For this exercise, we have a function called `youSayGoodbyeISayHello` where you pass in a `greeting`.

If the `greeting` is `"goodbye"`, we return `"hello"`, otherwise we return `"goodbye"`:

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

This all appears to work on the runtime level, but actually it's not working on the type level.

In the tests, we can see that there are errors.

For example, here we are expecting `result` to be `"goodbye"` because we're passing in `"hello"`, but actually we're getting a union type of `"goodbye"` or `"hello"`:

```typescript
const result = youSayGoodbyeISayHello("hello");

// ERROR
type test = [Expect<Equal<typeof result, "goodbye">>];

// hovering shows:
const result: "goodbye" | "hello"
```

The opposite test also errors.

## Challenge

Your challenge is to add a type argument and conditional type to `youSayGoodbyeISayHello` so that it works on both the type and runtime levels.

Hint: The solution uses something unexpected that might seem hacky!

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

Let's build this solution together from our starting point:

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

## Adding a Type Argument

We need to capture a type argument for `greeting` which we can call `TGreeting`, then replace the `unknown` type.

In order to make sure it's inferred properly we could have it `extend string`, but since we know we're only dealing with "hello" or "goodbye" we can be more specific:

```typescript
function youSayGoodbyeISayHello<TGreeting extends "hello" | "goodbye">(greeting: TGreeting) {
    ...
```

This prevents random strings from being passed in, which should be included in the texts.

This change also means that we have `TGreeting` being inferred properly, but we still are getting a union type back.

## Adding a Conditional Type

To fix the inference, we'll use a conditional type to check if `TGreeting extends "goodbye"`.

If it does, we'll return `"hello"`, otherwise we'll return `"goodbye"`. Notice that we have autocomplete working in VS Code at this point:

```typescript
function youSayGoodbyeISayHello<TGreeting extends "hello" | "goodbye">(
  greeting: TGreeting,
): TGreeting extends "hello" ? "goodbye" : "hello" {
  return (greeting === "goodbye" ? "hello" : "goodbye");
}
```

However, there's still an error:

```text
Type '"hello" | "goodbye"' is not assignable to type 'TGreeting extends "hello" ? "goodbye" : "hello"'.
```

This is a common error people see when using conditional types in return types, because TypeScript isn't smart enough to match the runtime code to the type level code.

To fix it, we need to add an `as any` to the return. It might look strange, but it's the only way we can tell TypeScript that we know better in this situation.

```typescript
function youSayGoodbyeISayHello<TGreeting extends "hello" | "goodbye">(
  greeting: TGreeting,
): TGreeting extends "hello" ? "goodbye" : "hello" {
  return (greeting === "goodbye" ? "hello" : "goodbye"); as any
}
```

## Adding a Type Helper

The above will solve the challenge, but it's also possible to use a type helper in order to clean up the code a bit.

The `TGreeting extends` check can be extracted into a `GreetingResult`:

```typescript
type GreetingResult<TGreeting> = TGreeting extends "hello"
  ? "goodbye"
  : "hello";
```

Then the `GreetingResult` replaces the `return`:

```typescript
function youSayGoodbyeISayHello<TGreeting extends "hello" | "goodbye">(
  greeting: TGreeting,
) {
  return (
    greeting === "goodbye" ? "hello" : "goodbye"
  ) as GreetingResult<TGreeting>;
}
```

## The Takeaway

Both solutions work.

The major difference is that when using a type helper, the typing is done in the return type where in the first solution it was being done in the "`as`".

The big takeaway here is that TypeScript isn't clever enough on its own to map a conditional type in the return of a function.

In these situations, adapt one of the solutions from above!

Ensure Runtime Level & Type Level Safety with Conditional Types

0:00 Let's build this one up from scratch and see where we get to. We know that this greeting here, which is currently typed as unknown, we're going to have to capture that in a type argument. We need to know whether it's hello or goodbye that they're passing in so that we can pass the opposite back.

0:16 What we're going to do is we can put in TGreeting here. We'll just capture it in TGreeting. Whenever you do that and you have some function call examples, you should check whether it's inferring properly. Actually, it's not. It's being inferred as string here.

