How does the JSON Validator highlight syntax errors?

If a JSON payload has syntax errors, our custom tokenizer analyzes the string character-by-character. It highlights the exact line in the editor, and displays a validation banner detailing the line number, column number, and the specific syntax violation.

What is checked during strict JSON validation?

The validator checks for RFC 8259 compliance, which forbids comments, single quotes, unquoted keys, duplicate keys, trailing commas inside arrays or objects, and incorrectly formatted numeric values.

Why does standard JSON.parse not catch duplicate keys?

JavaScript's native JSON.parse() resolves duplicate object keys by overwriting earlier occurrences with the final one. Our custom parsing engine strictly checks for duplicates to prevent silent configuration bugs.

How do I compare two JSON payloads?

Select the Diff tab in the workspace. Enter your original JSON in the first editor and the modified JSON in the second editor. The tool will parse, format, and display a line-by-line comparison highlighting additions and removals.

Is this validator secure for corporate API payloads?

Yes. All parsing, validation, linting, and diff calculations occur locally in your browser memory. No data is sent to external web servers, making it compliant with strict enterprise data privacy rules.

Standard JSON Validator - Validate JSON Compliance Online

JSON Validator

Validate JSON online against the strict RFC 8259 spec, catching trailing commas, duplicate keys, and mismatched brackets fast.

RFC 8259 Compliance Basics

RFC 8259 is the formal specification that defines valid JSON syntax: object keys and string values must use double quotes, arrays and objects cannot end with a trailing comma, and comments are not part of the standard. A payload that violates any of these rules will fail to parse in strict environments even if it looks correct at a glance.

Why a Custom Tokenizer Catches More Than JSON.parse

JavaScript's built-in JSON.parse() silently keeps only the last occurrence of a duplicate key and simply throws a generic error on malformed syntax without pointing to the cause. This validator instead tokenizes the text character by character, so it can flag single-quoted strings, unquoted keys, mismatched or unbalanced brackets, trailing commas, and duplicate object keys individually, each reported with the exact line and column number.

Validation Example

Given an input like {'id': 123, "title": 'Design Patterns'}, strict mode immediately flags the single-quoted values as an RFC compliance violation and suggests replacing them with double quotes. A payload with a repeated key such as {"theme":"dark","theme":"light"} is separately caught by the duplicate-key checklist item even though JSON.parse would accept it silently.

When to Use Related Developer Tools

Once a payload passes validation, switch to the JSON Formatter to beautify or minify it for storage or transmission. For tabular exports, the JSON to CSV Converter and CSV to JSON Converter convert validated records between spreadsheet and JSON formats.

JSON Lint Target

Awaiting Payload

Structure & Complexity Metrics

Recent Validation Drafts

AI JSON Debugger

How JSON Validation and Diagnostics Work

RFC 8259 Compliance Basics

Why a Custom Tokenizer Catches More Than JSON.parse

Validation Example

When to Use Related Developer Tools

Questions, answered

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