Let's look at the `youSayGoodbyeISayHello` problem that we've seen before:

```typescript
export const youSayGoodbyeISayHello = (greeting: "goodbye" | "hello") => {
  return greeting === "goodbye" ? "hello" : "goodbye";
};
```

The function takes in a greeting of `"goodbye"` or `"hello"`, then returns the correct greeting based on what's passed in.

For example, if we pass in `"hello"`, the result should be `"goodbye"`, but instead we get `"hello"` or `"goodbye"`.

## Challenge

Your challenge is to figure out how to solve this problem using function overloads instead of with conditional types.

There are often many ways to solve a problem. Here we revisit an earlier exercise that used conditional types and compare it to a function overload approach.

The first step in the solution is to convert `youSayGoodbyeISayHello` to use the `function` keyword.

Then we'll add a function overload for when `hello` is passed in and `goodbye` should be returned:

```typescript
function youSayGoodbyeISayHello(greeting: "hello"): "goodbye";
function youSayGoodbyeISayHello(greeting: "goodbye" | "hello") {
  return greeting === "goodbye" ? "hello" : "goodbye";
}
```

At this point, there's an error in the test where we pass in `goodbye`.

```typescript
// error under `goodbye`
const result = youSayGoodbyeISayHello("goodbye");
```

TypeScript tells us it was expecting `hello` bute we've passed in `goodbye`.

Doesn't it seem like the original function would have taken care of that?

## Implementation Signatures Are Not Exposed to Overloads

One of the most important things to know about function overloads is that the implementation signatures of functions are not exposed outside of the function.

As soon as we added an overload, the `greeting: "goodbye" | "hello"` of the original function became an internal signature.

When we add the second overload, the error goes away:

```typescript
function youSayGoodbyeISayHello(greeting: "goodbye"): "hello";
function youSayGoodbyeISayHello(greeting: "hello"): "goodbye";
function youSayGoodbyeISayHello(greeting: "goodbye" | "hello") {
  return greeting === "goodbye" ? "hello" : "goodbye";
}
```

## Comparing Function Overloads to Conditional Types

Here's what the solution to the conditional type version of this exercise looked like:

```typescript
type YouSayGoodbyeAndISayHello<T> = T extends "hello" ? "goodbye" : "hello";
```

I find the function overload version more declarative and simple to read. You can get a good idea what a function does with a quick look.

However, function overload implementations aren't as type safe as other techniques.

If we change the original function to return a number, we will get an error on our overload signature is not compatible:

```typescript
// error on first overload
function youSayGoodbyeISayHello(greeting: "goodbye"): "hello";
function youSayGoodbyeISayHello(greeting: "hello"): "goodbye";
function youSayGoodbyeISayHello(greeting: "goodbye" | "hello") {
  return 123;
}
```

However, if the return is a different string there won't be an error:

```typescript
function youSayGoodbyeISayHello(greeting: "goodbye"): "hello";
function youSayGoodbyeISayHello(greeting: "hello"): "goodbye";
function youSayGoodbyeISayHello(greeting: "goodbye" | "hello") {
  return "asdf";
}
```

## Making Function Overloads More Type Safe

A good way to increase type safety when using function overloads is to add the return types to the original function and make sure the types line up with your overloads:

```typescript
function youSayGoodbyeISayHello(greeting: "goodbye"): "hello";
function youSayGoodbyeISayHello(greeting: "hello"): "goodbye";
function youSayGoodbyeISayHello(
  greeting: "goodbye" | "hello"
): "goodbye" | "hello" {
  return greeting === "goodbye" ? "hello" : "goodbye";
}
```

Match Return Types with Function Overloads

0:00 The solution here is, obviously, we're going to need a function here instead of a const. Let's fix that. We're going to need to add overloads for each specific case. For this case, we're going to need to add an overload. For the next case, we need to add one. Let's go with that one first. Let's go with function youSayGoodbyeISayHello. Greeting when it's hello will return goodbye.

0:24 This is good, because now the first case is being handled, but there's a strange error here. Now we have youSayGoodbyeISayHello, this one, it's saying, "I was expecting hello, but you passed goodbye." That's really strange because, surely, it should be catching this one here.

