Property does not exist on selected type, but available via callback

[edyth933]

🔎 Search Terms

I was thinking this code works like how the addEventLister or on behavior happens. Are there other less complex solutions?

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAKgFhACgJwPZgM5QLxQN4BQUxUAZgJbIbABc+RJjAhsgOYAMd1y5AdqwG4GjYi1YBGLsB78hjAL7CoGCAGNUvACZ1CIkmM5ReAVwC2AIwjI5eqGIBMdc6lQAbCE143iixsDiU2vS2dmwAzFIygkrMbAAsdCYWVt5QiooEmmquLNDqvNRQ0iAAwuZ0ADwA0lAQAB7AEFpYANYQIKiksAgo6BgAfAAUYGhgAHJMphB0VQA0UKrlUMOjGHTwSKsA2lUAugCUOP1QAG6o5JqH2MdnFwQExWWDAOQq+ZrP8yN9RwsaGG4IAA6VyoVgrPpAhz7fb3fKFYoAMWMvFUOCg1VqDSamla7U63U2fSG33Gk2mUDmUFJa0JvUwOwOv10JHIXQhZKmOFwrzUGg+hxZenhgJBYI5GChbHssIUBEUQA

💻 Code

type TheProps = {
    first: {
        arg0: string;
        arg1: string;
    }
    second: {
        arg0: number;
        arg2: boolean;
    }
    third: {
        arg3: string;
        arg4: number;
    }
}

declare const tryCb: <K extends keyof TheProps>(propName: K, cb: (props: TheProps[K]) => void) => void

tryCb('second', props => console.log(props.arg2)) // => no error (although this should also be an error)

const tryFunc = <K extends keyof TheProps>(propName: K, props: TheProps[K]) => {
    if (propName == 'second') {
        console.log(props.arg2) // => error here
    }
}

🙁 Actual behavior

Property does not exist on selected type, but available via callback

🙂 Expected behavior

no error occurs same as using callback

Additional information about the issue

No response

[jcalz]

This is not a bug in TS. The caller chooses the type argument (which is why you can do this when you call your function). The implementation needs to handle any choice the caller makes, including things like unions:

tryFunc(Math.random() < 0.999 ? 'second' : 'first', { arg0: "abc", arg1: "def" }); // no compiler error

Here there is a 99.9% chance you'll see "second" as the first argument, but the second argument has no arg2 property.

I think you need #27808 if you want to be able to narrow type parameters this way. The feature at #56941 for #33014 doesn't address this because TheProps[K] is in an input position.

[edyth933]

so why don't tryCb and tryFunc make the same error or not?

[MartinJohns]

With tryCb the compiler infers "second" for the type argument based on the provided parameters. This is not possible within tryFuc, because within the function you can't know what type is being passed in. That's what jcalz means with "the caller chooses the type argument".