Enter JSON
Formatted Output
What Is a JSON Formatter & Validator?
JSON (JavaScript Object Notation) is the de facto standard data interchange format for Web APIs. It is used daily in server responses and configuration files such as package.json and tsconfig.json.
This tool pretty-prints compressed JSON into a readable format and detects syntax errors in real time, highlighting the line number where each error occurs.
Strict Syntax Rules of JSON
Although JSON resembles JavaScript object literals, it follows much stricter syntax rules. Violating any of the following will cause a parse error.
| Rule | OK | NG |
|---|---|---|
| Keys must be double-quoted | {"name": "John"} |
{name: "John"} |
| Strings must use double quotes | "hello" |
'hello' |
| Trailing commas are not allowed | {"a": 1, "b": 2} |
{"a": 1, "b": 2,} |
| Comments are not allowed | (no comments) | {"a": 1} // comment |
| Only 6 value types | string, number, boolean, null, object, array | undefined, functions, Date |
Why JSON Debugging Matters in API Development
In REST API and GraphQL development, the vast majority of requests and responses use JSON. Formatting and validation become essential in scenarios like the following:
- Inspecting API responses — Pretty-print compressed JSON fetched via curl or Postman to quickly understand the data structure
- Building request bodies — Create and validate JSON before sending POST requests to prevent 422 errors
- Editing config files — Validate package.json or tsconfig.json after edits to catch syntax errors early
- Analyzing logs — Expand single-line compressed JSON logs to pinpoint the root cause of errors
The most common causes of syntax errors are trailing commas and single quotes. These patterns are valid in JavaScript but strictly prohibited in JSON, so be careful.
Safe and Entirely Browser-Based
This tool runs entirely within your browser using JavaScript alone. Your JSON input is never sent to any external server. You can safely format and inspect response data that contains API keys or tokens.
- No external API calls — works even in offline environments
- No data storage or transmission — your input is completely gone once you close the page
- No account required — just open the URL and start using it immediately