0:44 This is a really confusing but important part of function overloads, which is when you have an overloaded function like this, the implementation signature is not exposed outside of the function. This then becomes an internal signature.

1:01 This is still important because if you have, let's say, this was a number instead here, then you would get an error here because this overload signature is not compatible with the implementation signature. In other words, hello has to be assignable to greeting. You can have this as string, for instance, if you want, but hello/goodbye makes a lot of sense.

1:20 We know now what we need to do because this is currently the only one being exposed to the real world. Now let's just swap these over, add another implementation signature there, or another signature there rather, and it starts working.

1:35 This is pretty cool, because before we solve this with conditional types, and it was pretty messy code-wise, this is actually pretty simple to read. It's quite declarative. You have a hello and that returns goodbye. Have goodbye and that returns hello.

1:51 Then the implementation signature is left a little bit to not worry too much about what it returns, because technically inside here, and I wonder how far we could push the limits here, if we return a number here, then this one is going to say, "This overload signature is not compatible." If we return just like a blah, blah, blah, blah, blah, then it's actually not quite type-safe, as you can see.

2:15 We can make it a little bit more type-safe, but we have to be pretty strict with ourselves. We can add this return type signature. You notice that function overloads are not a particularly type-safe form of TypeScript, or rather, it means that the implementation part of the function overload is not really held to a very high standard in terms of being super-duper type-safe.

2:41 What I would probably recommend whenever you're doing a function overload is in the implementation signature, type the return type and make sure manually that the return type matches up to this other stuff here. If we put something random in here, then this is going to yell at us. It's going to say, "This overload signature is not compatible with its implementation signature."

3:04 I really do like this code because these overloads are much easier to read. They give you a really good sense just by looking at the function, what it does. For someone who's not au fait with really advanced TypeScript, this is going to be a pretty nice way of handling things for someone just stumbling on this code. Function overloads, they're pretty good. They can pull their weight.

Function Overloads vs. Conditional Types

0:00 Let's now look at this problem, which we've also seen before, the youSayGoodbye and ISayHello function. We're taking in a greeting of goodbye or hello, and then we're returning the correct greeting based on what's passed in.

0:12 You can see here that result youSayGoodbyeISayHello should be goodbye because we're passing in hello, but, in fact, it's goodbye or hello. The same thing is 1 down here.

0:23 Your job is to figure out how we can solve this one, which we previously solved with conditional types with function overloads.

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Understanding generics in functions requires deep knowledge of the TypeScript compiler. You usually have multiple options when it comes to representing a generic in a function - and there's often not a right answer. We'll examine exercises which help illuminate these decisions.

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There's more to generics than inference. In this section we'll cover generics with conditional types, generic currying, and generics on interfaces. Each come with their own challenges.

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Sometimes, you don't need a generic. Function overloads provide another method for inferring types from arguments passed to a function. In this section, we'll compare and contrast overloads with generics - and even discuss how they can be combined.

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By the end of this workshop, you'll be a master of generics and have a good understanding of function overloads, inference, and all the tricks you need.

TypeScript Generics Workshop Welcome

Generics allow you to ensure that your functions are properly typed, often without having to specify the return type.

Replace the unknown Type with a Generic

Typing Functions with Generics

It is possible to specify what can be passed into a generic while still allowing its literal type to be inferred.

Add Constraints to a Generic

Restricting Generic Argument Types

Types passed into a function can be used to infer types in an object that gets returned.

Use Multiple Generics with a Function

Typing Independent Parameters

Generics work really well with Type Helpers. One use case is typing properties that are passed in as part of an object.

Approaches for Typing Object Parameters

In Excalidraw's codebase, we'll examine a sum function that accepts an array and a mapper function used to reduce values down to a number.

Generic Functions in Excalidraw

Here you will learn how generics can be used to allow you to pass in typed properties of any shape when instantiating a class.

Add Types to a Class

Generics in Classes

With generics you can specify that certain properties are present and of a specific type while also allowing for any additional properties to be present.

