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Jason5Lee/opaque-type.ts

Last active February 22, 2020 02:14
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TypeScript's opaque type.
Raw
opaque-type.ts
type USD = number & { readonly __brand: unique symbol }
type EUR = number & { readonly __brand: unique symbol }
type Currency = USD | EUR;
function addCurrency<T extends Currency>(a: T, b: T): T {
return (a + b) as T;
}
addCurrency(5 as USD, 6 as USD);
addCurrency(5 as EUR, 7 as USD); // Compilation error.
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