Here we have a`makeInfiniteScroll` function that is similar to a hook in React or another framework:

```typescript
const makeInfiniteScroll = (params: unknown) => {
  const data = params.initialRows || []

  const scroll = async () => {
    const rows = await params.fetchRows()
    data.push(...rows)
  }

  return {
    scroll,
    getRows: () => data,
  }
}
```

In our tests is a `table` constant that calls `makeInfiniteScroll` and passes in an object with a `key` of `"id"` as well as a `fetchRows` function.

The `fetchRows` function returns a `Promise.resolve` with an array containing a single object with an `id` of `1` and a `name` of `"John"`:

```typescript
it("Should fetch more data when scrolling", async () => {
  const table = makeInfiniteScroll({
    key: "id",
    fetchRows: () => Promise.resolve([{ id: 1, name: "John" }]),
  })

  await table.scroll()

  await table.scroll()

  expect(table.getRows()).toEqual([
    { id: 1, name: "John" },
    { id: 1, name: "John" },
  ])
})
```

If `fetchRows` does not include a property that was set by `key`, an error will occur. For example, if `fetchRows` returned `{ name: "John" }`, we would get an error.

## Challenge

Your challenge is to make `makeInfiniteScroll` generic, and to update it to also accept an `initialRows` property that is the same type as the `fetchRows`.

This will allow for the inference of `rows` as seen in the test:

```typescript
it("Should allow you to pass initialRows", () => {
  const { getRows } = makeInfiniteScroll({
    key: "id",
    initialRows: [
      {
        id: 1,
        name: "John",
      },
    ],
    fetchRows: () => Promise.resolve([]),
  })

  const rows = getRows()

  expect(rows).toEqual([
    {
      id: 1,
      name: "John",
    },
  ])

  type tests = [Expect<Equal<typeof rows, Array<{ id: number; name: string }>>>]
})
```

Refactor a function to handle generic data, enforce property existence, and ensure correct type inference with initial data.

The first thing we need to add is a type argument for `makeInfiniteScroll`.

We'll start by scaffolding a type called `MakeInfiniteScrollParams` that will be an object containing `initialRows`, and `fetchRows`.

For now we'll set `initialRows` to be an array of `unknown` and `fetchRows` to be a function that returns a `Promise` of `unknown`:

```typescript
type MakeInfiniteScrollParams = {
  initialRows: unknown[]
  fetchRows: () => Promise<unknown>
}
```

Next, we'll set the `params` argument to be of type `MakeInfiniteScrollParams`:

```typescript
const makeInfiniteScroll = (params: MakeInfiniteScrollParams) => {
  const data = params.initialRows || []

  const scroll = async () => {
    const rows = await params.fetchRows()
    data.push(...rows)
  }

  return {
    scroll,
    getRows: () => data,
  }
}
```

With these changes in place, the function shows an error for `data.push`:

```text
Type 'unknown' must have a '[Symbol.iterator]()' method that returns an iterator. 
```

To fix this we can set `fetchRows` to be a function that returns a `Promise` returning an array of `unknown`:

```typescript
type MakeInfiniteScrollParams = {
  initialRows: unknown[]
  fetchRows: () => Promise<unknown[]>
}
```

With that error fixed, there is now an error in the test with `key` since it doesn't exist on the `param`:

```typescript
it("Should fetch more data when scrolling", async () => {
  const table = makeInfiniteScroll({
    key: "id",
    fetchRows: () => Promise.resolve([{ id: 1, name: "John" }]),
  })

  await table.scroll()

  await table.scroll()

  expect(table.getRows()).toEqual([
    { id: 1, name: "John" },
    { id: 1, name: "John" },
  ])
})
```

To fix it for the time being, we'll add it to our type as as a string:

```typescript
type MakeInfiniteScrollParams = {
  key: string
  initialRows: unknown[]
  fetchRows: () => Promise<unknown[]>
}
```

But now we are getting more errors!

When we hover over the object we pass in to `makeInfiniteScroll` we see the following error:

```text
Property 'initialRows' is missing in type '{ key: string; fetchRows: () => Promise<{ id: number; name: string; }[]>; }' but required in type 'MakeInfiniteScrollParams'. 
```

This is happening because we aren't passing in `initialRows`, so let's make it optional:

```typescript
type MakeInfiniteScrollParams = {
  key: string
  initialRows?: unknown[]
  fetchRows: () => Promise<unknown[]>
}
```

## Adding TRow

Now that we have all this stuff, we can start to see relationships between all the pieces.

We can add `TRow` to the type, and set `initialRows` to be an array of `TRow` and `fetchRows` to be a function that returns a `Promise` of `TRow[]`:

```typescript
type MakeInfiniteScrollParams<TRow> = {
  key: keyof string
  initialRows?: TRow[]
  fetchRows: () => Promise<TRow[]>
}
```

We are able to safely represent this in an object, since we're dealing with arrays.

Then in `makeInfiniteScroll` we pass `TRow` as a type argument and make sure to add it to the `params` argument:

```typescript
const makeInfiniteScroll = <TRow>(params: MakeInfiniteScrollParams<TRow>) => { ... 
```

## Making Tests Pass

The initial rows test is now passing. Rows are getting inferred properly based on `name` and `id`.

