Typescript fails to identify the parameter type of callbacks

I am facing a challenge with the function below and its callback type:

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

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

}

The function works correctly except for one issue - the onSuccess function has a parameter of type string but TypeScript is unable to recognize it and considers it as any type even though I have explicitly set it to string.

myFunction("eventName", "bobo", 123, {onSuccess: (asd) => {
    // "asd" should be a string but TypeScript treats it as any
    // Parameter 'asd' implicitly has an 'any' type
}})

playground link

What adjustments can I make to enable TypeScript to identify the type of parameters in the callbacks without manually specifying them each time?

P.S. This example is a simplified version of the actual problem I am encountering.

typescript

Answer №1

The current situation in TypeScript presents a missing feature where a function with leading or middle rest elements in its parameter list does not benefit from contextual typing of its input parameters when called. There is an open request to address this issue at microsoft/TypeScript#45972, marked as "awaiting more feedback" as of 2022-07-06. If you believe this change is necessary, showing support by giving it a thumbs up and providing your use case could make a difference.

As a workaround until the feature is implemented, you can define the function t() as a generic function with a type parameter corresponding to the leading rest tuple. Generic types are better supported according to the original pull request at microsoft/TypeScript#41544.

Here's how you can implement it:

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

Once implemented, it will work as intended:

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

Check out the code on the playground

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