JSON Validator
JSON Validator
Paste JSON or upload a .json file
Paste JSON to Validate
Paste or upload a JSON document to instantly check for syntax errors, view statistics, explore the tree structure, and beautify or minify your data.
Validate, beautify, minify and repair JSON instantly in your browser
JSON (JavaScript Object Notation) has become the universal data interchange format of the modern web. From REST APIs to configuration files, from NoSQL databases to mobile app settings, JSON is everywhere. Yet even experienced developers make syntax errors: a missing comma, an extra brace, a trailing comma after the last element, or a single quote used where a double quote is required. These tiny mistakes can break an API call, cause a build to fail, or crash an application at runtime. Our free JSON Validator is a fast, privacy-first tool that helps you catch and fix those problems instantly. Paste your JSON into the editor and validation begins automatically — no button press required. You get a clear Valid or Invalid status, and if something is wrong, you see the exact error message, line number, column number, and an excerpt of the failing line with a caret pointing to the problem character. No more hunting through hundreds of lines of JSON to find a misplaced comma. Beyond basic validation, the tool offers a full suite of features that go far beyond what most online validators provide. The Auto-Fix button intelligently repairs the most common JSON errors in one click: it strips JSONC-style comments (// and /* */), removes trailing commas, converts single quotes to double quotes, adds missing quotation marks around unquoted object keys, replaces JavaScript-only literals like NaN, Infinity, and undefined with their JSON-safe equivalents, handles Python constants (True, False, None), and even unwraps JSONP callback wrappers. Once your JSON passes validation, use Beautify to format it with configurable indentation (2 spaces, 4 spaces, or tabs) or Minify to produce a compact single-line version for production use. The Statistics panel gives you a detailed structural profile of your JSON document: total key count, unique key names across all objects, maximum nesting depth, raw byte size, minified byte size, and a complete type-breakdown chart showing how many objects, arrays, strings, numbers, booleans, and null values are present. This is invaluable when auditing large API responses or debugging data pipelines where the shape of data matters as much as the content. The collapsible Tree View renders your JSON as a hierarchical explorer. Each node can be expanded or collapsed independently, and you can click any key name to copy its full dot-notation path (e.g. users.0.address.city) to your clipboard — perfect for writing JSONPath expressions, JavaScript accessors, or database queries. You can also hover any node to copy its value directly. All processing happens entirely in your browser using the built-in JSON.parse() API. Your data never leaves your device and is never sent to any server. The tool works offline once loaded, and supports files of any size that your browser can handle — simply switch to Upload mode to load a .json file from your disk. Whether you are a developer debugging an API response, a data analyst validating an export, a student learning JSON syntax for the first time, or a DevOps engineer checking a configuration file before deployment, this tool is designed to give you answers fast, clearly, and without friction. Strict mode enforces RFC 8259 compliance, ensuring your JSON will be accepted by any standards-compliant parser. Use the sample buttons to try a valid document or a deliberately broken one to explore the error reporting before working with your own data.
Understanding JSON Validation
What Is JSON?
JSON (JavaScript Object Notation) is a lightweight, text-based data interchange format originally derived from JavaScript object literal syntax. Defined by RFC 8259 and ECMA-404, it supports six value types: objects (key-value maps enclosed in curly braces), arrays (ordered lists enclosed in square brackets), strings (double-quoted Unicode text), numbers (integer or floating-point), booleans (true or false), and null. Keys in JSON objects must always be double-quoted strings — unlike JavaScript objects which allow unquoted identifiers. JSON does not support comments, trailing commas, single-quoted strings, or special numeric values like NaN or Infinity, which are valid in JavaScript but not in JSON. This strictness makes JSON highly portable and easy to parse in virtually every programming language.
How Does Validation Work?
JSON validation uses the browser's built-in JSON.parse() function, which implements a strict RFC 8259 compliant tokenizer and parser. The parser reads the input character by character, building a parse tree. If it encounters anything that does not conform to the JSON grammar — an unquoted key, a single-quoted string, a trailing comma, a comment, or a mismatched bracket — it throws a SyntaxError with a message and a character position. Our validator catches that error, converts the character position into a human-readable line and column number, and extracts the relevant line from your input to show you an excerpt with a caret marker. For Auto-Fix, a series of regular-expression transforms are applied to the raw input in a defined order (stripping comments first, then normalising quotes, then fixing commas) before re-parsing, so that fixes do not interfere with each other.
Why JSON Validation Matters
Invalid JSON is one of the most common sources of hard-to-diagnose bugs in web development. An API that returns malformed JSON causes fetch() to throw a parse error, which can crash a front-end application or produce a confusing 500 error in a backend pipeline. Configuration files with invalid JSON prevent applications from starting. Database import jobs with broken JSON records fail silently or discard rows. By validating JSON before using it — in an API response, a config file, a seed file, or a data import — you catch problems at their source rather than hours later in a production log. The type breakdown statistics help you verify data shape: if you expect an array of 500 objects but the stats show only 1 object, you know immediately that something went wrong upstream.
