Consider this `programModeEnumMap` with different values:

```tsx
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;
```

We are interested in the "planned" values, which are `planned1on1` and `plannedSelfDirected`. These values are linked to different cases in the map.

## Challenge

Without changing any other code, make it so that the `PlannedPrograms` type alias corresponds to either `planned1on1` or `plannedSelfDirected` as string literals.

```typescript
type PlannedPrograms = unknown;
```

There are a couple of solutions, but try to find the one that involves passing a union to an indexed access type.

Learn how to extract specific string literals using union and indexed access types.

The more obvious solution to this challenge is to use the `ProgramModeMap` type directly with the indexed access type:

```typescript
type PlannedPrograms =
	| ProgramModeMap["PLANNED_ONE_ON_ONE"]
	| ProgramModeMap["PLANNED_SELF_DIRECTED"];
```

This works, and our test case passes:

```typescript
type test = Expect<
	Equal<PlannedPrograms, "planned1on1" | "plannedSelfDirected">
>;
```

However, there's a more dynamic solution.

## Pass a Union to an Indexed Access Type

Indexed access types can receive a union of strings. This feature sets it apart from the dot notation, enabling a broader array of use cases.

First we'll make a `Key` type for the union of strings, then we will use it to index into `ProgramModeMap`:

```typescript
type Key = "PLANNED_ONE_ON_ONE" | "PLANNED_SELF_DIRECTED";
type PlannedPrograms = ProgramModeMap[Key];
```

This feature also enables another interesting use case, which we'll explore in the next exercise.

Passing a union to an indexed access type provides a nice alternative to dot notation.

Unions and Indexed Access Types in TypeScript

Pass a Union to an Indexed Access Type

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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 

A quick guide to installing Node.js and Visual Studio Code to create a stable environment for web development.

Set up Node.js and VS Code

The pnpm package manager is a faster and more disk-efficient alternative to npm.

Installing pnpm for Package Management

The Total TypeScript VS Code extension provides helpful hints, syntax guidance, and error translations for an improved TypeScript coding experience.

Installing the Total TypeScript VS Code Extension

Kickstart Your TypeScript Set Up

Troubleshoot an unexpected token error in the browser when trying to import a TypeScript file.

Browsers are only able to understand JavaScript syntax, so we need to convert TypeScript syntax to JavaScript syntax for browser execution.

Browsers Can't Understand TypeScript Syntax

Unexpected Token Error in the Browser

Use the TypeScript CLI to convert a TypeScript file into a JavaScript file.

The tsc command can be used to convert TypeScript files into JavaScript files.

Compile TypeScript to JavaScript with the TypeScript CLI

Convert a TypeScript File into a JavaScript File

Currently we have to manually run the TypeScript CLI every time we make a change. The tsc CLI features a watch mode to do this automatically.

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

Currently our TypeScript files are compiled to the same directory as our source files. Let's change that!

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

Vite provides a simple and efficient way to build front-end apps for web development.

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

Now that we are using Vite to transpile TypeScript files into JavaScript files, the tsc command doesn't seem to do anything at all.

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

Learn how to identify and fix common type errors in your functions.

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

Learn about annotating empty function parameters in TypeScript.

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

Annotating Function Parameters

Annotating Empty Parameters

TypeScript has several basic types that can be used to annotate variables.

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

Annotate the type of object literals to avoid TypeScript errors and improve inference.

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

Annotating with {}

Typing Object Literals

Object property types can be made optional, allowing for flexibility in the shape of objects.

TypeScript allows you to mark properties as optional within an object. This lets you be more expressive when defining types.

Specifying Optional Properties

Making Object Property Types Optional

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The `type` keyword allows us to reference a type by a different name. This is useful when we want to consolidate multiple types into a single type alias.

Creating `type` Aliases

A Single Source of Truth for Type Definitions

To add an array to an object, we need to define the type of items the array contains.

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

Two Syntaxes for Arrays

Add an Array to an Object

When working with arrays of objects in TypeScript, you need to define the type properly.

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

Tuples provide a way to enforce precise array structures in TypeScript.

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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Using "any" in TypeScript may seem like a quick fix, but it actually turns off type checking and can cause issues down the line.

The Dangers of Using "any"

The Controversial `any` Type

How to make sure a function and its parameters and return types fit certain criteria, and how to annotate functions passed as arguments

Type aliases can be used to annotate function types for arguments, callbacks, and return values to enforce type safety and enable proper inference.

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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Types in Maps ensure that only correct key-value pairs are added or retrieved.

Adding Types to Maps

Type Checking Maps

Learn how to parse JSON with TypeScript, and the common problems and gotchas that can occur.

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

Learn how TypeScript infers types when working with async functions and API responses.

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

Learn how the TypeScript server in your IDE helps you catch errors and enables features like autocomplete and hovers.

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

Hovering over a function call can give hints about what arguments a function requires.

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

Take advantage of TypeScript's powerful autocomplete feature by knowing when and where to trigger it.

Hitting Control + Space in VS Code will open a list of all sorts of available options such as properties, methods, and DOM events.

The Control + Space Shortcut

Manually Triggering Autocomplete

TypeScript's Approach to Errors

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Quick Renaming in VS Code

Learn how to utilize TypeScript's "Go-To Definition" feature to easily navigate and understand code across files, making it a valuable tool for managing large codebases.

Navigating Code with "Go to Definition"

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Manually importing variables can be time-consuming and tedious. TypeScript supports autocomplete and quick fix hotkeys to make this process faster.

Using Autocomplete and Quick Fix to Import Variables

Fast Imports in VS Code

Visual Studio Code provides an "Organize Imports" command to help clean up import statements in large files.

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

Prettier is a code formatting tool for JavaScript and other languages. It can be added to a project or used as a VS Code extension.

Automatic Code Formatting with Prettier

Learn how to restart the TypeScript Server in VS Code and ensure that errors are correct.

Restarting the TypeScript Server in VS Code

Integrated Development Environments Superpowers

Learn how to handle null values in TypeScript and avoid type errors in your code.

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

Introducing Union Types in TypeScript

Handling Null Values in TypeScript

Understanding union types and assignability is essential to working effectively with TypeScript.

Diving Deeper into Unions and Assignability

TypeScript allows you to specify exactly which values are allowed as function arguments.

By combining literal & union types, we can restrict what values are allowed in certain scenarios.

Combining Literal & Union Types

Restricting Function Parameters

Literal types are assignable based on their literal values.

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

TypeScript has a limit on the size of union types that it can handle.

How Big Can a Union Be?

By wrapping the wider type in parentheses and adding an empty object declaration, it's possible to improve autocomplete.