JSON Formatter & Validator Online — Beautify, Minify, Validate Free
Paste JSON to format it with proper indentation, validate for syntax errors, or minify it for production. Free, instant, runs entirely in your browser.
What is JSON?
JSON (JavaScript Object Notation) is a lightweight text format for storing and exchanging data. Despite the "JavaScript" in the name, JSON is language-independent — virtually every programming language has libraries to read and write it. It's the de facto standard for APIs, configuration files, and data exchange between web services.
JSON has exactly six data types: strings (in double quotes), numbers, booleans (true/false), null, arrays ([]), and objects ({}). That's the entire specification. Its simplicity is what made it replace XML as the dominant data exchange format over the past 15 years.
Common JSON Errors and How to Fix Them
The validator catches these specific errors — here's what they mean and how to fix them:
Trailing comma: {"key": "value",} — JSON does not allow a comma after the last item in an object or array. Remove the trailing comma. This is the most common JSON error, especially when hand-editing.
Single quotes instead of double quotes: {'key': 'value'} — JSON requires double quotes for both keys and string values. JavaScript allows single quotes in object literals, but JSON does not. Replace all single quotes with double quotes.
Unquoted keys: {key: "value"} — JSON requires keys to be quoted strings. Object keys must be wrapped in double quotes.
Comments: JSON does not support comments. // this is a comment or /* block comment */ will cause a parse error. Remove all comments before using JSON.
Undefined or NaN: JavaScript's undefined and NaN values are not valid JSON. Use null for missing values, or convert NaN to a number or null.
JSON vs XML — Why JSON Won
In the early 2000s, XML was the standard for data exchange. By 2015, JSON had effectively replaced it for most web API use. The reasons are practical: a typical data structure that takes 200 bytes in JSON takes 400-600 bytes in XML due to XML's verbose tag syntax. JSON also maps more naturally to data structures in most programming languages — an object in JSON is a dictionary in Python, a Map in Java, an object in JavaScript. XML requires more processing steps to reach a usable data structure.
XML still has legitimate uses — particularly for document markup where the "X" in XML makes more sense (XHTML, SVG, RSS feeds) and for enterprise systems with schema validation requirements. But for REST APIs and configuration files, JSON is the clear standard in 2025.
Frequently Asked Questions
What's the difference between formatting and minifying JSON?
Formatting (beautifying) adds indentation and newlines to make JSON human-readable. Minifying removes all whitespace to produce the smallest possible file size — identical data, fewer bytes. Formatted JSON is for reading and editing. Minified JSON is for production — sending over a network, storing in a database. A formatted 10KB JSON file might minify to 6KB, a 40% reduction for the same data.
Does JSON support comments?
No. Standard JSON (RFC 7159) does not support comments. This is a deliberate design decision — JSON is a data format, not a configuration file format. For configuration files that need comments, YAML (which supports comments) or JSONC (JSON with comments, used by VS Code) are common alternatives. If you receive JSON with comments, remove them before parsing.
What's the maximum size of a JSON file?
There's no technical limit specified in the JSON standard. Practical limits come from the parser — most JSON parsers in major languages handle files up to several GB without issues, though performance degrades with very large files. For browser-side parsing, files over 50MB may cause noticeable lag. Our formatter handles files up to a few MB comfortably in the browser.
Can JSON arrays contain mixed types?
Yes — JSON arrays can contain any mix of values: [1, "two", true, null, {"key": "value"}] is valid JSON. However, most real-world data structures use arrays of a single type because mixed-type arrays are harder to process programmatically.
Is JSON case-sensitive?
Yes, entirely. {"Name": "Alice"} and {"name": "Alice"} are different keys. True, False, and Null are not valid JSON — the correct values are lowercase: true, false, null.