Fiber CORS: Not Working for Cross-Origin

In this tutorial, you'll learn about Fiber CORS: Not Working for Cross. We cover key concepts, practical examples, and best practices.

CORS in Fiber -- Enable cross-origin resource sharing in Fiber applications using the built-in CORS middleware.

The Problem

Fiber's CORS middleware blocks cross-origin requests by default. Without AllowOrigins configuration, browsers reject API calls from different origins.

Wrong

app := fiber.New()
app.Get("/api/data", getData)

Output:

// Browser: No 'Access-Control-Allow-Origin' header

Right

import "github.com/gofiber/fiber/v2/middleware/cors"
app.Use(cors.New(cors.Config{
    AllowOrigins: "http://localhost:3000,https://app.example.com",
    AllowMethods: "GET,POST,PUT,DELETE",
    AllowHeaders: "Origin, Content-Type, Authorization",
    AllowCredentials: true,
}))

Output:

// Browser fetch works! CORS headers present

Prevention

• Only allow specific origins in production
• Use AllowCredentials: true for cookies/auth headers
• Preflight OPTIONS requests handled automatically
• Set MaxAge to reduce preflight requests
• Test CORS with curl -H "Origin: http://example.com" -I

Common Mistakes with fiber cors

1. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
2. Non-exhaustive pattern matches that compile with warnings then crash at runtime
3. Misunderstanding that String is [Char] with poor performance for large text operations

These mistakes appear frequently in real-world GO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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