Here is a function `remapPerson` that takes in a `key` and a `value` for that key:

```typescript
export function remapPerson<Key extends keyof Person>(
  key: Key,
  value: Person[Key],
): Person[Key] {
  if (key === "birthdate") {
    return new Date();
  }

  return value;
}
```

This problem has a dynamic `key` inside that is checked, and then the appropriate value returned.

For example, we could call `remapPerson` with `'age'` and a `value` of 123123 and it would be typed as a `number`:

```typescript
remapPerson("age", 123123);
// hovering would show remapPerson(key: "age", value: number): number
```

If we call it with `'birthdate'` then it would be typed as a `Date`:

```typescript
remapPerson("birthdate", 123123);
// hovering would show remapPerson(key: "birthdate", value: Date): Date
```

While the above works, we have an error to fix:

```
Type 'Date' is not assignable to type 'Person[Key]'.
Type 'Date' is not assignable to type 'never'.
```

## Challenge

Your challenge is to fix this error by making changes only at the type level. You will not be adding any runtime code.

Hint: It might feel like a hack, but it's not!

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

The solution here highlights a limitation of TypeScript that you need to build into your mental model.

Here's the `remapPerson` function we started with:

```typescript
export function remapPerson<Key extends keyof Person>(
  key: Key,
  value: Person[Key],
): Person[Key] {
  if (key === "date") {
    return new Date();
  }

  return value;
}
```

Having a return type of `Person[Key]` tells us the function that it can be called with any key, or a union of some of the keys.

For example, means that we can pass either `number` or `Date` into `remapPerson`, but TypeScript can't narrow the return type inside of a generic function. The return types will be resolved as `never`.

To get around these strange errors when narrowing inside of generic functions, you need to put an `as` on the return type.

In this case, the return type should be matched to `Person[Key]`:

```typescript
export function remapPerson<Key extends keyof Person>(
  key: Key,
  value: Person[Key],
): Person[Key] {
  if (key === "birthdate") {
    return new Date() as Person[Key];
  }

  return value;
}
```

It might seem annoying to use `as`, but it's often the right choice!

Fixing the "Not Assignable" Error

0:00 The solution here is a little bit annoying and shows a limitation of TypeScript's that you need to build into your mental model. When we have a remapPerson function like this and the return type is Person [Key] , we're basically saying, "OK, this can be called with any key."

0:19 Weirdly, it can also be called with a union type of some of the keys to. I could say remapPerson. If I wanted to, I could manually pass in "age" | "birthdate", something like this. What this means then is I can pass either number or dates into here, so I can pass new Date().

0:42 Because of this wrinkle, TypeScript can't trust any narrowing inside a generic function. What we've got here is we've got this, if key is birthdate, then return a new Date.

1:00 What I should be able to say inside here is it should give me a proper error, if I tried to return a number for the user's birthdate, because it doesn't meet the contract in my person up here, but it's not telling me that. It basically resolves any return types inside these narrowing's as never, because it can't trust that key breaks down at this level.

1:26 What this means, if you start getting these strange errors when you're doing this narrowing inside generic functions is you need to put an as on it. If you like it, then you should have put an as on it. What we've got here is return new Date() as any.

1:41 What's the actual proper type that we should be returning there, because as any feels a little bit gross? Well, we can slap on as Person [Key] . We can match the return type there, because here you can see that value is typed as Person [Key]. If I remove this now, this is still typed as Person [Key], because that's what the value is literally typed as there.

2:04 This can be a frustrating thing when you start to look at generics, especially the inside of generic functions and start to feel, "Oh dear, I'm starting to reach the levels of what TypeScript can do here.

2:17 Trust yourself, trust that when you're in these situations and as is usually fine and usually exactly what you want. These situations can be annoying, but just knowing that should make it feel a little bit easier.

Fixing Errors in Generic Functions

0:00 In this problem, we have most of a function handled for us. We've got a remapPerson thing. What we're doing here is we're saying, "Key extends keyof Person." We're taking in the key and then we're taking in the value of that key.

0:17 We can say, remapPerson, let's say, age and let's say, value 123123. This is then typed as a number there, which is pretty clever in itself. If we get birthdate here, then this is going to be a value: Date. But we're not interested in the usage of this. All we're interested in is solving this error, "Type 'Date' is not assignable to type 'Person[Key]'. Type 'Date' is not assignable to type 'never'."

0:45 I'm intrigued by what you think of this. This isn't a real function, necessarily. It's not one that I would find in my app, but this problem of you have a dynamic key inside here. You're checking if the key is a certain value and then you return a certain value based on that. What it's saying is, "If key is birthdate, then ignore the value that the user passes in."

1:09 I want you to try to find a solution to this error using purely only type level stuff. You're not going to be adding any runtime stuff here. Again, this is going to feel like a hack.

1:21 Good luck.

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 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

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

Refactor a function to handle generic data, enforce property existence, and ensure correct type inference with initial data.

Introduce a Type Parameter to Ensure Type Consistency

Make An Infinite Scroll Function Generic with Correct Type Inference

Create a function to accept dynamic arguments for "click" and "focus" events, adding an extra argument for "click" with a specified shape.

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.

Extracting Object Properties with Reduce and Generics

Create a Pick Function

Develop functions with complex nested structures that use generic types to validate user input based on specified rules.

Add Strong Typing and Proper Error Handling to a Form Validator

Create a Form Validation Library

Modify fetchData to handle missing type arguments to return clear error messages, without compromising its functionality or usability.

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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Fixing Errors in Generic Functions

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