0:32 We can help this along if we want to. We can say, "extends string." Really, we know that this is either going to be hello or goodbye here. What this does, it means you can't pass in something random like blah, blah, blah, blah, blah, which isn't part of the test but is probably something we should include here.

0:50 We've got our greeting TGreeting that's being inferred properly, but we're still getting the union type back. What we should do is type the return type. We're going to do this with a conditional type. TGreeting, we're going to check if it extends goodbye. If it does, then we'll return hello. Otherwise, we'll return goodbye.

1:13 Now, all of our tests are passing, actually. We have this result being typed as goodbye when we pass in hello. We've got hello being passed in there. Because of our constraint, we even get autocomplete here too, which is really nice. Now, if I pass in goodbye, I'm going to get hello. It's very, very pleasing, lovely, but there's an issue.

1:32 Blah, blah, blah, blah, blah is not assignable to type TGreeting extends good...There's a long type error here. There's a real issue here. A lot of people run into this when they first start doing conditional types in return types, which is that really this function...

1:53 TypeScript isn't smart enough to match this runtime stuff, all of these little nodes here, the flow of this code, onto your type. It can't tell that those things match. You need to do an as inside here.

2:09 The cleanest way to do it would be to wrap these in parentheses and say, "as any" here. This may look completely bonkers to you, but it's one of the only ways to actually tell TypeScript that this is what you were trying to do, that you know better than TypeScript in this situation.

2:25 If we look at the second solution here, then this just takes it and wraps it in a type helper there. What we get is you can actually just as that whole thing as the GreetingResult TGreeting. This actually replaces the return type

2:44 If we look at the other one again, actually pull this on the other side here, then what we can see is -- I'll zoom out just a touch -- on this side, we're doing the typing in the return type. On this side, we're doing the typing in the as. I can actually just swap this out if I want to. It still will work there. You get result being goodbye, result being blah, blah, blah.

3:09 The idea is that this raw statement here, TypeScript isn't clever enough to actually map a conditional type on it. If you're using a conditional type in the return type of a function, you're probably going to need to use as somewhere.

Generics with Conditional Types

0:00 For this exercise, we have a function called youSayGoodbye and ISayHello. We're passing in a greeting here. If the greeting is goodbye, we return hello. Otherwise, we return goodbye.

0:10 This all appears to work on the runtime level, but actually, it's not working on the type level. We're expecting result here to be goodbye because we're passing in hello, but we're getting a union type of goodbye or hello. That's interesting. Same is true in the reverse, of course.

0:27 We probably know that we're going to need to add a type argument to this function. We're probably also going to need to use a conditional type as well.

0:36 I'll give you a clue to the solution here, which is the solution uses something unexpected and something that you may feel is a little bit illegal or a little bit troublesome or, "Maybe I shouldn't be using this in the solution." The solution will feel hacky. That's the way of saying it.

0:54 Good luck. Your job here is to try to get this working on the type level as well as the runtime level.

The Generics workshop is your guide to mastering one of TypeScript's most complex features.

Through dozens of exercises, you'll get to grips with generics from the lowest level, up through examples of how your favorite TS libraries use them.





Master TypeScript's generics with exercises from basics to advanced, including conditional types and currying.

TypeScript Generics Workshop

Total TypeScript Complete Volume is a set of five full TypeScript workshops for the advanced-beginner to expert professional TypeScript programmer that wants to take their skills to the next level. This bundle includes all of the Total TypeScript self-paced workshops.

TypeScript Pro Essentials

Type Transformations

TypeScript Generics

Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

TypeScript Pro Complete

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 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

Improve a theme API in TypeScript by creating a factory that returns a function that already has its theme interface passed in.

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?

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.

Match Return Types with Function Overloads

Function Overloads vs. Conditional Types

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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Use a Tuple to Represent a Dynamic Number of Arguments

Create a Function with a Dynamic Number of Arguments

Create a function that chooses specific keys from an object, ensures type safety, and prevents invalid keys.

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Create a Pick Function

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Create a Form Validation Library

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Modify a Generic Type Default for Improved Error Messages

Improve a Fetch Function to Handle Missing Type Arguments

Correctly type a composition of up to five functions with arbitrary input and output types, using overloads and generics.

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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Generics with Conditional Types

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