Private properties make class instance types incompatible

[hollow laurel]

Can someone help me understand why the RedirectRequest class cannot be used in place of UrlBuilder class in the following example?

https://www.typescriptlang.org/play?#code/MYGwhgzhAECqBOIBCBXAliAJgU3tA3gFDQnQDEADmPGALYQBc0YAdgJ7GlU30AUAlAU6lS8bABcU8FsOgBfQgsKhIMAErZMaMcHEaAjimwRxQkWQjAA9hWNNWHEdzoQBZkaIlSZIpSMs2xm5EHp6S0tDiABZoEAB0Fta2ELIKSipQ0ACyYADW2Agg7iTWLCbwKLpW8LwARuhYuEyFqBg48IIhpGmEhCzYAO7ZeQWIvP1DGlo6etiGxuIC-EA

If I get it right, the public structure of both the classes is same and the private properties should not matter

[cursive pebble]

virk#0

Playground Link

Preview:```ts
class UrlBuilder {
#params: any
params() {
return
}
}

class RedirectRequest {
#scopes: any
params() {
return
}

scopes() {
return this.#scopes
}
}

class MakeUrl {
constructor(builder: UrlBuild
...```

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[finite kelp]

It works if you make RedirectRequest extend UrlBuilder

[hollow laurel]

Yeah, but why not otherwise? Following structural typing, the public structure of both the classes is same

[obtuse stump]

Yeah not sure why.

You can make both classes implement one interface, and take that interface as argument instead.

[hybrid moth]

@hollow laurel One reason it works this way is because the class could be passed somewhere that does have access to the private properties

Contrived example, but:

class UrlBuilder {
    #params: any
    params() {
        return
    }
    static doThing(builder: UrlBuilder) {
        console.log(builder.#params)
    }
}

class MakeUrl {
    constructor(builder: UrlBuilder) {
        UrlBuilder.doThing(builder)
    }
}

If MakeUrl accepts RedirectRequest there, then doThing breaks

Also: https://github.com/microsoft/TypeScript/issues/7755#issuecomment-204161372

[hollow laurel]

Okay, I think I get it