TypeScript: Two variables with unknown type, but the same

Question

Imagine I have a sorting function with

type sortingType = 
    string | number | boolean | { [key: string]: unknown }; // basically unknown

const mySortFun = (a: sortingType, b: sortingType) => { /* do the thing */ };

But I know, that if a is type X, then b is also type X. How can I tell typeScript?

Drag13 · Accepted Answer · 2021-12-21 09:28:39Z

2

You need to somehow bind a and b variables together, like this:

type SortingType =
    string | number | boolean | { [key: string]: unknown }; // basically unknown

type Func = <T extends SortingType>(a: T, b: T) => void;

const mySortFun: Func = (a, b) => { /* do the thing */ };
mySortFun(1, 2)

I introduced new type type Func = <T extends SortingType>(a: T, b: T) => void; where a and b have the same generic type T. Now they are coupled together.

If you don't want to have extra type, the option from @captain-yossarian will be really useful:

const sort = <T extends SortingType>(a: T, b: T) => {}

edited Dec 21, 2021 at 9:28

answered Dec 21, 2021 at 9:23

Drag13

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3 Comments

captain-yossarian from Ukraine Over a year ago

Or you can do const sort = <T extends SortingType>(a: T, b: T) => {}

Drag13 Over a year ago

Your options is much easier to read, don't you mind if I add it to the answer?

captain-yossarian from Ukraine Over a year ago

sure, this is why I left a comment and upvoted your answer

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