Two different jobs, often bundled into one tool
A formatter (or "beautifier") takes JSON and re-indents/re-spaces it for human readability — adding line breaks and consistent indentation to a minified or inconsistently-spaced document. It generally assumes the input is already valid and focuses purely on presentation.
A validator checks whether a document is actually correct — either syntactically (does it parse at all) or structurally (does it match an expected schema). A validator's job is correctness, not appearance; it can validate a minified, hard-to-read single-line JSON document just as well as a nicely formatted one.
| Formatter | Validator | |
|---|---|---|
| Job | Improve readability | Confirm correctness |
| Input assumption | Usually already valid | Might be invalid — that's the point |
| Typical output | Re-indented JSON | Pass/fail + error location |
| Useless if input is broken? | Often fails silently or partially | This is exactly what it's designed to catch |
Why formatting alone can hide problems
Some formatters are lenient — they'll attempt to reformat something close to valid JSON, silently "fixing" small issues (or producing confusing partial output) rather than clearly reporting an error. This can create a false sense that a document is fine, when a stricter validator would catch a real problem the formatter simply worked around or ignored.
Why most good tools do both
Since you almost always want to both confirm correctness and read the result comfortably, well-designed JSON tools run validation first (reporting a clear error if the document is broken) and only then format the confirmed-valid result — this ordering matters, since formatting invalid JSON either fails outright or produces misleading output.
Common mistakes
- Assuming a formatter validated your JSON. A lenient formatter that manages to produce readable output doesn't mean the input was actually correct — check for an explicit validation step or error report.
- Only checking that output "looks nicely formatted." Visual tidiness says nothing about whether required fields are present or types are correct — that's a schema-validation concern, not formatting.
- Skipping validation on data you already "know is fine." Data from an external API, another team, or a manual edit is exactly the case where a quick validation pass catches real, otherwise-invisible mistakes.
FAQ
If my JSON formats without error, is it valid?
Usually, but not guaranteed — some formatters are lenient and paper over minor issues rather than reporting them clearly; an explicit validation step is more reliable.
Do I need both a formatter and a validator?
For anything beyond trivial use, yes — validate first to confirm correctness, then format for readability; a good tool bundles both in the right order.
Can formatting fix invalid JSON?
No — a formatter changes whitespace and indentation, not structure; genuinely invalid JSON (a missing brace, a trailing comma) needs to be corrected, not just reformatted.
Validate and format JSON in one step with the JSON Toolkit — runs entirely in your browser.