Add Object Property Constraints to a Generic

Generic Mapper Function

Orta Therox, an ex-TypeScript team member, says generics are crucial to avoid lengthy types in its structural type system, unlike Objective-C's nominal types.

The Importance of Generics in TypeScript

The TypeScript docs for generics have info and examples on creating and using generic types.

Generics section of the TypeScript docs

Learn more about the unknown type in the TypeScript docs

The unknown Type from the TypeScript Docs

The "Generic Constraints" page from the TypeScript documentation provides detailed information on how to define constraints for generic types

Generic Constraints from the TypeScript Docs

The Classes section of the TypeScript docs provides detailed information on how to use generics with classes, including examples and best practices.

Classes section of the TypeScript docs

See a real-world example of how generics are used in a software project.

gesture.ts from the Excalidraw GitHub Repo

A refresher from the Type Transformations workshop lesson, "Support Function Type Constraints with Variable Arguments", showing extends in action.

Total TypeScript - The Extends Keyword in action

Intro to Generics

Many of TypeScript's built-in types support type arguments. With help from generics you can specify the types that a function accepts and returns.

Pass Type Arguments to a Function

Add Type Parameters to a Function

By adding a default generic, you ensure that your functions remain type safe. The generic context follows a similar pattern to the type helper context.

Specify a Default Value

Defaults in Type Parameters

Generic slots can be added to the type arguments of classes or type helpers.

Infer from the Type Arguments of a Class

Infer Types from Type Arguments

Generics are captured at different levels based on where they're called. Here we'll examine how generics are used in a reduce function.

Pass Type Arguments to a Reduce Function

Strongly Type a Reduce Function

Not all libraries support generics by default. Fortunately, there are some techniques for working around this with help from generics.

Use Generics to Type a Fetch Request

Avoid any Types with Generics

The repo for cal.com contains several examples of how passing type arguments allows for overriding the weaknesses of external libraries.

Passing Type Arguments in cal.com

Improving Code Maintainability

MDN reference for the Set object in JavaScript, which is a collection of unique values.

JavaScript Set from MDN

Documentation for the Record utility type in the TypeScript Handbook.

Record Types from the TypesScript docs

TypeScript's reduce method signature on GitHub in the type declaration file used as a reference for JavaScript's built-in objects available in TypeScript.

TypeScript's reduce type signature from its source on GitHub

TypeScript's DOM lib provides the types for interacting with the Document Object Model. 

fetch, Response, and Body from TypeScript's DOM lib source on GitHub

Documentation for the Awaited utility type in the TypeScript Handbook. This type can be used to extract the type of a Promise or Async value.

Awaited Utility Type from the TypeScript docs

A TypeScript class for interacting with the SendGrid email service in the cal.com repository.

Sengrid.ts from the Cal.com Github Repo

Passing Type Arguments

The approach you take to generics impacts how far you have to drill into objects, and the readability of your code on the whole.

Represent Generics at the Lowest Level

Generics at Different Levels

This section is all about figuring out how best to represent these generics in terms of the things that are being passed into functions.

Two Approaches for Typing Object Keys

Typed Object Keys

Generics allow you to type arguments for functions that are passed to other functions.

Constrain a Type Argument to a Function

Make a Generic Wrapper for a Function

Learn how to ensure the accurate literal inference of type arguments using constraints.

Understand Literal Inference in Generics

The choice of representing types in type arguments can impact literal inference. Constraints like "as const" can refine it.

Understand Generic Inference When Using Objects as Arguments

Restrict inputs and infer literal types in TypeScript by applying constraints on the type parameter.

To accept multiple literal types and infer them, T should be extended with union types like "string | number"

Accepting Multiple Literal Types

Inferring Literal Types from any Basic Type

Going with the lowest-level and tightest representation in your generics leads to good results and better inference.

Constrain to the Array Member, Not the Array

Infer the Type of an Array Member

It doesn't necessarily take hard work to get good inference. Choosing which generics to use and where to place them is key.

Two Approaches to Working with Class Names

Generics in a Class Names Creator

Tanner Lindsley explains how to use descriptive names for generics in React Query to ensure proper function flow and passing in the right slots.

