/**
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Create an opaque type, which hides its internal details from the public, and can only be created by being used explicitly.
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The generic type parameter can be anything. It doesn't have to be an object.
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[Read more about opaque types.](https://codemix.com/opaque-types-in-javascript/)
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There have been several discussions about adding this feature to TypeScript via the `opaque type` operator, similar to how Flow does it. Unfortunately, nothing has (yet) moved forward:
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- [Microsoft/TypeScript#15408](https://github.com/Microsoft/TypeScript/issues/15408)
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- [Microsoft/TypeScript#15807](https://github.com/Microsoft/TypeScript/issues/15807)
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@example
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```
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import {Opaque} from 'type-fest';
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type AccountNumber = Opaque<number, 'AccountNumber'>;
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type AccountBalance = Opaque<number, 'AccountBalance'>;
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// The Token parameter allows the compiler to differentiate between types, whereas "unknown" will not. For example, consider the following structures:
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type ThingOne = Opaque<string>;
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type ThingTwo = Opaque<string>;
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// To the compiler, these types are allowed to be cast to each other as they have the same underlying type. They are both `string & { __opaque__: unknown }`.
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// To avoid this behaviour, you would instead pass the "Token" parameter, like so.
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type NewThingOne = Opaque<string, 'ThingOne'>;
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type NewThingTwo = Opaque<string, 'ThingTwo'>;
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// Now they're completely separate types, so the following will fail to compile.
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function createNewThingOne (): NewThingOne {
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// As you can see, casting from a string is still allowed. However, you may not cast NewThingOne to NewThingTwo, and vice versa.
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return 'new thing one' as NewThingOne;
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}
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// This will fail to compile, as they are fundamentally different types.
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const thingTwo = createNewThingOne() as NewThingTwo;
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// Here's another example of opaque typing.
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function createAccountNumber(): AccountNumber {
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return 2 as AccountNumber;
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}
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function getMoneyForAccount(accountNumber: AccountNumber): AccountBalance {
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return 4 as AccountBalance;
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}
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// This will compile successfully.
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getMoneyForAccount(createAccountNumber());
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// But this won't, because it has to be explicitly passed as an `AccountNumber` type.
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getMoneyForAccount(2);
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// You can use opaque values like they aren't opaque too.
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const accountNumber = createAccountNumber();
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// This will not compile successfully.
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const newAccountNumber = accountNumber + 2;
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// As a side note, you can (and should) use recursive types for your opaque types to make them stronger and hopefully easier to type.
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type Person = {
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id: Opaque<number, Person>;
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name: string;
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};
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```
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*/
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export type Opaque<Type, Token = unknown> = Type & {readonly __opaque__: Token};
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