
    return result;
  }
}

8. Best Practices

  1. Always declare $schema: Pin the draft version to avoid ambiguity
  2. Use additionalProperties: false: Reject undefined fields
  3. Use $defs + $ref: Reuse common structures
  4. Prefer format over hand-written patterns: More readable, and validators handle it automatically
  5. Integrate validation into CI: Ensure API responses always conform to the schema
  6. Version your schemas: Keep them in sync with API versions and support backward-compatibility checks
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