Offline REST API Testing Client Desktop App with Collections, Environment Variables and Response History

Every backend a student builds — Node.js, Django, Flask, Spring Boot, PHP — needs its endpoints tested before any frontend touches them. The usual options are painful: curl commands that cannot be reused, or cloud API tools that demand sign-ups, sync workspace data to someone else's servers, and refuse to work on flaky lab internet. This project takes a different route: a desktop-style API client that runs entirely offline, with everything a real testing workflow needs — a request builder for all five main HTTP methods, collections that persist between sessions, environment variables that switch the same request between dev, staging and production, and a response viewer that shows status code, round-trip time, payload size, highlighted body and headers. Because every byte stays in local storage, the tool is private by design and works anywhere. The included sample workspace (collections against the public JSONPlaceholder test API) means the app demonstrates itself the moment it opens.

1. The student picks a saved request from a collection or composes a new one: method, URL (with {{variables}}), query params, headers and JSON body.
2. The variable resolver substitutes every {{variable}} from the active environment (Development, Staging or Production) before the request leaves the machine.
3. The request engine dispatches the call with the Fetch API under an abort-controller timeout taken from the environment's timeout_ms variable.
4. A profiler records the round-trip: HTTP status code, elapsed milliseconds and payload kilobytes, using high-resolution timers.
5. The response viewer pretty-prints and syntax-highlights the JSON body and lists every response header in a table.
6. The call is appended to the persistent history with its resolved URL, status, timing and timestamp — one click replays any past request.
7. If the network is unreachable, the app serves the matching stored sample response and shows an explicit offline notice rather than an error.

Tech stack:

• HTML5 · CSS3 · vanilla JavaScript (single-file app)
• Fetch API with AbortController timeouts
• Web Storage API (localStorage persistence)
• JSONPlaceholder test API (sample workspace)
• Electron / Tauri wrapper (optional desktop packaging)
• Git

• [Full request builder] Compose GET, POST, PUT, PATCH and DELETE requests with an editable method selector, URL bar, query-param table, headers table and a JSON body editor — the complete anatomy of an HTTP request in one screen.
• [Persistent collections] Save requests into named, collapsible collections (JSONPlaceholder API, Products Demo, Posts & Comments) that survive restarts via local storage — no account, no cloud sync.
• [Environment variables] Development, Staging and Production environments with variables like {{base_url}}, {{api_key}} and {{timeout_ms}}; switching environments re-points every saved request instantly with zero edits.
• [Response inspector] Every response shows a status pill, round-trip time in milliseconds, payload size, a syntax-highlighted JSON body and a full response-headers table.
• [Request history] Each sent call is recorded with method, resolved URL, status, timing and timestamp; clicking any entry reloads its response for comparison.
• [Offline-first by design] Collections, environments and history live in the browser's local storage. When a host is unreachable, the app falls back to its stored sample responses with a clear offline notice instead of failing silently.
• [Sample workspace included] Preloaded requests, three environments and seeded history against the public JSONPlaceholder test API, so the tool is demonstrable immediately after installation.

• Working offline REST API client app (single-file, runs in any modern browser)
• Preloaded sample workspace: 3 collections, 9 saved requests, 3 environments, seeded history
• Full source code with comments explaining the request engine and variable resolver
• Project report PDF (background, HTTP concepts, architecture, methodology)
• PPT presentation for final review
• Viva Q&A preparation document (HTTP methods, status codes, headers, REST vs SOAP, CORS, offline design)