Typescript does not recognize callback's param type

Question

I have the following function and its callback type:

type Callbacks = {
    onSuccess: (a: string) => void;
};

function t(event: string, ...args: [...any, Callbacks]) {

}

It works as expected but one thing, onSuccess function has a string param but TS can't recognize it and says that it has any type but I explicitly set it to string.

t("eventName", "bobo", 123, {onSuccess: (asd) => {
    // "asd" is a string but TS says that it's an any
    // Parameter 'asd' implicitly has an 'any' type
}})

playground link

What should I change in order to let TS recognize the callback's params type because manually specifying them every time is tedious?

P.S. it's a simplified example of my problem

I would maybe report this, it looks like a bug to me. TS knows it's a string too, as if you change asd:number it will error. I would post an issue here, github.com/microsoft/TypeScript/issues just double check a similar issue doesn't exist first. Handling implicit types in Typescript, is just as important as checking Types, otherwise we get Type Code bloat, (not good).. — Keith
– Keith, Commented Jul 6, 2022 at 11:24
This is a current limitation with contextual typing of function parameters with leading/middle rest elements; it is reported at ms/TS#45972. Until and unless this is changed, the workaround I'd use is to make t generic like this. Does this fully address your question? If so I can write up an answer explaining and elaborating. If not, what am I missing? — jcalz
– jcalz, Commented Jul 6, 2022 at 14:11
@jcalz hello, I've checked the playground and it fully addresses my question, thanks! Feel free to write up an answer and I'll approve it :) — qwerty qwerty
– qwerty qwerty, Commented Jul 6, 2022 at 14:58
As a workaround more appropriate to what the author was looking for, I removed my answer. — Renato
– Renato, Commented Jul 6, 2022 at 15:58

jcalz · Accepted Answer · 2022-07-06 21:10:34Z

This is currently a missing feature in TypeScript. A function whose parameter list has leading or middle rest elements does not seem to benefit from contextual typing of its input parameters when called. There is a request at microsoft/TypeScript#45972 to change this. It's (as of 2022-07-06) marked as "awaiting more feedback", so if you want to see this changed it wouldn't hurt to give it a 👍 and comment with your use case if you think it's compelling. It wouldn't necessarily help, but I wouldn't hurt.

In the meantime, if you want to work around it, you could make t() a generic function in a type parameter corresponding to the leading rest tuple. (The original pull request for leading/middle rest elements at microsoft/TypeScript#41544 mentions that generic types are better supported.)

That would look like

function t<T extends any[]>(event: string, ...args: [...T, Callbacks]) {}

And then it works as desired:

t("hello", "bobo", 123, {
    onSuccess: (asd) => asd.toUpperCase() // asd is seen as string
})

Playground link to code

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