Developer Tools2026-08-217 min readClient-Side Engine

How to Validate JSON: Syntax Diagnostics, JSON Schema (Draft 2020-12), and Runtime Validation

Learn how to validate JSON data: debugging syntax errors, defining JSON Schema (Draft 2020-12) contracts, and validating runtime payloads with Ajv, Zod, and Pydantic.

CoShareXCore Engineering Team

The Two Levels of JSON Validation

Validating JSON requires verifying two distinct layers of correctness: Syntactic Conformance (verifying that the string conforms to RFC 8259 grammar rules) and Semantic/Schema Conformance (verifying that the parsed data structure matches expected keys, types, ranges, and invariants).

Common Syntax Errors & Exact Fixes

Syntax Error	Invalid Example	Fixed Example	RFC 8259 Rule
Trailing Commas	`{"a": 1, "b": 2,}`	`{"a": 1, "b": 2}`	Trailing commas are strictly prohibited after the last element.
Single Quotes	{'name': 'Alex'}	{"name": "Alex"}	All keys and string values must use double quotes (`"`).
Unquoted Keys	`{name: "Alex"}`	`{"name": "Alex"}`	Keys must always be valid double-quoted strings.
Unescaped Control Characters	`{"text": "Line 1 \n Line 2"}` (literal newline)	`{"text": "Line 1\nLine 2"}`	Newlines, tabs, and backslashes must be escaped (`\n`, `\t`, `\\`).
Comments	`{"a": 1 /* note */}`	`{"a": 1}`	Standard JSON does not support inline or block comments.

Structural Validation with JSON Schema (Draft 2020-12)

JSON Schema provides an industry-standard declarative language to enforce data structure contracts across APIs and microservices:

json

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "UserRegistration",
  "type": "object",
  "properties": {
    "userId": { "type": "string", "format": "uuid" },
    "email": { "type": "string", "format": "email" },
    "age": { "type": "integer", "minimum": 18 },
    "roles": {
      "type": "array",
      "items": { "type": "string", "enum": ["viewer", "editor", "admin"] },
      "minItems": 1
    }
  },
  "required": ["userId", "email", "roles"],
  "additionalProperties": false
}

Runtime Type Validation: Zod & Pydantic

In TypeScript and Python backends, developers use runtime schema parsers that validate JSON payloads and infer static types simultaneously:

typescript

import { z } from 'zod';

const UserSchema = z.object({
  userId: z.string().uuid(),
  email: z.string().email(),
  age: z.number().int().min(18),
  roles: z.array(z.enum(['viewer', 'editor', 'admin'])).min(1)
});

// Infer static TypeScript type directly from schema
export type User = z.infer<typeof UserSchema>;

export function validateUserInput(data: unknown): User {
  return UserSchema.parse(data); // Throws structured ZodError on mismatch
}

High-Performance Schema Validation with Ajv

While Zod is ideal for client-side forms and API boundaries in TypeScript, Ajv (Another JSON Schema Validator) compiles JSON Schema into highly optimized JIT JavaScript functions, achieving over 1,000,000 validations per second on server backends:

typescript

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

const ajv = new Ajv({ allErrors: true });
addFormats(ajv);

const validate = ajv.compile({
  type: 'object',
  properties: {
    name: { type: 'string', minLength: 2 },
    email: { type: 'string', format: 'email' }
  },
  required: ['name', 'email']
});

const data = { name: 'Alex', email: 'alex@cosharex.com' };
const valid = validate(data);

if (!valid) {
  console.error('Validation errors:', validate.errors);
}

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Frequently Asked Questions

How do I locate the exact line number of a JSON syntax error in JavaScript?

Native `JSON.parse()` error messages in modern V8 engines report the exact character position (e.g. `Unexpected token at position 142`). You can calculate the line and column by splitting the preceding text substring by newlines.

What is JSONC vs standard JSON?

JSONC (JSON with Comments) is a non-standard superset used by VS Code (`tsconfig.json`, `settings.json`) that allows single-line (`//`) and block (`/* */`) comments. It must be stripped before passing to standard `JSON.parse()`.

