JSON Validator Online: Live RFC 8259 Linter & Error Fixer
Validate, lint, and debug JSON payloads against official RFC 8259 grammar specifications. Pinpoint exact line and column error coordinates, diagnose trailing commas or single-quote mistakes, and repair broken schemas with a single click.
To validate JSON online, parse the string against RFC 8259 grammar standards. Valid JSON requires double quotes around all keys and string values, forbids comments, prohibits trailing commas in objects or arrays, and ensures all structural braces {} and brackets [] are balanced.
The Strict Grammar of RFC 8259 JSON
Standardized by the IETF in RFC 8259, JSON (JavaScript Object Notation) is a strict data interchange format designed for cross-platform interoperability. Unlike standard JavaScript object literals, RFC 8259 strictly enforces four foundational rules:
- Double Quotes Only: All string literals and object keys must be enclosed in double quotes (
" "). Single quotes (' ') are forbidden. - No Trailing Commas: Commas function strictly as element separators. Placing a comma after the final item in an array (
[1, 2, 3,]) or object ({"a": 1,}) invalidates the payload. - Zero Comments: Douglas Crockford intentionally removed comments (
//or/* */) from the JSON specification to prevent developers from storing parsing directives inside configuration files. - Quoted Keys: Every property key in an object map must be a valid quoted string. Unquoted keys (such as
{ status: 200 }) are invalid JSON.
Common JSON Syntax Errors & Fixes Matrix
| Syntax Error Type | Invalid Snippet | RFC 8259 Valid Syntax | V8 / Parser Error Message |
|---|---|---|---|
| Trailing Comma | ["apple", "banana",] | ["apple", "banana"] | Unexpected token ']' |
| Single Quotes | {'id': 101} | {"id": 101} | Unexpected token ''', "{'id'..." |
| Unquoted Keys | {userId: 408} | {"userId": 408} | Unexpected token 'u', "{userId..." |
| Unescaped Control Chars | "line1\nline2" (literal raw break) | "line1\nline2" (escaped) | Bad control character in string literal |
The 64-Bit Integer Trap: Why Snowflake IDs Break in JSON
A frequent production bug occurs when transmitting 64-bit database integers (such as Twitter/Discord Snowflake IDs or PostgreSQL BIGINT primary keys) through JSON:
// Raw JSON Payload from Backend:
{"orderId": 9007199254740993}
// Parsed in JavaScript via JSON.parse():
console.log(data.orderId); // 9007199254740992 ← Corrupted!
JavaScript's numeric data type adheres to the IEEE 754 double-precision floating-point standard, with a maximum safe integer limit of $2^{53} - 1$ (9,007,199,254,740,991). Any integer exceeding this threshold silently rounds to the nearest even number.
Architectural Solution: Backend APIs should serialize 64-bit integers and unsigned 64-bit numbers as quoted strings (e.g., {"orderId": "9007199254740993"}) to preserve full bit precision across all frontend clients.
Related JSON & Developer Utilities:
- JSON Formatter & Beautifier — Format raw JSON payloads with clean 2-space indentation.
- JSON Minifier Tool — Compress JSON data into a single line to optimize network payloads.
- Base64 Encoder / Decoder — Translate binary payloads and strings into URL-safe formats.
- JSON Syntax Rules & RFC 8259 Guide — Complete escaping and grammar reference.
Frequently Asked Questions
What is the most common JSON syntax error?
The most common JSON error is the trailing comma (placing a comma after the final item in an array or object before the closing bracket). While permitted in standard JavaScript, trailing commas are strictly forbidden in RFC 8259 JSON.
Can a JSON file have comments?
No. Official RFC 8259 JSON does not support comments of any kind (neither // nor /* */). If you require comments for configuration files, consider using JSON5, HJSON, or YAML.
Why are single quotes invalid in JSON?
The JSON specification mandates double quotes for all string literals and object keys to ensure universal parsing consistency across different programming languages (such as C, Java, and Python) where single quotes denote single character literals.
How does the Auto-Fix button work?
The Auto-Fix feature uses regex heuristics to sanitize relaxed JavaScript objects: it strips single-line and multi-line comments, wraps unquoted keys in double quotes, replaces single-quoted values with double quotes, and removes trailing commas.
Is my JSON payload validated privately?
Yes. All parsing, linting, and error coordinate detection execute 100% locally inside your browser's runtime memory using JavaScript. Your JSON data is never sent across a network or logged to external servers.