We've looked at inheriting and combining types.

But what if we want to be more selective?

## Challenge

Given this `User` interface, create a new object type that only includes the `firstName` and `lastName` properties:

```typescript
interface User {
  id: string;
  firstName: string;
  lastName: string;
}
```

Read through <a href="https://www.typescriptlang.org/docs/handbook/utility-types.html" target="_blank" rel="noopener">the TypeScript Utility Type docs</a> to see what you can find.

TypeScript provides us with tools for being more selective when creating new types based on others.

There are a couple of solutions here that both make use of TypeScript's built-in helpers.

## Use Omit

The first solution is to `Omit`.

According to TypeScript, this constructs a type with the properties of `T` except for those in type `K`.

Here's what the syntax looks like:

```typescript
type MyType = Omit<User, "id">;
```

What we're saying here is create a type with everything that `User` has except for `id`.

## Use Pick

The second solution is to use `Pick`, which is the inverse of `Omit`:

```typescript
type MyType = Pick<User, "firstName" | "lastName">;
```

Here we're taking `User` and picking its `firstName` and `lastName` properties.

Note that `Pick` provides autocompletion while `Omit` does not.

This is because you are able to omit keys that aren't present on the original type, which means you end up with all of its properties. This is an advanced problem that we will tackle down the road.

For now, just know that both of `Omit` and `Pick` are globally available in TypeScript and are extremely useful tools.

Two Techniques for Creating New Types Based on Others

0:00 We've got two solutions here. Both of them use type helps from TypeScript's built-in selection. Here, we've got a type helper which is saying Omit. What an Omit is doing is constructing a type with the properties of T, except for those in type K. What on earth does that mean?

0:18 What it means is that we're creating a new type without ID in here. If we want to, we could omit firstName and we'd end up with just ID and lastName. We could even omit either firstName or lastName here, and we'd end up with just the ID. That's the first method of doing this. You don't need to import from anywhere. This is a globally available type in TypeScript, and it's extremely useful.

0:45 The second one is we've got the inverse of Omit, which is Pick here. Pick, just like Omit, constructs a set of properties, except it's the inverse. It just picks the ones you want. If we wanted to only pick the ID type, then we would get that here. If we wanted to just pick firstName or just pick lastName, then we'd end up with the whole thing.

1:13 You might be wondering why you get autocompletes on this one instead of on this one. That's something normal to notice. What you might see is that you can omit keys that aren't there, and you just end up with the whole thing. This is an advanced problem, so we will tackle this down the road. I promise.

Selectively Construct Types from Other Types

0:00 We're continuing our Odyssey here of creating types from other types. It's really, really useful in TypeScript to be able to extract bits of a type and then create new ones out of it. Here, we've got our user type. What we want to do is create a new type here which is just the firstName and lastName properties of the user type.

0:23 How do we create an object type which is just those? We've got our test down here which is saying we want it to be firstName, lastName. The thing I want you to google in the TypeScript doc is utility types. There are various built-in utilities in TypeScript that I want you to see if you can have a look at. See if you can piece them together to work out how to create this MyType here.

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## Instructions

This Beginner’s TypeScript tutorial is split into several exercises. Each features a `problem` that encourages you to take an active, exploratory learning approach to finding a `solution`.

To attempt a solution, you’ll need to:

1. Watch and read the `problem` introduction.
2. Check out [the TypeScript docs](https://www.typescriptlang.org/docs/).
3. Search for relevant keywords.
4. Give it a go to see if it solves the problem (there may be more than one solution!)

Whether you succeed or get stuck, watch & read the `solution` and check out the included code.

### Prerequisites

You’ll need a working understanding of JavaScript.

### Running the Exercises Locally

Clone [the Beginner’s TypeScript Tutorial repo](https://github.com/total-typescript/beginners-typescript-tutorial), and use npm or yarn to install dependencies.

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Follow the [instructions in the README](https://github.com/total-typescript/beginners-typescript-tutorial) for more.

Free interactive video tutorial that will help you get started with TypeScript.

Beginner's TypeScript Tutorial Repo

First thing’s first: dealing with the implicit ‘any’ type error.

Fix the Implicit 'Any' Error

The Implicit ‘Any’ Type Error

Not all of the functions you write will take primitive types. When you need to pass objects as parameters, there are a few different ways to type them properly.

Add Types to Object Params

Working with Object Params

TypeScript can’t tell by itself if your code will still work when object properties are missing. For these situations, you need to specify which are optional.

Denote an Optional Property

Set Properties as Optional

Just as we saw with object properties, function parameters can be set as optional as well.

Mark a Function Parameter as Optional

Optional Parameters

Assigning types to variables allows TypeScript to provide more useful error messages on the lines where they occur.

Make a Variable Conform to an Interface

Assigning Types to Variables

TypeScript allows us to constrain a property’s value to one of a set number of things. For example, a medal that could only be gold, silver, or bronze.

Delineate Types a Value Can Be

Constraining Value Types

Arrays are one of the most common data structures in development. In this lesson, find out how they are represented in TypeScript.

Two Ways to Represent Arrays

Working with Arrays

Being strict about what is returned from your functions ensures that your code is safe on the inside as well as the outside.

Annotate a Function to Specify its Return Type

Function Return Type Annotations

Almost every web application involves making API calls. But how can we tell TypeScript what to expect when we’re expecting data?

Update Return Type Annotations for Async Functions

Typing Promises and Async Requests

One of the most crucial TypeScript concepts to understand is how type arguments work with certain functions and constructors.

Pass a Type Argument to Restrict Potential Types

Passing Type Arguments

Seeing “index” in a type error message usually refers to the key of an object. In this lesson are a few techniques for properly typing dynamic object keys.

Techniques for Typing Dynamic Object Keys

Assigning Dynamic Keys to an Object

TypeScript offers powerful features for narrowing down branches when working with third party APIs or functions that support data in different forms.

Change Function Returns Based on Type

Narrowing Down Union Types

When using a try-catch in TypeScript, it’s important that you type the error appropriately. There are a few different techniques to accomplish this.

Techniques for Coercing an Unknown Type

Typing Errors in a Try-Catch

Interfaces can inherit properties from other interfaces. This allows us to DRY out our code and have a neater way to work with complex data models.

Extend Interfaces From Other Interfaces

Inheriting Interface Properties

TypeScript allows for composing types together to create new types. This is useful when returning data that is a combination of multiple different types.

Combine Inline or at the Type Level

Combining Types to Create New Types

TypeScript supports typing functions that are passed in as arguments, including the arguments that they take.

Specify the Type of a Function

Typing Functions

TypeScript supports typing the returns from asynchronous functions– great for real world processes like fetching from databases or other third-party APIs.

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Selectively Construct Types from Other Types

Challenge

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