Limitations and RFC Notes
This validator checks JSON syntax (RFC 8259) but does not validate JSON Schema — it will not tell you whether a field value matches a required type or pattern from a schema definition. The auto-fix heuristics handle the most common errors but cannot fix every possible malformed document; deeply nested structural corruption or severe encoding issues may require manual correction. The single-quote to double-quote conversion uses a regular expression that works for the vast majority of cases but may fail on strings containing escaped single quotes in unusual ways. Strict mode in this tool enforces standard JSON.parse() behaviour, which already rejects all non-RFC-8259 constructs. File size is limited by browser memory rather than by the tool; very large files (tens of MB) may cause the browser tab to slow down during parsing.
How to Use the JSON Validator
Paste or Upload Your JSON
Paste your JSON document directly into the textarea, or click Upload File to load a .json file from your disk. The validator begins checking automatically as you type or paste, with a 300ms debounce to keep the UI responsive. Your data never leaves your browser.
Review the Validation Result
A Valid or Invalid badge appears immediately. If invalid, the error panel shows the exact error message from the JSON parser, the line and column number, and an excerpt of the failing line with a caret (^) pointing to the problem character, so you can fix it precisely.
Use Auto-Fix for Common Errors
Click Auto-Fix to let the tool repair common mistakes automatically: it strips comments, removes trailing commas, converts single quotes to double quotes, adds missing quotes around unquoted keys, and replaces JavaScript-only values like NaN and undefined with null. The changes panel lists everything that was modified.
Explore, Beautify, Minify, and Download
Once valid, switch to the Tree View to explore the structure and click any key to copy its dot-notation path. Use Beautify to format with your chosen indentation, Minify for a compact production version, and Download to save the result as a .json file. The Statistics panel shows key counts, depth, type breakdown, and size savings.
Frequently Asked Questions
Is my JSON data sent to a server?
No. All JSON validation, parsing, beautification, and analysis happens entirely inside your browser using JavaScript's native JSON.parse() function. Your data is never transmitted to any server, never stored in a database, and never logged. The tool works fully offline once the page has loaded. This makes it safe to use with sensitive configuration files, API responses containing private data, or any document you would prefer not to share with a third-party service. The small 'Processed locally' badge in the input panel confirms this behaviour on every use.
What errors does the validator detect?
The validator detects every JSON syntax error defined by RFC 8259: missing or extra commas, mismatched brackets or braces, unquoted or single-quoted keys, trailing commas after the last item, missing colons between keys and values, invalid escape sequences in strings, numbers with leading zeros (e.g. 007), bare JavaScript values like NaN, Infinity, undefined, and True/False/None (Python constants), and comments (// or /* */) which are not part of the JSON specification. The error message from the browser's native parser is displayed along with the line number, column number, and a code excerpt with a caret marker.
What does Auto-Fix actually change?
Auto-Fix applies a series of transforms in a specific order designed to maximise repair success without introducing new errors. First it removes any UTF-8 BOM and JSONP wrappers. Then it replaces curly/smart quotes with straight quotes. Next it strips // line comments and /* block */ comments (JSONC format). Then it converts Python boolean and null constants (True/False/None) to their JSON equivalents (true/false/null). It replaces NaN, Infinity, and undefined with null. It converts single-quoted strings to double-quoted strings. It adds missing double quotes around bare object keys. Finally it removes trailing commas before ] and }. After all transforms the result is re-validated, and the changes panel lists every type of fix that was applied.
What is RFC 8259 Strict mode?
RFC 8259 (published 2017) is the current official JSON specification and is identical to ECMA-404. It mandates that all strings use double quotes, that object keys are strings, that there are no comments, no trailing commas, and that numbers do not have leading zeros. The browser's built-in JSON.parse() already enforces this standard. Enabling the Strict mode toggle in our validator is primarily informational — it signals your intent to produce RFC 8259 compliant output and ensures the Auto-Fix routine does not apply any leniency beyond the standard transforms. For maximum compatibility with all JSON parsers across all languages, always aim for RFC 8259 compliant JSON.
What are JSON statistics used for?
The statistics panel gives you a structural fingerprint of your JSON document. Key count and unique key count help you understand schema diversity — if two objects in an array have very different unique key counts it may indicate inconsistent data from an API. Max nesting depth tells you whether the structure is deeply recursive, which can affect performance in some parsers and serializers. The type breakdown (how many strings, numbers, booleans, objects, arrays, and nulls) helps validate that data arrived in the expected format — for example, confirming that a numeric field has not been serialised as a string. Raw vs minified size shows how much bandwidth you save by serving minified JSON in production APIs.
How does the Tree View path explorer work?
The Tree View renders your parsed JSON as a hierarchical explorer where every object and array node can be independently expanded or collapsed. Clicking any key name copies its full dot-notation path to your clipboard — for example, clicking the city key inside the first element of a users array copies users.0.address.city. Array indices use bracket notation (users[0]) and object keys use dot notation. This path can be used directly as a JavaScript property accessor, in JSONPath expressions for tools like jq, in MongoDB query selectors, or in any other context that expects a dot/bracket notation path. A hover-visible copy button on each leaf node also lets you copy the value itself.