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Type aliases

@RiccardoOdone•Suggest a tip•

TypeScript employs a structural type system, so type aliases are just aliases.

type Dollars = number
type Euros = number

const twoDollars: Dollars = 2
const pay = (euros: Euros) => {}

pay(twoDollars) // ⛔️

type Dollars = { _kind: 'dollars', amount: number }
type Euros = { _kind: 'euros', amount: number }

const twoDollars: Dollars = { _kind: 'dollars', amount: 2 }
const pay = (euros: Euros) => {}

pay(twoDollars) // ✅ Does not compile
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