opaque type

opaque.ts

type Brand <T> = T & { readonly type: unique symbol }

// type Foo = Brand<number>
type Foo = number & { readonly type: unique symbol }
type Bar = number & { readonly type: unique symbol }

function Foo (n: number): Foo
{
    return (n as Foo)
}

function Bar (n: number): Bar
{
    return (n as Bar)
}

var x: number = 1
var f: Foo = Foo(1)
var b: Bar = Bar(2)

x = 3
f = 3
b = 3

x = Foo(3)
f = Foo(3)
b = Foo(3)

x = Bar(3)
f = Bar(3)
b = Bar(3)

console.log(x == f)
console.log(f == b)
console.log(x == b)

https://typescript-play.js.org/#code/FAFwngDgpgBAQgJwIYDsAmMA8AVAfDAXhmxgDIYBvGBKJNAexQBswZxoAuGAVxQEsAjt1gBnMAFsARvSYwAvsGAB6JW0iwAYvXqF4ydJhTcpUBLlDqYWnUSMmEZStVoNmrdlC69BwmGKky8hbQ8EgOtsaSpo5UNHSMLGqcPPxCohLSsgrAAGa8AMYgfIxW2jAAFChcdlEIAJRc1sAUwDBtziDcCCgVPUgipfR1wNl5KIXFPXBhvdWRpg2hCM2t7TSd3b0w-UvD2QBuMwAec-a6AIzAhw45jWVE1uXnw9cwklzT4UvlAEzDwEddABmXLA4CSMEA3SPIHDHLQ7TlWHghH0JH-QFET7o0FYsI4iF4hA44D5RgiGRQAB0THoAHNypiiDlhmSUBSmNTaQz4QQiJJWeTKTT6YzCPzhkA