How to get argument types from function in Typescript [duplicate]

Question

I may have missed something in the docs, but I can't find any way in typescript to get the types of the parameters in a function. That is, I've got a function

function test(a: string, b: number) {     console.log(a);     console.log(b) } 

I want access to the types string and number, likely as a tuple.

I know I can get the type of the function itself, as typeof test, or the return type via ReturnType<test>.

When I tried keyof typeof test, it returned never, which I also couldn't explain.

Other answers like this one point to extends, but I don't really understand how that works and don't give me an easy way to access the set-of-all-params as a type.

Answer 1

Typescript now comes with a predefined Parameters<F> type alias in the standard library which is almost the same as ArgumentTypes<> below, so you can just use that instead of creating your own type alias.

type TestParams = Parameters<(a: string, b: number) => void> // [string, number] 

Then to get for example the second parameter's type you can use the numeric indexing operator:

type SecondParam = TestParams[1] // number 

Original answer:

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Yes, now that TypeScript 3.0 has introduced tuples in rest/spread positions, you can create a conditional type to do this:

type ArgumentTypes<F extends Function> = F extends (...args: infer A) => any ? A : never; 

Let's see if it works:

type TestArguments = ArgumentTypes<typeof test>; // [string, number] 

Looks good. Note that these beefed-up tuples also capture things like optional parameters and rest parameters:

declare function optionalParams(a: string, b?: number, c?: boolean): void; type OptionalParamsArgs = ArgumentTypes<typeof optionalParams>;  // [string, (number | undefined)?, (boolean | undefined)?]  declare function restParams(a: string, b: number, ...c: boolean[]): void; type RestParamsArgs = ArgumentTypes<typeof restParams>; // [string, number, ...boolean[]] 

Hope that helps. Good luck!