JSON Validator & Formatter

About JSONLint.app

JSONLint.app is a fast, free validator and formatter for JSON, a lightweight data-interchange format. Copy and paste, directly type, or input a URL in the editor above and let JSONLint.app tidy and validate your messy JSON code.

What Is JSON?

JSON (JavaScript Object Notation) is a human-readable and compact format for representing complex data structures. It's used extensively in web APIs, configuration files, and data exchange between systems. JSON is language-independent, making it a universal data format understood by virtually all modern programming languages.

How to Validate JSON

Validating JSON ensures your data follows the correct syntax before it's processed by an application. Here's how to validate JSON using JSONLint.app:

1. Paste your JSON into the editor above
2. Click the Validate JSON button (or press Ctrl+Enter)
3. JSONLint.app will highlight any errors with line numbers and descriptions
4. Fix the errors and re-validate until your JSON is valid

Example of Valid JSON

Here is a simple example of valid JSON:

{
  "name": "John Doe",
  "age": 30,
  "email": "john@example.com",
  "isActive": true,
  "address": {
    "street": "123 Main St",
    "city": "New York",
    "country": "US"
  },
  "hobbies": ["reading", "coding", "hiking"]
}

Common JSON Errors

• Trailing commas — An extra comma after the last item: { "a": "b", }
• Single quotes — JSON requires double quotes, not single: { 'key': 'value' } is invalid
• Unquoted keys — Keys must be in double quotes: { key: "value" } is invalid
• Missing commas — Forgetting a comma between items: { "a": 1 "b": 2 }
• Comments — JSON does not support comments. Use JSONC format if you need them.
• Wrong boolean/null casing — Must be lowercase: true, false, null (not True, FALSE, NULL)

Proper JSON Format Rules

• Data is in name/value pairs
• Data is separated by commas
• Objects are wrapped in curly braces {}
• Arrays are wrapped in square brackets []
• Keys must be strings in double quotes
• String values must use double quotes
• Boolean values use true or false (lowercase)
• Null values use null (lowercase)
• Numbers should not have leading zeroes
• No trailing commas allowed
• File extension: .json — MIME type: application/json

Why Use JSONLint.app?

JSONLint.app helps you quickly find and fix errors in your JSON data. It highlights the exact line and character where the error occurs, saving you hours of manual debugging. All processing happens entirely in your browser — your data never leaves your machine.

Tips & Tricks

• Load JSON from a URL: https://www.jsonlint.app/?url=https://www.jsonlint.app/datasets/programming-languages.json
• Pass JSON directly via URL parameter: https://www.jsonlint.app/?json=%7B%22hello%22:%20%22world%22%7D
• Use keyboard shortcuts: Ctrl+Enter to validate, Ctrl+Shift+F to format.
• Drag and drop a .json file directly into the editor.

In-Depth JSON Guides

Beyond validating and formatting, we write practical, developer-focused guides on the JSON problems that actually cost you time — with real reproductions and code samples in JavaScript, Python, and Go:

• Unexpected token < in JSON at position 0 — why fetch().then(r => r.json()) fails when an API returns an HTML error page, and how to surface the real response.
• JSON numbers silently corrupt large IDs — integers above 2⁵³ lose digits in JavaScript. How to spot it and fix it with BigInt, Python Decimal, and Go json.Number.
• Parsing JSON files too big for memory — streaming multi-GB files with stream-json, ijson, Go's json.Decoder, and NDJSON.
• JSON Schema validation in practice — going from "is it valid JSON" to "is it the right shape" with Ajv, jsonschema, and Go.
• Reading and locating any JSON parse error — decode the error message and find the exact byte/line/column across JavaScript, Python, Go, and Java.

Browse all tutorials and free datasets on the Learn JSON hub.

Frequently Asked Questions