Simple CORS middleware for Go as a http.Handler

cors_middleware.go

func HandlePreFlight(h http.HandlerFunc, methods ...string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        methods = append(methods, http.MethodOptions)
        w.Header().Set("Content-Type", "application/json")
        w.Header().Set("Access-Control-Allow-Credentials", "true")
        w.Header().Set("Access-Control-Allow-Origin", config.APIAccessControlAllowOriginDomain)
        w.Header().Set("Access-Control-Allow-Methods", strings.Join(methods, ", "))
        w.Header().Set("Access-Control-Allow-Headers", "Accept, Accept-Endcoding, Content-Type, Content-Length, Authorization, X-CSRF-token")
        w.Header().Set("Access-Control-Expose-Headers", "Session-Token")
    
        if r.Method == http.MethodOptions {
            return
        }
    
        // Pass on the request.
        h(w, r)
    }
}