Modern JavaScript Web Development Cookbook

Opaque types for safer coding

In Flow (and TypeScript as well), types that are structurally the same are considered to be compatible and one can be used instead of the other. Let's consider an example. In Uruguay, there is a national identification card with a DNI code: this is a string that's formed by seven digits, a dash, and a check digit. You could have an application that lets you update people's data:

// Source file: src/opaque_types.jstype dniType = string;type nameType = string;function updateClient(id: number, dni: dniType, name: nameType) {    /*        Talk to some server        Update the DNI and name for the client with given id    */}

What could happen? If you don't define better types, there's nothing preventing you from doing a call such as  ...