The JSON Validator, Formatter, and Toolkit for Developers

Overview

JSONLint is an online JSON validator, formatter, converter, and learning toolkit. It combines a multi-diagnostic parser, more than 40 browser-based tools, JSON Schema support, error-code references, practical guides, and free datasets.

Chrome Extension: Check out our JSON Formatter Chrome Extension for formatting JSON directly in your browser. (GitHub Repo)

Open-source validation engine

The main validator is powered by @jsonlint/core, the open-source diagnostics engine extracted from JSONLint. One pass returns every error and warning with stable codes, precise offsets, and line/column locations.

npm install @jsonlint/core

JSONLint documents all 34 diagnostics at jsonlint.com/errors, with an explanation, invalid and corrected examples, and a suggested fix for each code.

Features

Core Tools (40+ Total)

Validators & Formatters

• JSON Validator — Reports every error and warning in one pass, with stable diagnostic codes, precise line/column locations, and linked explanations
• Pretty Print — Format JSON with customizable indentation (2/4 spaces, tabs)
• Minify — Compress JSON by removing all whitespace
• JSON Sorter — Sort object keys alphabetically (ascending/descending)
• Escape/Unescape — Escape JSON for embedding in strings or unescape stringified JSON
• Stringify — Convert JSON to escaped string format
• JSON Repair — Auto-fix broken JSON (trailing commas, single quotes, comments, truncated data)
• JSONC to JSON — Strip comments and trailing commas from JSONC/JSON5 files
• Error Analyzer — Detailed error explanations with suggestions and auto-fix

Viewers & Query Tools

• Tree Viewer — Interactive collapsible tree visualization with color-coded types
• Table Viewer — Display JSON arrays as sortable HTML tables
• JSON Diff — Compare two JSON objects with highlighted differences
• JSON Path Query — Extract data using JSONPath expressions
• JSON Search — Find keys and values with full-text search
• Size Analyzer — Analyze JSON size, depth, and structure complexity
• Flatten/Unflatten — Convert nested JSON to dot notation and back

LLM & AI Tools

• Token Counter — Count tokens for GPT-4, Claude, and other LLMs

Encoding Tools

• Base64 Encode/Decode — Encode JSON to Base64 or decode Base64 to JSON
• JWT Decoder — Decode and inspect JWT tokens (header, payload, signature)

Data Converters

Code Generators Generate typed, production-ready code from JSON:

• TypeScript — Interfaces with optional/required properties
• Python — Dataclasses with type hints
• Java — POJOs with Jackson/Gson annotations
• C# — Classes with JsonProperty attributes
• Go — Structs with json tags
• Kotlin — Data classes with kotlinx.serialization/Moshi/Gson
• Swift — Codable structs with CodingKeys
• Rust — Serde-compatible structs with derive macros
• PHP — Classes with typed properties (PHP 8+)

Schema Tools

• Schema Validator — Validate against JSON Schema Draft 7, 2019-09, or 2020-12; the validator selects the matching Ajv implementation from $schema
• Schema Generator — Auto-generate JSON Schema from sample data

Free JSON Datasets

JSONLint provides 47 free, open-source JSON datasets for testing, learning, and development:

• Reference Data — Countries, languages, currencies, timezones, HTTP status codes
• Development — Programming languages, file extensions, config templates (package.json, tsconfig, ESLint)
• Mock Data — Users, products, orders, transactions, comments, notifications
• Geography — US states, continents, airports, mountains
• Fun — Emojis, emoticons, colors, lorem ipsum

Browse all datasets at jsonlint.com/datasets

Editor Features

• Monaco Editor (VS Code's editor engine) with Shiki syntax highlighting
• Syntax highlighting and bracket matching
• Error highlighting on specific lines
• Drag and drop file upload
• Copy to clipboard
• Keyboard shortcuts (Ctrl/Cmd+Enter to validate, Ctrl/Cmd+Shift+F to format)
• Dark/light theme with system detection
• Full-width toggle for larger workspaces
• IME support for Chinese (Bopomofo/Zhuyin), Japanese, and Korean input
• Mobile responsive

Smart Validation

The validator provides structured feedback beyond a single JSON.parse() exception:

• Multi-diagnostic parsing — Reports all detectable errors and warnings in one pass
• Stable error codes — Links each diagnostic to a dedicated /errors/{CODE} reference page
• Helpful hints — Covers uppercase literals, single quotes, unquoted keys, invalid escapes, missing separators, and more
• JSONC-aware guidance — Recognizes comments and trailing commas and suggests the appropriate conversion tools
• Warnings — Flags duplicate keys, precision loss, lone surrogates, and suspicious Windows-path escapes without marking otherwise-valid JSON as invalid
• Preserves formatting — Validation doesn't modify your input (no more 1.0 → 1 conversion)

Privacy

🔒 JSON documents are processed entirely in your browser. Document contents are never included in telemetry or sent to JSONLint's servers.

