In this exercise we're going to take our first look at infer.

Before you go any further, please take some time to carefully [read over this section of the TypeScript docs](https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#inferring-within-conditional-types).

It's important that you start building an understanding of what's going on with infer before moving on to this challenge.



Here we have a type function called `GetDataValue`:

```tsx
type GetDataValue<T> = unknown; 
```

The goal is to be able to pass an object with a `data` attribute into `GetDataValue` and have it return whatever value is there, as seen in our tests:

```tsx
type tests = [
  Expect<Equal<GetDataValue<{ data: "hello" }>, "hello">>,
  Expect<Equal<GetDataValue<{ data: { name: "hello" } }>, { name: "hello" }>>,
  Expect<
    Equal<
      GetDataValue<{ data: { name: "hello"; age: 20 } }>,
      { name: "hello"; age: 20 }
    >
  >
];
```

If the `data` attribute isn't there, the function should return `never`.

Hint: You probably need to do some conditional check to see if the `T` is being passed in as `data`. If it is, extract the information out of it in order to return it.

If you find yourself in some complex conditional logic, harness the power of infer.

This challenge has two solutions.

## Solution 1

The first solution might look pretty similar to pieces that you've kind of understood before:

```tsx
type GetDataValue<T> = T extends { data: any } ? T["data"] : never; 
```

First we use `extends` to see if `T` has an object with a `data` attribute.

If it does, then we can just return it by using an index access type. Otherwise, we return `never`.

## Solution 2

While that solution is fine, in the exercise intro we mentioned that this challenge can be solved by using `infer`.

Here's what the `infer` solution looks like:

```tsx
type GetDataValue<T> = T extends { data: infer TData }
  ? TData
  : never;
```

Let's break it down.

The `infer` in `T extends { data: infer TData }` says "Whatever is passed in to the `data` key, infer its type".

Then, the `infer` declares `TData` for the true branch. If we try and access TData in the 'false' branch, we won't be able to. In other words, the `TData` variable is only defined for one branch.

## `GetDataValue` in Action

To check our work, we can create a new `Example` type that passes `{data: 1}` to `GetDataValue`:

```tsx
type Example = GetDataValue<{ data: 1 }>; 
```

This would extract the `1`.

If we change `GetDataValue` to return `1` or `undefined`:

```tsx
type GetDataValue = T extends { data: infer TData }
  ?  TData | undefined
  : never;
```

Our `Example` type would show `1 | undefined` when hovered over.

## Which Solution to Choose?

So, when you're choosing these two possibilities, which one should you pick?

I tend to prefer the second solution because infer gives us an opportunity to name a new variable.

To make our code more clear, we could rename `TData` to `TInferredData`:

```tsx
type GetDataValue<T> = T extends { data: infer TInferredData }
  ? TInferredData
  : never;
```

Compare this to the first solution, where we had `data` typed as `any` then extracted it with `T["data"]`:

```tsx
type GetDataValue<T> = T extends { data: any } ? T["data"] : never; 
```

I prefer using `infer` because the logic is expressed inside of the conditional check.

The next time you find yourself inside of a conditional check and you need to extract out something, I suggest you harness the power of `infer`!

Infer Elements Inside a Conditional with Infer

0:00 This problem has multiple solutions. The first solution might look pretty similar to pieces that you've understood before. We're first checking if T extends data any here. If it does, then we know that T has a data attribute on it, and so we can just return that data attribute. Otherwise, we return never.

0:20 This, we're using an index access type, which we've already seen. This is pretty cool. The one I wanted to point your attention to though is this infer here. What's going on here instead, now what we're saying is T extends data infer TData. Instead of this any, we've now got this infer attribute in the middle, which is saying whatever is passed in here, then just infer its type.

0:50 What this does is it declares a TData just for the positive condition. This means we can't access TData here because it says cannot find name TData. That variable isn't defined in that branch. It's only defined in this branch here. This is a way of adding another variable into the function.

