Typescript generic Rest operator with corresponding callback-CodePudding

i have recently picked up Typescript because it seemed interesting. I have used generics in C# before and managed to do quiet a few things with generics in Typescript but now im stumped for the following use case:

I have a generic function that takes in any number of Rest arguments. These arguments are than passed to a callback function which is also provided. But i cannot seem to figure out how to make the typescript compiler accept the types i provide to implicitly be passed into the callback function.

async SampleFunction(
    cb: () => void,
    ...args: any[]
)

with this as sample usage:

let stringVariable = "lorem";
let numberVariable = 8;

SampleFunction(
    (stringArg, numberArg) => {},
    stringVariable,
    numberVariable,
)

This example results in arguments that think they are implicitly type 'any'.

my question hereby is: how can i make this function definition so that i could pass in any aditional arguments which then get their type passed into the callback function as they are passed in the same order.

CodePudding user response：

You can use the Parameters<T> utility type. The (...args: any[]) => any type in the extends allows any function to be passed as the callback regardless of its signature, and infers the rest from the passed function.

function sampleFunction<T extends (...args: any[]) => any>(
  callback: T,
  ...args: Parameters<T>
) {}

sampleFunction((a: string, b: number) => null, "foo"); // Doesn't pass
sampleFunction((a: string, b: number) => null, "foo", "bar"); // Doesn't pass
sampleFunction((a: string, b: number) => null, "foo", 42); // Passes
sampleFunction((a: string, b: number) => null, 42, "bar"); // Doesn't pass

CodePudding user response：

Thanks to the help of @Etheryte i got onto the right search track.

I found the sollution i wanted which is:

function SampleFunction<T extends any[]>(
    cb: (...args: T) => void,
    ...args: T
): void {}

let stringVariable = 'lorem';
let numberVariable = 8;

SampleFunction(
    (a, b) => {}, 
    stringVariable, 
    numberVariable
);

This implicitly passes the right argument types from the function into the callback