Here is an async function `fetchData` that takes in a type argument of `TResult` and a real argument of `url: string`, and then returns a `Promise<TResult>`:

```typescript
const fetchData = async <TResult>(url: string): Promise<TResult> => {
  const data = await fetch(url).then((response) => response.json())
  return data
}
```

It should be similar to any fetch wrapper that you've seen before.

For this exercise, when calling `fetchData` you'll pass in `{ name: string }` and you get back `{ name: string }` after awaiting it.

Then, `data.name` should equal `"Luke Skywalker"`:

```typescript
it("Should fetch data from an API", async () => {
  const data = await fetchData<{ name: string }>(
    "https://swapi.dev/api/people/1"
  )
  expect(data.name).toEqual("Luke Skywalker")

  type tests = [Expect<Equal<typeof data, { name: string }>>]
})
```

Currently, this data is coming back as `unknown`, but it would be great if we had an error message saying `"You must pass a type argument to fetchData"`:

```typescript
it("Should force you to add a type annotation with a helpful error message", async () => {
  const data = await fetchData("https://swapi.dev/api/people/1")

  type tests = [
    Expect<Equal<typeof data, "You must pass a type argument to fetchData">>
  ]
})
```

## Challenge

Your challenge is to find a way to make it so that if the user doesn't pass a type argument to `fetchData`, the `typeof data` would equal `"You must pass a type argument to fetchData"`.

The solution is simpler than you expect!

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

This solution doesn't involve any complex conditional types, or odd mapping. All it requires is a basic understanding of how type arguments work in TypeScript.

When you don't pass a type argument, it gets inferred as `unknown`.

This means that `fetchData` without a type argument returns a `Promise` with `unknown`, which is why the call to `fetchData` in our second test ends up as `const data: unknown`:

```typescript
it("Should force you to add a type annotation with a helpful error message", async () => {
  // const data: unknown
  const data = await fetchData("https://swapi.dev/api/people/1")

  type tests = [
    Expect<Equal<typeof data, "You must pass a type argument to fetchData">>
  ]
})
```

If we remove the `await`, it would be `Promise<unknown>`.

To make sure `unknown` is never available, we can change the default of `TResult` in the type argument to be the error message:

```typescript
const fetchData = async <
  TResult = "You must pass a type argument to fetchData"
>(
  url: string
): Promise<TResult> => {
  const data = await fetch(url).then((response) => response.json())
  return data
}
```

Now in the second test, we see that the type of `data` is now `const data: "You must pass a type argument to fetchData"`.

This is helpful because it allows the user to understand what's wrong.

Notice that back in the first test, we can see that we can pass a type argument to `fetchData` and it will work as expected, but the result can't be constrained.

```typescript
it("Should fetch data from an API", async () => {
  /*
    const data: {
      name: string
    }
  */
  const data = await fetchData<{ name: string }>(
    "https://swapi.dev/api/people/1"
  )
  expect(data.name).toEqual("Luke Skywalker")

  type tests = [Expect<Equal<typeof data, { name: string }>>]
})
```

This means that if you are sure that there will always has a `data` attribute or a `data: any`, then this solution won't work.

But, if you find yourself wanting to pass in a type argument and the result is unconstrained, then this is a neat solution.

Modify a Generic Type Default for Improved Error Messages

0:00 The solution here is probably simpler than you expect. It doesn't involve any crazy conditional types. It doesn't involve any weird mapping. It just involves a little bit of understanding of how type arguments work in TypeScript.

0:13 When you don't pass a type argument, how does it get inferred? It gets inferred as unknown. Now fetchData, when you don't pass in a type argument, basically returns a promise with unknown, which is how this ends up as data unknown. If we remove the await, this would be Promise unknown.

0:34 How do we make it so the unknown is never available? We change the default of TResult and we say TResult equals "You must pass a type argument to fetchData." Now this data is technically typed as any and actually, it's any inside here too, and we end up with Promise TResult.

0:57 What you get is now data is "You must pass a type argument to fetchData," when there's no type argument specified, and data up here is name string. This is really neat actually. What it does is it allows the user to understand what's going wrong, and as soon as we pass a type argument to it, then we say whatever string, then you're good to go.

1:22 Now one thing that's interesting here is that you then can't really constrain TResult. You can't say if you're sure that it always has a data attribute or like a data any for instance, then this wouldn't match that constraint. If TResult has to be of a certain shape, then this doesn't work.

1:43 For unconstrained generics, if you want the user to pass a type argument there, then this is actually a really neat little solution. Well done if you found the solution. Good stuff.

Improve a Fetch Function to Handle Missing Type Arguments

0:00 This is a fun little exercise. We have a fetchData function, which is an async function which takes in a type argument of T results, a real argument of URL string, and then returns promise T results. Very, very similar to any fetch wrapper that you've seen before.

0:16 FetchData here, basically, you're passing name string, and you get back name string after awaiting it. Then data.name = "Luke Skywalker." But we want to make this API pretty bulletproof because currently, this data is coming back as unknown, which is fine, but we would love to have a error message here saying how to get it not unknown.

0:41 Because one thing you could do is just pass a name string here, for instance, and this would be fine, but it would be great if we could have a more idiomatic way to do it and have a way that was consistent across the whole codebase.

0:55 I would like you to find a way where if the user doesn't pass a type argument to fetchData, I want type of data to equal "you must pass a type argument to fetchData." Nasty one. This is an interesting little challenge. You won't need to do anything too fancy here. The solution is simpler than you expect. I'll leave that with you. 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

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

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

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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Improve a Fetch Function to Handle Missing Type Arguments

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