Typescript generic Rest operator with corresponding callback

Question

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.

jetenergy · Accepted Answer · 2022-06-10 21:34:30Z

5

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

answered Jun 10, 2022 at 21:34

jetenergy

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Comments

Etheryte · Accepted Answer · 2022-06-10 21:00:12Z

1

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

answered Jun 10, 2022 at 21:00

Etheryte

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1 Comment

jetenergy Over a year ago

This answer helps a lot but i only have a slight problem with it. A missmatch in argument length is getting properly reported but the implicit argument types is still any. i would love to have them be the type of the arguments implicitly.

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