JSON Formatter & Validator
Paste JSON to pretty-print it, minify it, or validate it with a clear line-and-column error message. Live as you type, completely free, and entirely in your browser.
Output
Nothing to format yet — paste some JSON above.
What this tool does, and why it stays local
This formatter does three jobs. It pretty-prints — adding indentation so a dense API response becomes readable. It minifies — stripping whitespace down to the smallest valid string for transport. And it validates — telling you the moment your input stops being legal JSON, with the parser's message and an approximate line and column to point you at the offending character.
Everything happens in your browser with the native JSON.parse and JSON.stringify functions. There is no server round-trip, which is the whole point: API responses, environment configs, and webhook payloads frequently contain access tokens, customer data, or internal hostnames you should never paste into a remote service. Because this tool issues zero network requests, you can format that data without it ever leaving your machine.
The errors that trip everyone up are the JavaScript-isms that are not legal JSON: trailing commas after the last array element or object property, single quotes instead of double quotes, unquoted keys, and // comments. Strict JSON forbids all of these. If you are hand-editing a config and it will not parse, that is almost always the cause — and the line-and-column hint will land you within a character or two of it.
A precision gotcha worth internalizing: JSON numbers are IEEE 754 doubles in every JavaScript engine, so an integer larger than 2^53 silently loses precision when parsed. If your API returns 64-bit IDs (think Twitter-style snowflakes or large database keys), serialize them as strings. We see this bite teams when a perfectly valid-looking ID quietly rounds off in transit. We design APIs to avoid exactly this class of bug — our Next.js and Stripe integration guide walks through robust request and response handling end to end.
Need to go from a sample JSON payload to a typed schema or database model? Pair this with our schema generator, and learn the fundamentals in our glossary entry on what an API is.
How to use it
Paste your JSON into the input box. Pick an indent style — two spaces, four spaces, or minify — and the formatted output updates live. If the input is valid you will see a green confirmation and a character and line count; if it is not, you will get the exact parser error and the approximate position to fix. Use the copy button to grab the result. Nothing is stored, so a refresh clears everything.
FAQs
Is my JSON sent to a server?
No. All parsing, formatting, and validation happen in your browser using the native JSON.parse and JSON.stringify functions. Nothing is uploaded, logged, or stored. You can confirm this by opening your browser's network tab — formatting produces zero network requests. That makes it safe for config files and API responses you would not want to paste into a remote service.
Why does my JSON fail to validate?
The most common causes are trailing commas, single quotes instead of double quotes, unquoted object keys, and comments — all of which are valid in JavaScript object literals but illegal in strict JSON. JSON requires double-quoted keys and strings, no trailing commas, and no comments. This tool surfaces the parser's error message along with the approximate line and column so you can jump straight to the problem character.
What is the difference between pretty-printing and minifying?
Pretty-printing adds indentation and line breaks so JSON is readable by humans — ideal when you are debugging an API response or editing a config file. Minifying strips all non-essential whitespace to produce the smallest valid representation, which is what you want when shipping JSON over the wire or embedding it in code. Both produce semantically identical data; only the whitespace differs.
Does formatting change my data?
No. The tool round-trips your input through JSON.parse and JSON.stringify, which preserves every value exactly. One subtlety worth knowing: object key order is preserved by modern engines, but JSON itself does not guarantee key order, so never rely on it for correctness. Also, very large integers beyond JavaScript's safe range can lose precision in any JSON parser — if you handle 64-bit IDs, transmit them as strings.
Can it handle large JSON files?
It handles most everyday payloads — API responses, config files, exports of a few megabytes — without trouble, since everything runs locally on your machine's CPU. Extremely large documents (tens of megabytes) may feel sluggish because the browser must parse, re-serialize, and render the whole string. For files that big, a streaming parser in a build step is the better tool than any in-browser formatter.
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Drowning in undocumented JSON APIs?
We build APIs with strict schemas, predictable error shapes, and typed clients — so your payloads validate the first time and stay stable as you ship. Talk to us about API design, integration work, or untangling a legacy service that returns surprises.
Or reach us directly: (770) 652-1282 · beltz@quantlabusa.dev