Here were have an `programModeEnumMap` object that keeps different groupings in sync:

```typescript
export const programModeEnumMap = {
	GROUP: "group",
	ANNOUNCEMENT: "announcement",
	ONE_ON_ONE: "1on1",
	SELF_DIRECTED: "selfDirected",
	PLANNED_ONE_ON_ONE: "planned1on1",
	PLANNED_SELF_DIRECTED: "plannedSelfDirected",
} as const;
```

We have a `ProgramModeMap` type that uses `typeof` to represent the entire enum mapping:

```typescript
type ProgramModeMap = typeof programModeEnumMap;
```

The goal is to have a `Group` type that is always in sync with the `ProgramModeEnumMap`'s `group` value. Currently it is typed as `unknown`:

```typescript
type Group = unknown;

type test = Expect<Equal<Group, "group">>; // red squiggly line under Equal<>
```

## Challenge

Your challenge is to find the proper way to type `Group`.

To do this, you'll need to look into the Indexed Access Type.

TypeScript provides a way to access specific values in an as const object that will stay in sync.

In order to access the `group` property of `ProgramModeMap`, we need to use indexed access types.

```typescript
export const programModeEnumMap = {
	GROUP: "group",
	ANNOUNCEMENT: "announcement",
	ONE_ON_ONE: "1on1",
	SELF_DIRECTED: "selfDirected",
	PLANNED_ONE_ON_ONE: "planned1on1",
	PLANNED_SELF_DIRECTED: "plannedSelfDirected",
} as const;

type ProgramModeMap = typeof programModeEnumMap;
```

Think about it as indexing into an object.

In JavaScript, we can use the `.` syntax to access a property, but that won't work in TypeScript:

```typescript
type Group = ProgramModeMap.GROUP; // red squiggly line under ProgramModeMap.GROUP
```

The error message tells us that we can't access `ProgramModeMap.GROUP` because `ProgramModeMap` is a type, not a namespace:

```typescript
Cannot access 'ProgramModeMap.GROUP' because 'ProgramModeMap' is a type, but not a namespace. Did you mean to retrieve the type of the property 'GROUP' in 'ProgramModeMap' with 'ProgramModeMap["GROUP"]'?
```

However, the error message does suggest that we use the square bracket syntax instead:

```typescript
type Group = ProgramModeMap["GROUP"];
```

With this change, we can now access the `Group` property of `programModeMap` in order to get its value.

This syntax provides autocompletion for all of the members inside of the `progamModeMap`. This is useful because if the object keys ever change, TypeScript will automatically update the type aliases for us.

It's a great way to have a single source of truth for all of your types throughout your app.

Indexed access types allow you to access properties within types.

Use an Indexed Access Type to Access a Property

Access Specific Values in an as const Object

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Discover the history of TypeScript, how it evolved from JavaScript, and how it can drastically improve your development experience with better tooling.

TypeScript's Relationship with JavaScript

Learn the differences between TypeScript and JavaScript in terms of build processes, file extensions, and how code is run in browsers and Node.js.

JavaScript vs. TypeScript in the Build Process

To get set up for TypeScript development, you'll need an IDE, Node.js, and an execution environment.

Essential Tools for TypeScript Development 

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Set up Node.js and VS Code

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Installing pnpm for Package Management

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Installing the Total TypeScript VS Code Extension

Kickstart Your TypeScript Set Up

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Unexpected Token Error in the Browser

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Compile TypeScript to JavaScript with the TypeScript CLI

Convert a TypeScript File into a JavaScript File

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Running tsc in watch mode allows you to automatically compile TypeScript files as you make changes.

Use tsc's Watch Mode to Automatically Compile TypeScript Files

Automating TypeScript Compilation

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Using the outDir setting in tsconfig.json, you can configure the output directory for all emitted files.

Configure the Output Directory for Compiled Files

Compiling TypeScript Files to a Directory

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Create a new project with Vite and TypeScript to improve upon our current setup.

Configure Vite for a Vanilla TypeScript Project

Use Vite for a Frontend App with TypeScript

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When the noEmit option is set to true in the tsconfig.json file, TypeScript acts more like a linter than part of the build process.

TypeScript as a Linter

Comparing Vite with tsc

Integrating TypeScript into a CI/CD system allows it to catch errors before they make it to production.

TypeScript in a CI/CD System

There is some controversy around whether or not TypeScript should block your dev server when there are errors. Let's explore the pros and cons of different approaches.

Should TypeScript Block Your Dev Server?

