Free Online JWT Decoder & Debugger

Overview

This single-file web application serves as a robust, client-side tool for developers to decode and inspect JSON Web Tokens (JWT). By utilizing the browser's native capabilities, it provides an instant visual breakdown of the Token Header, Payload, and Signature without ever transmitting data over the network.

Feature Set

• Instant Parsing: Real-time decoding of JWT strings as the user types or pastes.
• Syntax Highlighting: Clean, readable JSON visualization for headers and payloads.
• Signature Verification: Visual separation of the signature section.
• Smart Timestamp Conversion: Automatically detects 'iat', 'exp', and 'nbf' claims and displays them in a human-readable local date format.
• One-Click Copy: Convenient buttons to copy the raw header or payload JSON.
• Error Handling: Graceful UI feedback for invalid JWT formats or malformed Base64Url strings.

Technical Architecture

• Single File: Pure HTML5, Vanilla JavaScript, and Tailwind CSS (via CDN).
• Sandboxed Execution: The logic is 100% in-memory. Zero local storage, zero cookies, zero network requests.
• Responsiveness: Fluid grid system ensuring readability on mobile, tablet, and desktop.

UI/UX Specification

• Theme: Vibrant Light Mode. Use a clean color palette (White backgrounds, slate-700 for text, blue-600 for primary actions).
• Layout:
  • Header: Simple, centered title area.
  • Input: A prominent, multi-line textarea designed for pasting long tokens.
  • Results Section: A split-panel view. The left side displays the decoded 'Header' and 'Payload' (JSON format). The right side displays the signature and raw claims breakdown.
• Micro-interactions:
  • Subtle hover effects on buttons.
  • Smooth fade-in transitions when the result panels appear.
  • Copy-to-clipboard feedback (a temporary "Copied!" tooltip).

Directives for Implementation

• No Persistence: Do not use localStorage or any tracking mechanism. The tool should reset state on reload.
• Security: Ensure all Base64Url decoding logic accounts for standard padding requirements.
• Styling: Use a sans-serif system font stack (e.g., -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto). Avoid dark mode entirely; maintain a high-contrast, clean professional aesthetic.
• Performance: Use requestAnimationFrame for UI updates to ensure the interface remains snappy even with very large tokens.