# REST API Testing Guide 2026: Master cURL, Postman & Axios ## Why REST API Testing Matters in 2026 **Broken APIs cost businesses millions.** Stripe lost $500K+ in one hour due to API failures. GitHub experienced cascading failures from bad endpoints. In 2026, API testing isn't optional—it's survival. This guide covers essential REST API testing techniques, authentication strategies, debugging methods, and production-ready workflows for developers. --- ## Understanding REST API Basics ### The 5 Essential HTTP Methods

Method	Purpose	Idempotent	Safe	Body Required
GET	Retrieve data	✅	✅	❌
POST	Create resource	❌	❌	✅
PUT	Replace resource	✅	❌	✅
PATCH	Partial update	❌	❌	✅
DELETE	Remove resource	✅	❌	❌

**Key Terms:** - **Idempotent**: Same request multiple times = same result - **Safe**: Doesn't modify server state (read-only) --- ## Method 1: cURL - Command Line Testing ### Basic Requests ```bash # GET request curl https://api.example.com/users # GET with query parameters curl "https://api.example.com/users?role=admin&status=active" # POST with JSON body curl -X POST https://api.example.com/users \ -H "Content-Type: application/json" \ -d '{"name":"Alice","email":"alice@example.com"}' # Bearer token authentication curl https://api.example.com/users/me \ -H "Authorization: Bearer YOUR_JWT_TOKEN" ``` 💡 **Pro tip**: Decode JWT tokens before making requests with [JWT Token Decoder](https://devconverter.dev/jwt-decoder) to inspect expiration and claims. ### Advanced cURL Options ```bash # Show response headers curl -i https://api.example.com/health # Detailed debugging curl -v https://api.example.com/users # Follow redirects curl -L https://api.example.com/redirect # Save response to file curl https://api.example.com/data -o output.json # Upload file curl -X POST https://api.example.com/upload \ -F "file=@document.pdf" ``` --- ## Method 2: Postman - Visual API Client ### Why Postman? - Visual interface for building requests - Collections for organizing tests - Environment variables (dev, staging, prod) - Automated testing with JavaScript - Mock servers and documentation generation ### Example: Login Endpoint Test ```javascript // Postman test script pm.test("Login successful", function () { pm.response.to.have.status(200) }) const response = pm.response.json() pm.test("Response has token", function () { pm.expect(response).to.have.property("token") }) // Save token for next requests pm.environment.set("auth_token", response.token) ``` 💡 Use [JWT Decoder](https://devconverter.dev/jwt-decoder) to inspect auth tokens and verify expiration times. ### Environment Variables ```json // Dev Environment { "base_url": "https://dev-api.example.com", "api_key": "dev_key_123" } // Usage in requests GET {{base_url}}/users X-API-Key: {{api_key}} ``` --- ## Method 3: Axios - JavaScript/Node.js Testing ### Basic Implementation ```javascript const axios = require("axios") // GET request async function getUsers() { try { const response = await axios.get("https://api.example.com/users") console.log(response.data) } catch (error) { console.error("Error:", error.response?.status) } } // POST with authentication async function createUser() { const response = await axios.post( "https://api.example.com/users", { name: "Bob", email: "bob@example.com" }, { headers: { Authorization: "Bearer YOUR_TOKEN", "Content-Type": "application/json", }, } ) return response.data } ``` ### Configured Axios Instance ```javascript const api = axios.create({ baseURL: "https://api.example.com", timeout: 5000, headers: { Authorization: `Bearer ${process.env.API_TOKEN}`, "Content-Type": "application/json", }, }) // Request interceptor api.interceptors.request.use(config => { console.log(`${config.method.toUpperCase()} ${config.url}`) return config }) // Error handling interceptor api.interceptors.response.use( response => response, error => { if (error.response?.status === 401) { console.error("Token expired!") } return Promise.reject(error) } ) ``` --- ## Authentication Strategies ### 1. Bearer Token (JWT, OAuth2) ```bash # cURL curl https://api.example.com/protected \ -H "Authorization: Bearer eyJhbGc..." ``` ```javascript // Axios const response = await axios.get("https://api.example.com/protected", { headers: { Authorization: `Bearer ${token}` }, }) ``` **Common Issues:** - Token expired (check `exp` claim) - Missing "Bearer" prefix - Wrong token type 💡 **Debug tokens**: [JWT Token Decoder](https://devconverter.dev/jwt-decoder) shows expiration in readable format. ### 2. API Key Authentication ```bash # Header method curl https://api.example.com/data \ -H "X-API-Key: abc123" # Query parameter curl "https://api.example.com/data?api_key=abc123" ``` ### 3. Basic Authentication ```bash # Using -u flag curl -u username:password https://api.example.com/data # Manually encoded curl https://api.example.com/data \ -H "Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ=" ``` 💡 **Encode credentials**: [Base64 Encoder/Decoder](https://devconverter.dev/base64-encode-decode) for safe encoding. ### 4. OAuth 2.0 Flow ```bash # Step 1: Get token curl -X POST https://oauth.example.com/token \ -H "Content-Type: application/x-www-form-urlencoded" \ -d "grant_type=client_credentials" \ -d "client_id=YOUR_ID" \ -d "client_secret=YOUR_SECRET" # Step 2: Use token curl https://api.example.com/data \ -H "Authorization: Bearer ACCESS_TOKEN" ``` --- ## Debugging Failed Requests ### HTTP Status Codes Quick Reference