To make sure that our test for the key is a property of the row is passing, we can check if `key` is a `keyof` `TRow`:

```typescript
type MakeInfiniteScrollParams<TRow> = {
  key: keyof TRow
  initialRows?: TRow[]
  fetchRows: () => Promise<TRow[]>
}
```

Now if we don't pass in a promise to `fetchRows`, it is going to throw an error. To fix this we can update it so `Promise` returns a `Promise<TRow[]>` or `TRow[]`:

```typescript
type MakeInfiniteScrollParams<TRow> = {
  key: keyof TRow
  initialRows?: TRow[]
  fetchRows: () => Promise<TRow[]> | TRow[]
}
```

Introduce a Type Parameter to Ensure Type Consistency

0:00 The first thing that comes to mind is we're obviously going to need a type argument for makeInfiniteScroll. The question is what we should be expecting in that type. For now, let me just scaffold a couple of things. I'm going to say, "interface MakeInfiniteScrollParams."

0:17 I like extracting this out to an interface or a type. Let's go for a type, actually. MakeInfiniteScrollParams and just scaffold this here. We know that we have initialRows, which is probably going to be an array of something, and we have fetchRows. It looks like fetchRows is going to be a function. It doesn't actually take in any arguments, which is good. Let's say it just returns unknown.

0:44 In fact, I think it might be best if it returns a Promise unknown. We know that, basically, it's going to fetch something. We need to actually pass in a Promise here. I guess maybe we could do a maybe Promise. Anyway, we'll get to that.

0:59 We've got this unknown here. We can see that data.push...Type unknown must have a Symbol.iterator method that returns an iterator. What? It turns out the data.push expects it to be an array of something. Because we're spreading this in, we know that this is probably going to be an array of something, which makes sense because we're fetching more rows here.

1:25 We've now got this key here. Key doesn't exist on the params. Let's add that in just as a string, for now. Let's see if things have improved at all. Oh no. No, they really haven't. Why isn't this erroring? Oh yeah. Because we don't need to pass in the initial rows. This one can actually just be an optional here.

1:48 That's good. We've removed some errors, but this table, for instance, it's now just...fetchRows is returning Promise unknown. That's good. table here, getRows is returning an unknown array, which is going to be complaining down here. It's supposed to be an array of the things that you pass into initialRows.

2:10 Now we can see that we have a couple of unknowns here. We have this key string, which I'm just going to type as unknown as well. Now that we have all this stuff, we can start to see relationships between all the pieces. We can see that this type probably needs to have a T on it, like TRow, for instance, that represents all of the pieces here.

2:31 initialRows, it makes sense for that to be an array of TRow. fetchRows is also going to be an array of TRow. We can represent this pretty safely in an object here because we know we're going to be dealing in arrays. We know that TRow array is going to be there, and Promise TRow is going to be there.

2:50 Now we need to find some way of passing it into the MakeInfiniteScrollParams. We've got TRow here. From all of our pain that we've been through, we know that we can add TRow here, and it's going to be in InfiniteScroll there.

3:07 How are we doing now? We're getting pretty nice functionality here. We're getting getRows. We're actually passing this test down at the bottom. Our rows are now inferred properly based on name and id. If we add age, 23 there, let's say, then this is going to be added to this too, which is fantastic. This function is now inferring properly. Brilliant.

3:29 What's erroring at us now? We have a "Should ensure that key is one of the properties of the row." We know how to do this. We know that TRow, we can use keyof TRow to make sure that this key is one of those members.

3:45 Now, if we go down to the bottom here, we've got key id. We'll get id and name here. If we pass in age, it's going to yell at us. If we pass in name, it's going to work. Pass in id. It's going to work. That is fabulous. This is erroring properly because we don't have name on this particular object that we're inferring from.

4:04 What can we take from this? We know that we can pass in...There's multiple inference sites here, so the initialRows and the Promise TRow. This is a really, really neat one, the fact that you can infer from a Promise that's passed in.

4:19 I'm just wondering, if we don't pass in a Promise here...We have fetchRows. Let's just say that we return this, for instance. This is actually going to yell at us because it's expecting an async function here. This doesn't seem, to me, to make sense. We could say this returns a Promise TRow or TRow array here.

4:41 Now this will work because it doesn't need to return a Promise. It can return a Promise if it wants to, but it doesn't need to. This will work as an async or a sync function. There you go. A little bonus for you. This is pretty nice. I'm pretty happy with how this API turned out. Well done, if you found the solution.

Make An Infinite Scroll Function Generic with Correct Type Inference

0:00 In this exercise, we're taking this makeInfiniteScroll function, and it's completely untyped right now. We're trying to capture some reusable stuff inside a single function. You can think of this makeInfiniteScroll as a hook in React or a similar thing in another framework.

0:20 We have this table here, makeInfiniteScroll. We're expecting this key to be ID and fetchRows, we're returning promise.resolve ID1 named John. Now, we're expecting that if we fetchRows here and the promise.resolve doesn't include the key, then it's going to error at us. This key has to be a key of the thing that we're fetching from fetchRows.

0:48 It should also allow you to pass in initialRows. These initialRows should be the same type as this fetchRows here. It should also infer the rows when the rows get returned. Currently, the rows are any. It lets you pass any key inside here, and currently, there's no inference going on whatsoever.

1:08 Your job is to try to type this function to make it generic and make sure that all of the pieces get inferred properly. Good luck.

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Make An Infinite Scroll Function Generic with Correct Type Inference

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