Free online JSON validator and formatter
Paste your JSON and instantly check if it is valid. The online JSON validator detects syntax errors, formats code with indentation, and minifies for production. Everything runs in your browser — your code is never sent to any server.
Validation uses the browser's native engine — no external libraries. Valid JSON per RFC 8259.
How to use the JSON validator step by step
Paste your JSON in the left panel
Click «Validate» to check the syntax
Format or minify as needed
Most common JSON syntax errors
JSON has strict rules. These are the mistakes the validator catches most frequently:
Trailing commas at the end of an object or array
JSON does not allow trailing commas. If you have {"key": "value",} you will get an error. Unlike JavaScript, JSON does not accept that last comma before the closing brace.
Single quotes instead of double quotes
In JSON all keys and string values must be wrapped in double quotes ("). Single quotes (') are not valid. A common mistake when copying JavaScript code directly.
Unmatched braces or brackets
Every { needs its } and every [ its ]. The validator pinpoints exactly which line is missing the closing character or has an unexpected one.
Disallowed values: undefined, NaN or functions
JSON only accepts these value types: string, number, boolean (true/false), null, object and array. The values undefined, NaN, Infinity and functions are valid in JavaScript but not in JSON.
What is JSON used for?
JSON (JavaScript Object Notation) is the most widely used data interchange format on the web. You will find it in REST APIs, configuration files, NoSQL databases and in the communication between the frontend and backend of almost every modern application.
Its popularity comes from being human-readable, compact and easy to generate or parse in any programming language. When working with external APIs, project configurations or data exports, ensuring JSON is valid before using it prevents unexpected errors in production.
When to use this validator
This free online JSON validator is useful in many day-to-day situations for any developer or person working with data:
- Debug API responses — Copy a response from an API and verify the JSON has the expected structure before processing it.
- Review configuration files — Files such as
package.json,tsconfig.jsonormanifest.jsonmust be valid JSON. A syntax error can prevent your project from compiling. - Prepare data for import — Before importing JSON into a database or tool, validate it to avoid rejections or corrupted records.
- Format minified JSON — APIs often return compact JSON without spaces. The formatter converts it into readable code in one click.
Frequently asked questions
Is the JSON validator completely free?
Yes, it is 100% free and no sign-up required. You do not need to create an account or enter your email. You can validate, format and minify as many JSON files as you need with no usage limits.
Is my JSON sent to any server?
No. All validation and formatting happens directly in your browser using native JavaScript. Your JSON code never leaves your device and is never stored on any external server.
What errors does the validator detect?
The validator detects all JSON syntax errors: trailing or missing commas, unclosed quotes, unmatched braces or brackets, invalid values (undefined, functions), and any unexpected characters. It also reports the exact line and column where the error occurs.
What is the difference between formatting and minifying JSON?
Formatting (beautify) adds line breaks and indentation to make JSON easy for humans to read. Minifying removes all unnecessary whitespace to reduce file size, improving transfer speed in APIs and web applications.
Can I validate JSON with nested arrays and objects?
Yes, the validator supports any valid JSON structure: deeply nested objects, arrays of objects, null values, booleans, integers and decimals, and strings with special or Unicode characters.
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