Code	Meaning	Fix
400	Bad Request	Check JSON syntax
401	Unauthorized	Verify token
403	Forbidden	Check permissions
404	Not Found	Verify URL
429	Rate Limited	Implement retry
500	Server Error	Check logs
503	Unavailable	Retry later

### Debugging 401 Unauthorized ```javascript // Check token expiration const token = 'eyJhbGc...'; const payload = JSON.parse(atob(token.split('.')[1])); const now = Math.floor(Date.now() / 1000); if (payload.exp < now) { console.error(`Token expired ${now - payload.exp} seconds ago`); } // Common mistakes // ❌ Wrong headers: { 'Authorization': token } // ✅ Correct headers: { 'Authorization': `Bearer ${token}` } ``` 💡 **Decode visually**: [JWT Decoder Tool](https://devconverter.dev/jwt-decoder) for readable format. ### Debugging 400 Bad Request ```javascript // ❌ Wrong: String instead of object axios.post("/users", '{"name":"Alice"}') // ✅ Correct axios.post("/users", { name: "Alice" }) // Validate JSON first try { const data = JSON.parse(jsonString) await axios.post("/users", data) } catch (error) { console.error("Invalid JSON:", error.message) } ``` 💡 **Validate JSON**: [JSON Formatter](https://devconverter.dev/json-formatter) catches syntax errors. --- ## Production Best Practices ### 1. Use Environment Variables ```javascript // ❌ Never hardcode const API_KEY = 'abc123'; // ✅ Use env vars const API_KEY = process.env.API_KEY; if (!process.env.API_KEY) { throw new Error('API_KEY required'); } ``` ### 2. Implement Retry Logic ```javascript async function apiRequestWithRetry(url, maxRetries = 3) { for (let attempt = 1; attempt <= maxRetries; attempt++) { try { return await axios.get(url) } catch (error) { const status = error.response?.status // Don't retry 4xx errors if (status >= 400 && status < 500) throw error if (attempt < maxRetries) { const delay = Math.min(1000 * Math.pow(2, attempt), 10000) console.log(`Retry in ${delay}ms...`) await new Promise(resolve => setTimeout(resolve, delay)) } else { throw error } } } } ``` ### 3. Set Timeouts ```javascript const api = axios.create({ baseURL: "https://api.example.com", timeout: 5000, headers: { "Content-Type": "application/json" }, }) // Different timeouts for operations api.get("/health", { timeout: 2000 }) // Quick check api.get("/report", { timeout: 30000 }) // Long operation ``` ### 4. Validate Responses ```javascript function validateUserResponse(data) { const required = ["id", "email", "name"] for (const field of required) { if (!(field in data)) { throw new Error(`Missing: ${field}`) } } if (!data.email.includes("@")) { throw new Error("Invalid email") } return data } ``` --- ## Security Best Practices ### 1. Never Log Secrets ```javascript // ❌ Never console.log("Token:", token) // ✅ Sanitize function sanitize(obj) { const sensitive = ["password", "token", "apiKey"] const safe = { ...obj } for (const key in safe) { if (sensitive.some(s => key.toLowerCase().includes(s))) { safe[key] = "[REDACTED]" } } return safe } ``` ### 2. Always Use HTTPS ```javascript // ❌ Insecure axios.get("http://api.example.com/users") // ✅ Secure axios.get("https://api.example.com/users") ``` ### 3. Test Authorization ```javascript // Test what users CANNOT do try { await api.delete("/admin/users/123") console.error("❌ Security issue!") } catch (error) { if (error.response?.status === 403) { console.log("✅ Auth enforced") } } ``` --- ## API Testing Checklist - ☑ Health endpoint tested - ☑ Auth success + failure tested - ☑ CRUD operations validated - ☑ Error responses verified - ☑ Rate limits tested - ☑ Timeouts configured - ☑ Secrets in env vars - ☑ Logs sanitized - ☑ HTTPS enforced - ☑ CI tests automated --- ## Common Mistakes to Avoid ❌ Only testing 200 responses ❌ Hardcoding tokens ❌ Ignoring error responses ❌ Not validating schemas ❌ Logging secrets ✅ Test failures ✅ Automate critical flows ✅ Validate auth & permissions ✅ Use proper tooling --- ## Essential Developer Tools When testing REST APIs, use these tools to debug faster: **[JWT Token Decoder](https://devconverter.dev/jwt-decoder)** - Decode and inspect JWT tokens, check expiration, verify claims **[JSON Formatter & Validator](https://devconverter.dev/json-formatter)** - Format and validate JSON before sending requests **[Base64 Encoder/Decoder](https://devconverter.dev/base64-encode-decode)** - Encode credentials for Basic Auth **[URL Encoder/Decoder](https://devconverter.dev/url-encode-decode)** - Handle URL query parameters correctly **[Hash Generator](https://devconverter.dev/hash-generator)** - Generate API signatures and checksums --- ## Quick Summary ✅ REST APIs power modern applications ✅ Test with cURL, Postman, and Axios ✅ Master authentication (Bearer, API Key, OAuth) ✅ Debug with proper tools ✅ Always test auth, errors, and rate limits ✅ Automate API tests in CI/CD ✅ Use developer tools for debugging **Master REST API testing in 2026 and ship with confidence.** Visit [DevConverter.dev](https://devconverter.dev) for more developer tools and guides.

Posts tagged with #curl

REST API Testing in 2026: Complete Guide for Developers (cURL, Postman, Axios)