Why I Built a JSON Formatter That Never Sends Your Data Anywhere

By Deepak Kumar Developer & builder of JSON Formatter Hub · About the author

Published July 23, 2026 · 4 min read · JSON Formatter Hub

Every JSON formatter online does the same basic job: parse, validate, pretty-print. But most of them make the same architectural choice I decided not to make — they send your JSON to a server first.

I'm Deepak Kumar, a full-stack developer, and I built JSON Formatter Hub specifically to avoid that. Here's why it matters, and how the client-side version actually works.

The problem with server-side formatting

Think about what people actually paste into a JSON formatter: API responses, auth tokens, database exports, config files, sometimes entire request payloads with PII in them. A tool that uploads that to a backend — even briefly, even without storing it — is a tool you have to trust with data you probably shouldn't be sharing with a third party at all.

This isn't hypothetical. Newer "AI-powered" JSON tools go a step further: they pipe your JSON to an LLM API to generate explanations or fix errors. That means your data now also passes through a third-party AI provider's infrastructure. For anything containing credentials or user data, that's a meaningfully larger exposure surface than people realize.

Doing it entirely in the browser

The alternative is straightforward in principle, less so in practice: do everything with JSON.parse, JSON.stringify, and vanilla DOM rendering, entirely client-side.

A few specifics from building it:

Validation with useful error location. JSON.parse throws on invalid input, but the error message alone isn't enough for a developer to fix a 2000-line payload. I wrap it and back-calculate line/column from the character position in the error:

function validateJSON(input) {
  try {
    JSON.parse(input);
    return { valid: true };
  } catch (err) {
    const match = /position (\d+)/.exec(err.message);
    let line = null, column = null;
    if (match) {
      const pos = parseInt(match[1], 10);
      const lines = input.slice(0, pos).split('\n');
      line = lines.length;
      column = lines[lines.length - 1].length + 1;
    }
    return { valid: false, message: err.message, line, column };
  }
}

Tree and table views without a parsing library. Since the input is already a JS object after JSON.parse, tree view is just recursive DOM generation over that object — no separate parser needed. Table view is a flatten step: collect the union of keys across array entries, then map each row against that column set.

Large files. Pretty-printing a multi-megabyte JSON file synchronously can freeze the tab. Deferring the heavy formatting work and rendering progressively keeps the UI responsive without needing a server round-trip to do the same job slower.

The full implementation is open on GitHub: jsonformatterhub/jsonformatterhub.

Try it

JSON Formatter Hub is free, has no login, and — because of everything above — never sees your data. Format, validate, beautify, minify, or convert JSON to XML/CSV/YAML, all in your browser.

If you build developer tools yourself, I'd be curious whether you've made the same client-side-first call, or where it hasn't been worth it for you.