1:12 We can say, if we had TData up the top here, then this is kind of the same thing, except it would be declared in the whole scope. What infer does is it allows you, inside the scope of a conditional check, to pattern-match against the thing you're checking and extract out a new type variable.

1:31 What this means is that TData now, we've extracted it, and it could be anything that's inside this TData or inside this data slot. We just then return it. If we say type Example = GetDataValue -- let's say we have a data, and let's say it's just a 1 here -- this is going to extract the 1. We could even do TData or undefined here, and we end up with 1 or undefined.

1:57 When you're choosing these two possibilities, which one should you pick? I tend to prefer the second solution because this infer TData gives me a new opportunity to name a variable. We could even call this TInferredData or something here. For me, this reads pretty nicely in terms of the syntax here.

2:21 With this one, we're saying data.

2:23 any and then extracting it there. Whereas here, all of the logic is expressed inside the conditional check. This infer, it's really, really powerful. I suggest that the next time you're inside a conditional check, and you need to extract out something interesting, you use infer because it is extremely powerful.

Introducing infer for Conditional Logic

0:01 In this section, we're going to learn about something called infer. Now, infer can confuse a lot of people, so make sure you read the docs really, really carefully and try to understand what's going on with it before you process even what this task is doing.

0:16 Here, we're creating a function called GetDataValue -- sorry, a type function -- where we should be able to pass in something that has a data attribute here. This data attribute, we basically want to return from this type function whatever is in that data slot.

0:34 Here, this hello is being extracted out there. Here, we have name hello. That's being extracted out there too. In here, we have data name hello age 20, blah, blah, blah, blah, blah. If it doesn't conform to that, we should be returning never as well.

0:52 That's your job. We probably need to do some kind of conditional check to make sure that the T here is being passed in as data. If it is being passed in as data, we need to then extract out that information in order to return it.

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## Inference Basics 

The TypeScript compiler uses type inference to automatically determine the data type of a value depending on the context in which it is used. We can use several utilities and operators to define our own types more precisely.

## Unions & Indexing Unions

Union types play a key role in the type transformation process. In this section we will work at unions and discriminated unions, as well as how to access their properties with indexed access types. Several of TypeScript’s built-in utility types will also be explored.

## Template Literals 

As of TypeScript 4.1, template literals can be used to power amazing inference on strings. In this section we’ll practice using template literals to help create object types, split strings into tuples, and other useful transformations.

## Type Helpers

Type helpers allow you to manipulate and work with types in a more convenient way. They allow you to set constraints that ensure your types are as you expect while retaining flexibility for reuse. The type helper pattern is important to understand, as it forms the basis of many of TypeScript's core features.

## Conditional Types & Infer 

In this section, we will further explore how to leverage the type helpers pattern, paying special attention to conditional types and inference when working with functions, objects, and Promises.

## Mapped Types 

With TypeScript's mapped types, you can transform any type into and out of objects, unions, and tuples. It's an amazingly flexible tool for creating all sorts of different types. You’ll get practice with all of these, as well as using template literals to dynamically customize your types. One learner wrote they'd been using TS for 2 years and finally understood Mapped Types after this section!

## Challenges

To tie up the previous exercises, we've put together several real-world challenges that will put your Type Transformation skills to the test.

Learn to manipulate types for cleaner, maintainable code with over 50 hands-on exercises, from string manipulation to advanced unions and mapped types.

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The 'keyof' Operator

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Simple Syntax Used to Access Parts of a Discriminated Union

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The Annotation Used to Infer an Object's Values as Literal Types

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Template Literal Types

Template Literals

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Introducing Type Helpers

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Let's create a Maybe type that can handle null and undefined

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Use Constraints to Limit Type Parameters

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Explainer: Type Helpers in Redux

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Compare and Return Values with Extends and the Ternary Operator

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Transcript

Introducing infer for Conditional Logic

Transcript