JSON Validator

When JSON fails to parse, the error message is often unhelpful: "unexpected token at position 347" tells you nothing about what to fix. The JSON Validator finds and explains syntax errors with precise line and column numbers, shows the problematic character highlighted in context, and suggests specific fixes for common mistakes — missing commas, trailing commas, unescaped quotes, invalid Unicode sequences, mismatched brackets. Goes beyond a simple "valid/invalid" check: explains why and where, and walks you through fixing each error.

Also supports JSON Schema validation — check that your JSON matches a specified schema (types, required fields, enum values, patterns, nested structure). Essential for validating API request/response bodies, configuration files, and data pipelines. The validator uses the Ajv JSON Schema library with support for Draft 7, Draft 2019-09, and Draft 2020-12. All validation runs in your browser — your JSON and schemas never upload anywhere. Perfect for validating sensitive API payloads, internal configs, or schema files under active development.

JSON validation has two layers: syntax validation (is it valid JSON per RFC 8259?) and schema validation (does the structure match expected fields and types?).

Syntax errors the validator catches:

Most common JSON errors with explanations:

| Error | Example | Fix |
|---|---|---|
| Missing comma | {"a":1 "b":2} | Add , between properties |
| Trailing comma | {"a":1,} | Remove final , |
| Unquoted key | {a: 1} | Quote key: {"a": 1} |
| Single quotes | {'a': 1} | Use double quotes: {"a": 1} |
| Unescaped quote in string | {"a": "hello "world""} | Escape: "hello \\"world\\"" |
| Unescaped newline | "line 1\\nline 2" literal newline | Use \\n escape |
| Control characters | {"a": "text<CR>"} | Escape as \\r |
| Leading zero | {"a": 007} | Remove leading zeros: {"a": 7} |
| BOM at start | \\uFEFF{"a":1} | Strip byte-order mark |
| Duplicate keys | {"a":1, "a":2} | RFC 8259 allows but discourages; parser-dependent behavior |
| NaN, Infinity | {"a": NaN} | Use string "NaN" or null |
| Comments | // comment | JSON doesn't allow — use JSON5 or strip before validating |

JSON Schema validation (Draft 7 / 2020-12):

JSON Schema describes the expected structure:

``json
{
"type": "object",
"required": ["name", "email"],
"properties": {
"name": { "type": "string", "minLength": 1 },
"email": { "type": "string", "format": "email" },
"age": { "type": "integer", "minimum": 0, "maximum": 150 }
}
}
``

Checks against a JSON payload:

``json
{ "name": "", "email": "not-an-email" }
``

Reports:
- name fails minLength constraint (empty string).
- email is missing @ (format validation).

Schema drafts:

- Draft 4 (2013) — legacy, widely supported.
- Draft 7 (2019) — widely adopted, supported by most validators.
- Draft 2019-09 — added unevaluatedProperties, $anchor.
- Draft 2020-12 — current standard, matches OpenAPI 3.1.

JSON Schema vs TypeScript types:

TypeScript types give compile-time checks; JSON Schema gives runtime validation. Use JSON Schema to validate data coming from outside your code (API inputs, config files, user-submitted data). Generate TypeScript types from JSON Schema with json-schema-to-typescript for both.

Validation performance:

The Ajv library compiles schemas into optimized JavaScript functions — validation is typically faster than JSON.parse itself. For high-throughput APIs, use Ajv in production.

JSON Validator vs JSON Formatter: Validator tells you if JSON is valid and why it's not. Formatter pretty-prints valid JSON (and also reports syntax errors). For error-finding with schema validation, use the validator. For formatting clean JSON, use our JSON Formatter.

JSON Schema vs TypeScript types: TypeScript gives compile-time checks within your code. JSON Schema gives runtime validation for data crossing boundaries (API inputs, config files). Use both — generate TypeScript types from JSON Schema for end-to-end type safety.

JSON Schema vs Joi / Yup / Zod: JavaScript-specific validation libraries (Joi, Yup, Zod) are more convenient for runtime validation in JavaScript apps but aren't portable. JSON Schema is language-agnostic — use it for API contracts shared across languages.

JSONLint vs this tool: JSONLint is the original web-based validator. This tool adds schema validation, better error messages, and runs entirely client-side (JSONLint uploads to their server).

Validator vs Linter: A validator checks if JSON is *valid*. A linter enforces *style* (indentation, key ordering). Both are useful. For style, see Prettier or jsonlint with style config.