Here we have a function called `returnBothOfWhatIPassIn` with type arguments `T1` and `T2`. It takes in a params of `a: T1` and `b: T2` that is then returned as a tuple:

```typescript
 const returnBothOfWhatIPassIn = <T1, T2>(params: {
  a: T1;
  b: T2;
}): [T1, T2] => {
  return [params.a, params.b];
};
```

As seen in the test, when calling `returnBothOfWhatIPassIn` with `{a: "a", b: 1}`, we should get back `string` and `number`:

```typescript
it("Should return a tuple of the properties a and b", () => {
  const result = returnBothOfWhatIPassIn({
    a: "a",
    b: 1,
  });

  expect(result).toEqual(["a", 1]);

  type test1 = Expect<Equal<typeof result, [string, number]>>;
});
```

And you'll notice there's no errors in this file, and the tests are all passing.

However, there's a way that we can remove one of the generic parameters from the function.

## Challenge

Your challenge is to try to find a way to refactor this file so that one of these generic parameters is removed but the tests will still pass.

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

The solution here is to reduce it into one generic parameter `TParams` that represents the entire object of params being passed in. Then in the tuple is `TParams["a"]` and `TParams["b"]`:

```typescript
const returnBothOfWhatIPassIn = <
  TParams extends {
    a: unknown;
    b: unknown;
  },
>(
  params: TParams,
): [TParams["a"], TParams["b"]] => {
  return [params.a, params.b];
};
```

For comparison, here's what the starting point was:

```typescript
const returnBothOfWhatIPassIn = <T1, T2>(params: {
  a: T1;
  b: T2;
}): [T1, T2] => {
  return [params.a, params.b];
};
```

It is debatable which one is better– the original looks a bit cleaner, but the refactored solution has more constraints.

## The Big Takeaway

The big idea here is that you should be on the hunt for useless generics in your projects and functions.

In the original, we don't need to use both `T1` and `T2`. When hovering over them it's nice to see that `a` is a string and `b` is a number, but we don't get to see the entire parameters being passed in.

Here's what we see hovering over the original `returnBothOfWhatIPassIn`:

```typescript
const returnBothOfWhatIPassIn: <string, number>(params: {
  a: string;
  b: number;
}) => [string, number]
```

Compared to seeing the entire params when we hover in the refactored version:

```typescript
const returnBothOfWhatIPassIn: <{
  a: string;
  b: number;
}>(params: {
  a: string;
  b: number;
}) => [string, number]
```

Ultimately it's up to you which one you use, but experience has taught me that you should try to reduce the number of generic functions and type arguments as much as possible.

This will make it easier to refactor and you're less likely to end up with useless generics cluttering up your functions.

Refactoring Functions with Unnecessary Type Arguments

0:00 The solution here is to basically reduce it into one generic parameter, where you have TParams here. That represents the entire object of params that are being passed in. Then, in the tuple here, you have TParams a and TParams b.

0:15 It's actually debatable which one is better here. This one here is maybe actually a little less cleaner. You're having to do a little bit less mapping here. Whereas this one, it's like there's a bit more constraint going on. You're having to index into it both times.

0:32 I want to put into your mind the idea that you should be on the hunt for useless generics in your projects, in your functions. Really, you don't need to use both of these. When you hover over them here, you're going to see them as string and number being locked in, which is pretty nice.

0:54 When you're in this function, what you see when you hover over is actually the entire parameters that are being passed. This is basically dealer's choice. It's up to you which one you use. Experience has taught me that you should try to reduce the number of type arguments that you possibly can.

1:14 In general, fewer, it's going to be easier to refactor. You're less likely to end up with useless ones cluttering up your functions. This is something you should be aware of whenever you're writing generic functions, is that you sometimes end up with a little bit of cruft, a little bit of crap that you need to get rid of.

Spotting Useless Generics

0:00 In this exercise, we have a function called returnBothOfWhatIPassIn. What we've got here is T1 and T2, the params: { a: T1; b: T2 }, and we're returning them in a tuple, basically.

0:13 What we've got here is they're being inferred as string and number, and we're getting back string and number in this tuple slot. We don't care about being too strict here. We're not looking for the literal "a" or the literal 1 here. We just want string and number.

0:29 You notice there's no errors in this file. The tests are all passing. Everything's great, but we can remove one of the generic parameters here. It seems a little bit clumsy the way that we've done it, and one of these generics is a little bit useless and we can condense them down.

0:49 That's your challenge, is to try to find a way to refactor this file so that we don't have one of these generic parameters.

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.

<Testimonial author={{name: "Tal Faran"}}>Just finished all the Generics sections. Really enjoyed it! It covers realistic problems from our day to day work, and when you reach the end you have strong grasp of how to use generic more efficiently, and in a prettier way that is more accurate of what you wanted to achieve in the first place.</Testimonial>

## Intro to Generics

The best place to start is the start. We'll learn the syntax for assigning generics to functions and classes. We'll learn what's possible with the syntax, and build our very first runtime generic function.

## Passing Type Arguments

TypeScript can infer generics based on arguments passed to functions- but there's an alternative. You can manually pass types as type arguments to functions. We'll cover exactly why this is useful, _when_ it's useful, and what limitations this approach has.

## The Art of Generic Inference

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.

## Advanced Generics

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.

## Function Overloads

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.

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

TypeScript Generics Workshop

Total TypeScript Core Volume is a set of four full TypeScript workshops for the intermediate to expert professional TypeScript programmer that wants to take their skills to the next level. This bundle also includes the Advanced React with TypeScript workshop that will give you expert-level knowledge for typing your React apps.

Type Transformations

TypeScript Generics

Advanced TypeScript Patterns

Advanced React with TypeScript

TypeScript Expert Interviews

Core Volume

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

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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Spotting Useless Generics

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