To improve the validator, JSONLint records a document-free 10% sample containing only aggregate signals: document length, diagnostic count and codes, validation result, mode, and engine latency. The collector clamps and allowlists every field before writing to Cloudflare Analytics Engine. See lib/shadow-telemetry.ts and workers/shadow/README.md.

Tech Stack

• Framework: Next.js 15 (App Router)
• Language: TypeScript
• Styling: Tailwind CSS
• Editor: Monaco Editor with Shiki syntax highlighting
• JSON syntax validation: @jsonlint/core
• JSON Schema validation: Ajv 8 with draft-specific 7, 2019-09, and 2020-12 builds
• Utilities: jsonpath-plus, fast-xml-parser, diff, and jsonrepair

Local Development

# Clone the repository
git clone https://github.com/circlecell/jsonlint.git
cd jsonlint
# Install dependencies
npm install
# Start development server
npm run dev
# Build for production
npm run build
# Start production server
npm start

Visit http://localhost:3000 to see the app.

Environment Variables

Create a .env.local file:

NEXT_PUBLIC_BASE_URL=https://jsonlint.com

Project Structure

jsonlint/
├── app/                    # Next.js App Router pages
│   ├── page.tsx           # Home (JSON Validator)
│   ├── tools/             # Tools directory page
│   ├── datasets/          # Datasets directory page
│   ├── learn/             # Learning resources
│   ├── errors/            # Diagnostic index and 34 error-code pages
│   ├── json-to-csv/       # Converter tools
│   ├── json-to-typescript/ # Code generators
│   ├── json-schema/       # Schema tools
│   ├── json-formatter/    # Formatter and Chrome-extension landing page
│   ├── jwt-decoder/       # JWT decoder tool
│   ├── json-base64/       # Base64 encode/decode
│   ├── about/             # About page
│   ├── sitemap.ts         # Dynamic sitemap generator
│   └── [...slug]/         # Dynamic content pages
├── components/            # React components
│   ├── Header.tsx         # Site header with navigation
│   ├── Footer.tsx         # Site footer
│   ├── JsonEditor.tsx     # Monaco editor wrapper
│   ├── JsonValidator.tsx  # Main validator
│   ├── ToolNav.tsx        # Tool navigation bar
│   ├── DatasetCard.tsx    # Dataset preview card
│   └── ArticleLayout.tsx  # Learn article layout
├── docs/                  # Documentation & content
│   ├── content/           # Article markdown files
│   └── datasets/          # Dataset documentation (47 datasets)
├── lib/                   # Utility functions
│   ├── jsonlint-core-integration.ts # Main validator adapter
│   ├── error-codes.ts     # Diagnostic reference catalog
│   ├── shadow-telemetry.ts # Document-free sampled metrics
│   ├── json-utils.ts      # Secondary parsing/formatting operations
│   ├── dataset-utils.ts   # Dataset loading utilities
│   └── themes/            # Custom Shiki themes
├── public/               # Static assets
│   ├── datasets/         # JSON dataset files (47 datasets)
│   └── images/           # Logos, favicons, screenshots
├── workers/shadow/       # Cloudflare telemetry collector and query helpers
└── styles/
    └── globals.css       # Global styles & theme variables

URL Parameters

Pre-populate the editor using URL parameters:

• ?json={encoded-json} — Load JSON directly
• ?url={url} — Fetch JSON from a URL

Example:

https://jsonlint.com/?json=%7B%22hello%22%3A%22world%22%7D

Contributing

We welcome contributions! If you have suggestions, feature requests, or found a bug:

1. Create an Issue
2. Submit a Pull Request

Adding a New Tool

1. Create a new folder in app/ with the route name
2. Add the converter/tool component
3. Add the page to navigation in components/ToolNav.tsx and components/Footer.tsx
4. Add SEO content below the tool

Adding Content Pages

1. Create a .md file in docs/content/completed/
2. Include frontmatter with title and description
3. The page will be available at the corresponding URL and added to Learn and the sitemap automatically

Adding Datasets

1. Create a JSON file in public/datasets/
2. Create a corresponding .md file in docs/datasets/ with frontmatter
3. The dataset will appear at /datasets/{name} and in the datasets directory

License

This project is licensed under the MIT License - see the LICENSE file for details.

Author

Created by Todd Garland — Founder of BuySellAds

Credits

• @jsonlint/core — Multi-diagnostic JSON parser used by the main validator
• Monaco Editor by Microsoft
• Douglas Crockford — JSON creator
• Zach Carter — Original jsonlint implementation

jsonlint.com — Making JSON easier since 2010.