Typescript: Obtain the type parameters of a function with all declarations

Question

I'm trying to get an array of type parameters of a function with all the declarations (Overloads) with or concatenation.

For example, I can use the Parameters<T> but this function only get the last declaration.

interface FOO {
    foo(a: string, b: string): string;
    foo(a: boolean, b: string): string;
    foo(a: boolean, b:string, c: number): string;
};

type params = Parameters<FOO["foo"]>; // [boolean, string, number]

Actually, the params type is [boolean, string, number] but I expect the type as [string, string] | [boolean, string] | [boolean, string, number].

hackape · Accepted Answer · 2019-04-23T17:14:43.080

2

TS doesn't support infer union param type from overloaded function. The way TS function overload works is like switch-case, it eventually collapses to one of the signatures. Check this section:

When inferring from a type with multiple call signatures (such as the type of an overloaded function), inferences are made from the last signature (which, presumably, is the most permissive catch-all case). It is not possible to perform overload resolution based on a list of argument types.

If such params type is important to you, you could add:

interface FOO {
    foo(a: string, b: string): string;
    foo(a: boolean, b: string): string;
    foo(a: boolean, b: string, c: number): string;
    foo(...args: [string, string] | [boolean, string] | [boolean, string, number]): string;
};

Edit: OK, I was wrong. turns out Titian the TS wizard always finds a way, even when it's officially claimed "not possible". Be sure to checkout his brilliant answer.

edited Apr 23 '19 at 17:14

answered Apr 23 '19 at 15:58

hackape

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It can be done with some caveats.. I have an answer already but can't seem to find it :-( – Titian Cernicova-Dragomir Apr 23 '19 at 16:15
@TitianCernicova-Dragomir Do show us some wizardry!! – hackape Apr 23 '19 at 16:52
@TitianCernicova-Dragomir guess you mean [this answer](https://stackoverflow.com/questions/51650979/type-infererence-with-overloaded-functions/51654917#51654917) – hackape Apr 23 '19 at 17:05
Thank you for digging up that answer, and thank you for the upvote, that upvote just earned be my legendary badge ! :D – Titian Cernicova-Dragomir Apr 23 '19 at 18:05
1

^ wooo, congrats! I just got to 3000 rep today – hackape Apr 23 '19 at 18:26

Typescript: Obtain the type parameters of a function with all declarations

1 Answers1