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API Testing Tool (Postman-lite) ​

DifficultyIntermediate
Team SizeSolo
Time~25-30 hours
Demo-ready byStep 6
PrerequisitesHTML, CSS, JavaScript, HTTP fundamentals
Built byPostman, Insomnia, Hoppscotch, Thunder Client

Skills you'll earn: HTTP methods, request/response handling, environment variables, code generation, collection management

Start with a fetch call. End with a full-featured API workbench.

(Assumes basic HTML/CSS/JS and fetch API knowledge)

Step 1: Send a GET request and show the response (~30 min) ​

One input field for a URL. One button. Click it, fetch() the URL, display the response body in a <pre> tag.

  • One HTML file
  • fetch(url).then(r => r.text()).then(text => pre.textContent = text)

You now have: The simplest API client.

Step 2: Show response details (~1 hour) ​

You got a response but don't know if it was 200 or 500.

  • Display status code, status text
  • Display response headers
  • Show response time (measure before and after fetch)

You now have: Useful response inspection.

Step 3: Choose the HTTP method (~1 hour) ​

You can only GET. APIs need POST, PUT, DELETE, PATCH.

  • Add a dropdown for method selection
  • For methods with a body (POST, PUT, PATCH), add a textarea for the request body
  • Set Content-Type: application/json when sending JSON

You now have: Full HTTP method support.

Step 4: Custom headers (~1 hour) ​

Some APIs need Authorization, X-API-Key, or custom headers.

  • Add a key-value input for headers
  • "Add header" button to add more rows
  • Include all headers in the fetch call

You now have: Complete request control.

Step 5: Pretty-print responses (~1-2 hours) ​

Raw JSON is hard to read.

  • Detect JSON responses and format them with JSON.stringify(data, null, 2)
  • Add syntax highlighting (highlight.js or Prism)
  • Tab between "Pretty", "Raw", and "Headers" views

You now have: A readable response viewer.

Step 6: Save requests to collections (~2-3 hours) ​

You keep retyping the same URL.

  • Save requests to localStorage: name, method, URL, headers, body
  • Show a sidebar with saved requests
  • Click to load a saved request into the editor
  • Organize into folders/collections

You now have: Reusable request collections.

Step 7: Environment variables (~2 hours) ​

Your API URL is different in dev vs production.

  • Define environments: { "base_url": "http://localhost:3000" }, { "base_url": "https://api.example.com" }
  • Use in URLs and headers
  • Replace variables before sending
  • Switch environments from a dropdown

You now have: Environment management.

Step 8: Authentication helpers (~2-3 hours) ​

Manually copying tokens is tedious.

  • Add auth presets: Bearer token, Basic auth, API key
  • For OAuth2: build a flow that gets and refreshes tokens
  • Auto-attach auth headers to requests

Step 9: Response history (~2 hours) ​

  • Log every request/response with timestamp
  • Browse history, re-run old requests
  • Compare responses over time

Step 10: Request chaining (~3 hours) ​

Your login endpoint returns a token. You want to use it in the next request automatically.

  • Extract values from responses using JSONPath or dot notation
  • Set variables from response: token = response.body.data.token
  • Reference in subsequent requests: Authorization: Bearer

Step 11: Code generation (~2 hours) ​

  • Generate curl, fetch, axios, requests (Python) code from the current request
  • Copy to clipboard

Step 12: Import/export (~2-3 hours) ​

  • Export collections as JSON
  • Import from Postman (parse their collection format)
  • Share collections with teammates via file

Step 13: Make it look good (~3 hours) ​

  • Split pane: request on the left, response on the right
  • Tabbed interface for multiple open requests
  • Keyboard shortcuts (Ctrl+Enter to send)

Step 14: Deploy (~2-3 hours) ​

  • Containerize and deploy to the cluster
  • Add user accounts so collections sync across devices
  • Store collections in a database

Useful Resources ​

Where to go from here ​

  • WebSocket testing
  • GraphQL support
  • Mock servers
  • Automated test assertions (expect status 200, expect body contains "success")
  • CI integration (run collections from CLI)