Conditional type of `get` util

[haughty shell]

I would like to be able to have a simple get function defined as: const get = (key, obj) => obj[key]; I'm struggling to annotate it properly so that it properly infers that get("x", {"x": 2}) is a number. So far I have:

function get<K extends PropertyKey, T extends object>(key: K, obj: T): K extends keyof T ? T[K] : undefined {
    const hasKey = (obj: T, key: PropertyKey): key is keyof T => key in obj;
    if (hasKey(obj, key)) return obj[key];
    return undefined;
}

But I have 2 errors:

1. Type 'T[K & keyof T]' is not assignable to type 'K extends keyof T ? T[K] : undefined'.
2. Type 'undefined' is not assignable to type 'K extends keyof T ? T[K] : undefined'

How should I be able to fix them and could you help me to understand this error messages? Here is a link to the playground.

[rapid grail]

I think it's not possible to get this to work without type casting.

See https://github.com/Microsoft/TypeScript/issues/13995#issuecomment-363265172

[wraith dirge]

I had an idea, I'm not sure if it covers every possibility or not

[midnight hill]

webstrand#0

Playground Link

Preview:ts function prop<T extends object, K extends PropertyKey>( key: K, obj: T & {[P in K]?: undefined | T[P & keyof T]} ): typeof obj[K] { return obj[key] } ...

[wraith dirge]

but it at least solves all of the problems you mentioned and every test case I can think of right now

now, if you wanted to return something other than undefined when the key is missing, I don't know that's possible with this trick

[haughty shell]

It is mentioned later in the discussion that the issue you linked to should be fixed by this.

[haughty shell]

Thanks @wraith dirge that does the trick 🙂

!resolved

@wraith dirge any idea why conditional type K extends keyof T ? T[K] : undefined does not work here? 🤔

[wraith dirge]

yeah conditional types involving unresolved generics aren't able to be executed. So in the body of the generic function, they're basically treated as unique types (usually called nominal types around here) that aren't assignable from anything else.

a more intelligent compiler could execute some special cases, but typescript isn't there yet

so my personal rule of thumb is: if the generic function has a conditional return type, expect to be forced to use as assertions or ignore compiler errors.

[haughty shell]

that make sense, thanks!