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Classy Type-Wrapper for Flow (npm)

A tiny module that gives you the TypeWrapper class + static methods bundle, for quick definition of type wrappers.

What are Type Wrappers?

A way of declaring a new type, that is simply a wrapper around another type, to make mix-ups easier to prevent. For example, declaring a wrapper around string called UserID, to stop User IDs from being mixed up with other strings, like usernames or descriptions.

This is elaborated on in this blog post.


// @flow 
import { TypeWrapper } from 'flow-classy-type-wrapper';
// Declare a new type wrapper: 
class UserID extends TypeWrapper<string> {}
// String -> UserID-wrapped String 
const id = UserID.wrap("123-abcd");
// $ExpectError - Can't use like a string without unwrapping. 
const wrong = "Hello, user #" + id;
function greet(userId: UserID, name: string): string {
  // UserID-wrapped String -> String 
  const userIdAsString = UserID.unwrap(userId)
  // Unwrapped; fine to use now. 
  return "User #" + userIdAsString + "" + name;
console.log(greet(id, "Bert"));

See the examples/ directory for more.

Inspired by @mkscrg's original "class bundle" pattern from this Flow issue.