How to Format and Validate JSON Online

JSON (JavaScript Object Notation) is the most widely used data interchange format on the web. It represents structured data as human-readable text using key-value pairs, arrays, and nested objects. Almost every web API — from Twitter to Stripe to OpenAI — returns data in JSON format. A simple JSON object looks like this:

{
  "name": "Alice",
  "age": 30,
  "skills": ["JavaScript", "Python"]
}

JSON is language-independent — it can be read and written by every programming language, making it the universal glue between different systems.

JSON from APIs and log files is often minified — stripped of all whitespace to reduce file size. This is efficient for transmission but nearly impossible to read:

{"user":{"id":42,"name":"Bob","roles":["admin","editor"],"active":true}}

Formatting (also called "pretty-printing") adds indentation and line breaks to make the structure clear:

{
  "user": {
    "id": 42,
    "name": "Bob",
    "roles": ["admin", "editor"],
    "active": true
  }
}

Formatting is essential when debugging API responses, reading config files, or reviewing data returned by a service.

The ToolsHub JSON Formatter is instant and works entirely in your browser: 1. Paste your JSON. Copy your raw JSON from an API response, log file, or code and paste it into the input box. 2. Click "Format". The tool validates your JSON and displays it with proper indentation and colour-coding. 3. Spot errors instantly. If your JSON is invalid, the tool highlights the exact line and character where the error occurs. 4. Copy or download. Copy the formatted JSON to your clipboard or download it as a .json file. 5. Minify when needed. Switch to "Minify" mode to strip all whitespace for embedding in code or APIs.

These two operations are related but distinct: Formatting (pretty-printing) adds indentation and line breaks to make JSON readable. It assumes the JSON is already valid. Validation checks whether the JSON is syntactically correct — every string is quoted, every bracket is matched, no trailing commas, correct data types. Invalid JSON will cause errors in applications. The ToolsHub formatter does both simultaneously: it validates first, then formats. If validation fails, you see the error location before any formatting is applied.

Here are the most frequent JSON errors and their fixes: Trailing comma: JSON does not allow a comma after the last item in an array or object. Remove the trailing comma before the closing bracket. Unquoted keys: Object keys must be quoted strings. { name: "Alice" } is invalid; it must be { "name": "Alice" }. Single quotes: JSON requires double quotes. {'name': 'Alice'} is JavaScript object syntax, not valid JSON. Undefined / NaN / Infinity: These JavaScript values are not valid in JSON. Replace them with null or a numeric string. Comments: JSON does not support comments. If you need comments in config files, consider JSONC or JSON5 formats (or strip comments before parsing).