Generics in React Query

The official TypeScript documentation for the Keyof Type Operator.

Keyof Type Operator from the TypeScript Docs

The official TypeScript documentation for Type Assertions.

Type Assertions from the TypeScript Docs

The official TypeScript documentation for the Record Utility Type.

Record Utility Type from the TypeScript Docs

Study the source code to gain a deeper understanding of how the Object.keys() method works.

Object.keys() from the TypeScript Source Code

The official TypeScript documentation for Tuple Types.

Tuple Types from the TypeScript Docs

The official TypeScript documentation for the Parameters Utility Type.

Parameters Utility Type from the TypeScript Docs

The official TypeScript documentation for the ReturnType Utility Type.

ReturnType Utility Type from the TypeScript Docs

The Art of Type Arguments

TypeScript isn't clever enough on its own to map a conditional type in the return of a function.

Ensure Runtime Level & Type Level Safety with Conditional Types

Generics with Conditional Types

TypeScript has a limitation of not trusting narrowing inside of a generic function. Using a technique to add better inference gets us around this.

Fixing the "Not Assignable" Error

Fixing Errors in Generic Functions

Curried functions accept arguments that return functions. It's possible to use generic arguments to ensure these returned functions are properly typed.

Fix Type Inference in Curried Functions

Generic Function Currying

Understanding how generics work at different levels of functions will help you when structuring chained arguments and other complicated types.

Understanding Generics at Different Levels of Functions

Generic Interfaces with Functions

Sometimes it's possible to refactor a function with multiple generics to maintain the same functionality with a single generic.

Refactoring Functions with Unnecessary Type Arguments

Spotting Useless Generics

On the flip side of the previous exercise, sometimes additional generics should be added to allow for more specific types.

Improving Type Inference with Additional Generics

Spotting Missing Generics

How tRPC Handles Inheritable Generics

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With factory functions, you can avoid passing the same type argument repeatedly, creating cleaner theming systems.

Create a Factory Function to Apply Type Arguments to All Child Functions

Refactoring Generics for a Cleaner API

In TypeScript, passing multiple type arguments to functions can cause partial inference issues.

A workaround to TypeScript's lack of partial inference can be achieved by splitting type arguments over two function calls.

A Workaround for The Lack of Partial Inference

The Partial Inference Problem

Advanced Generics

Function overloads can be used to handle different input and return types, similar to generics.

Understanding Function Overloads

What is a Function Overload?

It can be difficult to be type safe with a function overload. Do your best to ensure the user gets what they expect.

Specifying Types for an Overloaded Function 

Debugging Overloaded Functions

Function overloads work best when used correctly. Think carefully about what technique you should use when writing your functions.

When to Use Overloads and Unions

Function Overloads vs. Union Types

Function overloads can work together with generics to make functions behave differently based on their input.

Typing Different Function Use Cases

Generics in Function Overloads

Knowing how to read and diagnose problems related to function overloads on interfaces is very important. Here, you'll troubleshoot an overloaded interface.

The Inference Mystery Solved

Solving an Inference Mystery

Function overloads allow for different ways to call your function. This is great for scenarios with different type signatures.

Split Functions Into Two Different Call Signatures

Use Function Overloads to Infer Initial Data

Function overloads are great for preventing one of TypeScript's oddest errors where TypeScript can't infer what type a function should be called with.

Handling Default Arguments with Function Overloads

The "Instantiated with Subtype" Error

TypeScript declaration file for the DOM API that helps you write type-safe and efficient code.

TypeScript DOM Source Code on GitHub

Function Overloads

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Introduce a Type Parameter to Ensure Type Consistency

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Create a Pick Function

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Create a Form Validation Library

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Improve a Fetch Function to Handle Missing Type Arguments

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Using Overloads and Generics to Type Function Composition

Typing a Function Composition with Overloads and Generics

It's possible to dynamically type arguments based on what is being passed in, allowing for stronger typing and better inference.

Adding conditional types and type constraints ensure good inference and strong typing for dynamic arguments.

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Function Overloads vs. Conditional Types

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