Conclusion

For large JSON payloads, mismatched braces or brackets can be tough to debug. A good JSON validator parses your structure step-by-step and points you directly to the offending line number. When debugging, look for missing commas or unclosed arrays within large nested objects.

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Common Syntax Errors & Exact Fixes

Syntax Error	Invalid Example	Fixed Example	RFC 8259 Rule
Trailing Commas	`{"a": 1, "b": 2,}`	`{"a": 1, "b": 2}`	Trailing commas are strictly prohibited after the last element.
Single Quotes	{'name': 'Alex'}	{"name": "Alex"}	All keys and string values must use double quotes (`"`).
Unquoted Keys	`{name: "Alex"}`	`{"name": "Alex"}`	Keys must always be valid double-quoted strings.
Unescaped Control Characters	`{"text": "Line 1 \n Line 2"}` (literal newline)	`{"text": "Line 1\nLine 2"}`	Newlines, tabs, and backslashes must be escaped (`\n`, `\t`, `\\`).
Comments	`{"a": 1 /* note */}`	`{"a": 1}`	Standard JSON does not support inline or block comments.

Structural Validation with JSON Schema (Draft 2020-12)

JSON Schema provides an industry-standard declarative language to enforce data structure contracts across APIs and microservices:

json

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "UserRegistration",
  "type": "object",
  "properties": {
    "userId": { "type": "string", "format": "uuid" },
    "email": { "type": "string", "format": "email" },
    "age": { "type": "integer", "minimum": 18 },
    "roles": {
      "type": "array",
      "items": { "type": "string", "enum": ["viewer", "editor", "admin"] },
      "minItems": 1
    }
  },
  "required": ["userId", "email", "roles"],
  "additionalProperties": false
}

Runtime Type Validation: Zod & Pydantic

In TypeScript and Python backends, developers use runtime schema parsers that validate JSON payloads and infer static types simultaneously:

typescript

import { z } from 'zod';

const UserSchema = z.object({
  userId: z.string().uuid(),
  email: z.string().email(),
  age: z.number().int().min(18),
  roles: z.array(z.enum(['viewer', 'editor', 'admin'])).min(1)
});

// Infer static TypeScript type directly from schema
export type User = z.infer<typeof UserSchema>;

export function validateUserInput(data: unknown): User {
  return UserSchema.parse(data); // Throws structured ZodError on mismatch
}

High-Performance Schema Validation with Ajv

typescript

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

const ajv = new Ajv({ allErrors: true });
addFormats(ajv);

const validate = ajv.compile({
  type: 'object',
  properties: {
    name: { type: 'string', minLength: 2 },
    email: { type: 'string', format: 'email' }
  },
  required: ['name', 'email']
});

const data = { name: 'Alex', email: 'alex@cosharex.com' };
const valid = validate(data);

if (!valid) {
  console.error('Validation errors:', validate.errors);
}

How to Validate JSON: Syntax Diagnostics, JSON Schema (Draft 2020-12), and Runtime Validation

The Two Levels of JSON Validation

Common Syntax Errors & Exact Fixes

Structural Validation with JSON Schema (Draft 2020-12)

Runtime Type Validation: Zod & Pydantic

High-Performance Schema Validation with Ajv

Code Formatter & Converter Suite

Frequently Asked Questions

How do I locate the exact line number of a JSON syntax error in JavaScript?

What is JSONC vs standard JSON?

Conclusion

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How to Validate JSON: Syntax Diagnostics, JSON Schema (Draft 2020-12), and Runtime Validation

The Two Levels of JSON Validation

Common Syntax Errors & Exact Fixes

Structural Validation with JSON Schema (Draft 2020-12)

Runtime Type Validation: Zod & Pydantic

High-Performance Schema Validation with Ajv

Code Formatter & Converter Suite

Frequently Asked Questions

How do I locate the exact line number of a JSON syntax error in JavaScript?

What is JSONC vs standard JSON?

Conclusion

Related Articles

Streaming & Parsing Large JSON Files Without Crashing Memory: Node.js, Python & Web Workers

JSON Minification: Performance Gains, Gzip Interaction, and Streaming Parsers

JSON vs XML: Structural Architecture, Parsing Performance, and Modern Use Cases

Ready for faster, account-free developer tools?