[00:00] In this exercise, we're declaring a PromiseFunc type. And PromiseFunc is kind of like a reusable type that we've used in a bunch of different places in this imaginary application. But we want to modify it to make it a generic type. That's because currently PromiseFunc just takes in a single input as the parameter, and this is currently typed as any.

[00:19] And then it returns PromiseAny here, okay? And so this isn't great, because we want to be able to say, okay, our first example is a PromiseFunc that takes in first argument of string and returns a string. And we're expecting example one to be of that type, but it's not. And we're getting an error here saying,

[00:38] type PromiseFunc is not generic. Ha, interesting. Your job is to try to work out how to make PromiseFunc work as a generic type. Good luck.


Here we have the `PromiseFunc` that you may recognize from previous exercises:

```tsx
type PromiseFunc = (input: any) => Promise<any>;
```

Currently `PromiseFunc` takes in a single input as the parameter, which is typed as `any`. Additionally, it returns a `Promise<any>`.

However, this isn't ideal because we want to be able to define a `PromiseFunc` that takes in a specific type as its first argument and returns a specific type.

As seen in the tests, we might want a `PromiseFunc` that takes in a `string` and returns a `string` or a `PromiseFunc` that takes in a `boolean` and returns a `number`.

However, when we try to do this we get an error due to the `any` typing:

```tsx
type Example1 = PromiseFunc<string, string>; // red squiggly line under PromiseFunc<>

type test1 = Expect<Equal<Example1, (input: string) => Promise<string>>>;

type Example2 = PromiseFunc<boolean, number>; // red squiggly line under PromiseFunc<>

type test2 = Expect<Equal<Example2, (input: boolean) => Promise<number>>>;
```

## Challenge

Your task is to transform `PromiseFunc` into a generic type that takes in two type parameters.

You'll know you've succeeded when the errors go away and the tests pass.


Understand how to specify multiple parameters for a generic function.

[00:00] Okay, so we know that we need PromiseFunc as a type to be generic here. Now, the way that we need to do that is we need to basically say PromiseFunc now takes in some type parameters. You might think, and you might have seen this syntax before if you vaguely looked at this, that you might need to put the syntax here.

[00:18] So you might say we need to say T input, T output here. But this doesn't seem to be making any difference because type PromiseFunc is not generic. This actually makes the declared function generic. And so these type parameters actually get applied to the function, but we need to put them before the equal sign.

[00:39] So we end up like this. We have PromiseFunc and we say T input. And now as soon as we've declared T input here, it's going to instead yell at us for a different reason. So it is a generic type now, but now it just requires one type argument instead of two. So we need T input and T output. Let's have a look at this.

[00:58] OK, great. So now it stopped yelling at us here, but now the example is actually input any, promise any. We're not using T input and T output. And of course, I can call these anything. I can call these T mat is great or something, you know, like it doesn't matter. These are just like variable names.

[01:15] So T input, we're going to put that here because we know that T input, anything passed into the T input type parameter, that then needs to be put as the input. So now we can see that input string is working, but it's still returning promise any. So let's take the T output and stick it in the result of the promise. And now everything's working.

[01:36] So we've got input string returns promise string. And inside here, we've got input Boolean returns promise number. Beautiful. So the lesson here is that you can have multiple type parameters, call them different things, and they're all going to be required by default.


The first thing we need to do is make `PromiseFunc` generic by adding type parameters to it. We do this by adding the angle brackets and choosing what to call the type parameters.

In this case, since we want it to have two parameters we call them `TInput` and `TOutput` and separate them with a comma:

```tsx
type PromiseFunc<TInput, TOutput> = (input: any) => Promise<any>;
```

Next, we need to replace the `any` types with the type parameters.

In this case, the `input` will use the `TInput` type, and the `Promise` will use the `TOutput` type:

```tsx
type PromiseFunc<TInput, TOutput> = (input: TInput) => Promise<TOutput>;
```

With these changes in place, the errors go away and the tests pass.

The `PromiseFunc` is now a fully generic type. Any type passed as `TInput` will serve as the input type, and any type passed as `TOutput` will act as the output type of the Promise.


Modify a generic type to accept multiple type arguments.

Add Multiple Type Arguments to a Generic Type

Generics with Multiple Type Parameters

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Should TypeScript Block Your Dev Server?

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Generics with Multiple Type Parameters

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