JSON 整形・検証ツール (オンライン無料)

What is JSON formatting?

JSON (JavaScript Object Notation) is the most common format for exchanging structured data between programs. Formatting (also called beautifying or pretty-printing) adds consistent indentation and line breaks so a dense, machine-generated JSON string becomes readable, with nested objects and arrays clearly stepped. This tool formats, minifies, and validates JSON with a collapsible tree view.

Format vs. minify

Formatting and minifying are opposites applied to the same data. Formatting expands JSON for humans to read and debug. Minifying removes all unnecessary whitespace to make the smallest possible payload for transmission, which is what you send over the network. Neither changes the data itself, only its presentation.

Validating JSON and common errors

Invalid JSON is a frequent source of bugs. The validator pinpoints the line where parsing fails. The usual culprits are: trailing commas after the last item, single quotes instead of double quotes, missing quotes around keys, unescaped characters inside strings, and comments (which standard JSON does not allow). Fixing the flagged line usually resolves the whole document.

Your data stays local

All parsing, formatting, and validation happen in your browser using the native JSON engine. Nothing you paste is sent to a server, so it is safe to format API responses, config files, or other data you would not want to upload. The tree view lets you collapse large structures to focus on the part you care about.