Our `getName` function has been refactored.

Now instead of taking in a single `params` object, it takes in `first` and `last` as individual parameters:

```typescript
export const getName = (first: string, last: string) => {
  if (last) {
    return `${first} ${last}`;
  }
  return first;
};
```

This time TypeScript is telling us that in the first name test that it expected two arguments but only got one:

```typescript
it("Should work with just the first name", () => {
  const name = getName("Matt");

  expect(name).toEqual("Matt");
});
```

If we update the first name test to call `getName` with "Pocock" as a second argument, the error will go away. However, the test would fail at runtime.

## Challenge

Your challenge is to work out how to mark the `last` parameter as optional.

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

Similar to last time, adding a `?` will mark `last` as optional:

```typescript
export const getName = (first: string, last?: string) => {
```

There is a caveats, however.

You can't put the optional argument _before_ the required one:

```typescript
// this will not work! 
export const getName = (last?: string, first: string) => {
```

This would result in "Required parameter cannot follow an optional parameter".

As before, you could use `last: string | undefined` but you would then have to pass `undefined` as the second argument.

Mark a Function Parameter as Optional

0:00 Here we go. We now have a little extra character in there. Very similar to exercise number three, you can make arguments optional by just adding this little character there. If I remove this, I obviously get the same error there before.

0:16 There are some caveats too. For instance, I can't do this. I can't make the optional argument go before the required argument, and it gives me a nice little nice, little error message here. "Required parameter cannot follow an optional parameter."

0:31 This is very, very similar to how we declared it in the object type. Using it here, we can use the same trick if we want to, string or undefined, like this, but I would need to then pass undefined as the second argument. Whereas using this, it means you just don't need to pass it at all.

Optional Parameters

0:00 We've got a similar setup to our previous exercise, but the API is slightly different. Instead of parsing a single object, we're actually parsing two function parameters here.

0:10 We're still getting the same problem, which is it should work with just the first name but it's not. It's saying you need to parse in the last parameter.

0:19 Here, the error message is a little bit different. It's saying it expected two arguments but got one. If you add in the second property here then, of course, it's going to work but it's going to fail at runtime.

0:31 Your challenge is to work out how we can make this argument optional.

Get hands-on interactive practice with this free TypeScript tutorial and learn the foundational knowledge and skills you need to become a TypeScript Wizard.

In this TypeScript tutorial you will:

* Learn to type variables, functions, and errors
* Become familiar with the TypeScript documentation
* Gain experience with modern TypeScript tooling

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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.

If you [follow this link to use gitpod](https://gitpod.io/#https://github.com/total-typescript/beginners-typescript-tutorial), you can work in a full VS Code coding environment running in your browser.

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

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

Two Techniques for Creating New Types Based on Others

Selectively Construct Types from Other 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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Optional Parameters

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