branded-types

Flippin' great branded types for TypeScript!

Use them like opaque types in Flow, except you get to control visibility.

Simple example

Define a branded type:

import {Branded} from 'branded-types'
export class OrderId extends Branded<string, 'my-package.OrderId'>() {}

Import and use it

import {OrderId} from './other-module';

OrderId.brand('someUuid') === 'someString' // error

Advanced example

Want to hide the branding and un-branding functions?

• Declare your branded type in namespace
• Use the static functions within your module to safely create instances
• Export only the type instead of the entire class

import { Branded } from "branded-types";

namespace my {
  export class OrderId extends Branded<string, 'my-package.OrderId'>() {}
}

export function safeCreateId() {
  return my.OrderId.brand('x')
}

export type OrderId = my.OrderId;

The other module can only use the safe ID creation functions now:

import {safeCreateId, OrderId} from './other-module';

safeCreateId() == 'someString'; // error

let m: Map<OrderId, string> = new Map;

m.set(safeCreateId(),  '1');
m.set('someString', '2'); // error

Features

• Not nominal. The same package in different places in the filesystem will still be compatible with other instances (unless you bump the brand name - you control version compatibility)
• Nice error messages e.g. the equality comparison error looks like this:

  This condition will always return 'false' since
  the types 'string' and 'OrderId' have no overlap.

  and the incorrect map insertion above produces the error:

  Argument of type 'string' is not assignable to parameter of type 'OrderId'.

• No runtime overhead. Despite all the class machinery, the converted values have NO runtime representation. The conversion functions do nothing except return the same value. The machinery is there for the nice errors and API only.
• No built in string / number operators allowed. If for example you try to concat two OrderIds with +, its not going to work. You have to use an explicit cast to the source type via e.g. String(brandedVal) or Number(brandedVal) etc, then perform the operation