Here we have an interface that represents a user within our system:

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

There's a function called `getUserId` that takes in a user, and returns its id.

Our test is currently failing, because it calls `getUserId` and passes in a `defaultUser` that doesn't match the `User` contract.

## Challenge

Reference <a href="https://www.typescriptlang.org/docs/handbook/typescript-in-5-minutes.html" target="_blank" rel="noopener">the TypeScript docs</a> and determine how to change `defaultUser` so that our test will pass.

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

There's a little bit of syntax we need to add to our declaration to solve this problem.

By adding `: User` to `defaultUser`, we're telling TypeScript that we want it to conform to our `User` interface.

```typescript
const defaultUser: User = {}
```

Now TypeScript will display errors at the line where `defaultUser` is declared.

We also benefit from autocompletion for the properties!

The same `: Type` syntax can be used with other types, including numbers and functions:

```typescript
let a: number = 1
```

As you write TypeScript, you need to think about _where_ you want your contracts to be, and _what_ needs to be done in order to meet them.

Make a Variable Conform to an Interface

0:00 The syntax right here. This little colon just before defaultUser there. What this means is if I don't parse in isAdmin, then it's going to give me an error at this line. "Property 'isAdmin' is missing in type blah blah blah but required in type 'User'." If I add that back, I can say isTrue here.

0:20 If I remove this, then nothing errors, but if I get this wrong then the error's not going to be where the actual error is. It's going to be further down in the code. And so, you need to think, when you're using TypeScript, where do I want my contracts to be? What exact thing do I want to meet to my contract?

0:40 It's so useful to have it here because, of course, the error appears in the correct place, but you also get autocomplete here, too. Without this, I don't get autocomplete on isAdmin. I've made a deliberate typo there. It's going to say, "Property 'isAdmin' is missing in type blah blah blah." I'm going to get a really confusing, big error message.

1:03 Whereas, just like this, if I say it's right there, then I get to clear it up immediately and it actually tells me exactly where the error is.

1:12 This syntax is really, really useful. You can use it for let. Let A is a number. We can say one. You can use it with const. You can use it with functions, as well. There's a lot you can do with this little syntax. It's really, really useful for making sure that your variable match a certain contract.

Assigning Types to Variables

0:02 We've got a tricky problem here. We're using an interface user as a way to represent the user within our system. We've got a function down here called getUserId, which takes in a user and returns its ID.

0:17 Now, this getUserId (defaultUser) is failing. It's failing because this defaultUser is not meeting this user contract. If I were to change it...I would say id, 1, firstName, Matt, lastName, Pocock, isAdmin, true, then it's going to pass.

0:43 I don't want the error to be down here. I ideally want the error to be at this line or around here, because I want to make sure that my defaultUser is matching that user contract. Your job is to go through the TypeScript docs and work out how you would do this.

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

[Start now](https://totaltypescript.com/tutorials/beginners-typescript/implicit-any-type-error), and see why TypeScript’s an essential tool for modern web developers!

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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

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

Mark a Function Parameter as Optional

Optional Parameters

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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Assigning Types to Variables

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