How do you implement JWT authentication middleware in Go?

JWT (JSON Web Token) authentication middleware validates the token on every request, extracts claims, and attaches them to the request context for use by downstream handlers.

import "github.com/golang-jwt/jwt/v5"

type Claims struct {
    UserID int    `json:"user_id"`
    Role   string `json:"role"`
    jwt.RegisteredClaims
}

type contextKey string
const claimsKey contextKey = "claims"

func jwtMiddleware(secret []byte) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            // Extract token from Authorization: Bearer 
            auth := r.Header.Get("Authorization")
            if !strings.HasPrefix(auth, "Bearer ") {
                writeJSON(w, http.StatusUnauthorized,
                    map[string]string{"error": "missing bearer token"})
                return
            }
            tokenString := strings.TrimPrefix(auth, "Bearer ")

            // Parse and validate
            var claims Claims
            token, err := jwt.ParseWithClaims(tokenString, &claims,
                func(token *jwt.Token) (any, error) {
                    if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
                        return nil, fmt.Errorf("unexpected signing method: %v",
                            token.Header["alg"])
                    }
                    return secret, nil
                })

            if err != nil || !token.Valid {
                writeJSON(w, http.StatusUnauthorized,
                    map[string]string{"error": "invalid or expired token"})
                return
            }

            // Attach claims to context
            ctx := context.WithValue(r.Context(), claimsKey, &claims)
            next.ServeHTTP(w, r.WithContext(ctx))
        })
    }
}

// Helper to extract claims in handlers
func claimsFromCtx(ctx context.Context) (*Claims, bool) {
    c, ok := ctx.Value(claimsKey).(*Claims)
    return c, ok
}

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