The tsx CLI tool allows you to quickly run scripts written in TypeScript.

Quickly Create Scripts with tsx

TypeScript in The Build Process

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Common sense tells us that the booleans in the `add` function should be replaced with some sort of `number` type.

Fixing Errors in the `add` Function

Understanding Function Errors

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As a general rule, you should always annotate the parameters with their respective types when writing functions.

Annotating Function Parameters

Annotating Empty Parameters

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TypeScript comes with a set of basic types to help define what kind of values can be assigned to variables.

TypeScript Basic Types

Basic Types in TypeScript

Learn how to make function parameters optional in TypeScript to handle different argument formats.

Using the `?` , add optional parameters in TypeScript functions.

Optional Function Parameters in TypeScript

Optional Function Parameters

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Default parameters in TypeScript are specified using the `=` syntax.

Add a Default Parameter with `=`

Default Parameters in JavaScript Functions

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The ({ }) Syntax is used for objects in TypeScript so, this doesn't need to be shown.

Annotating with {}

Typing Object Literals

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Specifying Optional Properties

Making Object Property Types Optional

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Creating `type` Aliases

A Single Source of Truth for Type Definitions

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Learn about the two different syntaxes for defining arrays in TypeScript.

Two Syntaxes for Arrays

Add an Array to an Object

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There are few different ways in which an array of objects can be expressed

Representing an Array of Objects

Arrays of Objects in TypeScript

Update rest parameters to specify the type of arguments being passed into a function in TypeScript.

Learn how to use rest parameters to pass an indefinite number of arguments of the same type to a function in TypeScript.

Using Rest Parameters in TypeScript

Rest Parameters in TypeScript

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Tuples in TypeScript allow you to define fixed-length arrays with specified types for each member.

Using Tuple Syntax in TypeScript

Tuples for Precise Array Structures in TypeScript

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Learn how to specify optional members in a tuple using named tuples and the optional operator in TypeScript.

Working with Optional Tuple Members in TypeScript

Using Optional Tuple Members in TypeScript

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The Dangers of Using "any"

The Controversial `any` Type

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Function Types for Type Aliases in TypeScript

Function Types

An example of strictly typing a webpage event listener with no return value.

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The `void` Type

Typing an Event Listener

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Generics are used specify the type of data that can be used in a Set.

Creating Typed Sets

Restricting `Set` Types

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Adding Types to Maps

Type Checking Maps

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The parsed data returned by JSON.parse is expected to be any. To resolve this, we remove the generic type arguments.

Decoding the JSON.parse Error

Debugging JSON Parsing

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Unlike regular functions, you can't pass type arguments to the methods called inside an async function.

Adding Types to an Async Function

Typing Fetch API Responses in Async Functions

Essential Types And Notations

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Understanding How TypeScript Works in Your IDE

One of the coolest features of the TS server is the ability to introspect variables and declarations in TypeScript.

Introspecting Variables and Declarations in TypeScript

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By using hovers effectively, you can quickly understand and fix errors in your code without having to read lengthy error messages.

Fixing Errors with Hovering

Hovering Over a Function Call

Learn how to add comments that appear when a function is hovered over in TypeScript.

TSDoc comments allow you to document the purpose and usage of your code, providing a powerful way to communicate with others.

TSDoc Comments

Adding Documentation for Hovers

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The Control + Space Shortcut

Manually Triggering Autocomplete

TypeScript's Approach to Errors

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Quick Renaming in VS Code

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Navigating Code with "Go to Definition"

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Using Autocomplete and Quick Fix to Import Variables

Fast Imports in VS Code

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VS Code provides a a hotkey for the Organize Imports command, making it quick and easy to clean up your code.

The Organize Imports Hotkey in VS Code

Organizing Imports in Large Files

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Targeting different sections of your code in this editor allows for a wide range of Quick Fixes.

Refactoring with Quick Fixes

Quick Fix Refactoring

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Automatic Code Formatting with Prettier

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Restarting the TypeScript Server in VS Code

Integrated Development Environments Superpowers

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Understand how union types allow you to express an "or" relationship between types.

Introducing Union Types in TypeScript

Handling Null Values in TypeScript

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Diving Deeper into Unions and Assignability

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Combining Literal & Union Types

Restricting Function Parameters

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Literal Type Assignability

Learn how to combine unions in TypeScript to create type aliases with as little duplication as possible.

Learn how to create new types by combining existing union types in TypeScript.

Create New Types By Combining Unions

Combining Union Types in TypeScript

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How Big Can a Union Be?

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