This exercise is a little bit tricky.

Here's a function called `returnWhatIPassInExceptFor1`:

```typescript
function returnWhatIPassInExceptFor1(t: unknown): unknown {
  if (t === 1) {
    return 2;
  }
  return t;
}

```

When I pass in anything other than `1`, the function should return what I pass in (as the name suggests).

For example, if I pass in `a`, `b`, or `c`, the function should return `a`, `b`, or `c` respectively.

However, when I pass in `1`, the result should be of type `2`.

## Challenge

Your challenge is to use function overloads and a generic to replace the `unknown` so the tests pass as expected.

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

Let's work this solution out together.

We'll start by adding a function overload to handle the case of returning `2` when we pass in `1`.

Since the function implementation signature is not exposed to the outside world, we'll create a function overload that matches it:

```typescript
function returnWhatIPassInExceptFor1(t: 1): 2;
function returnWhatIPassInExceptFor1(t: unknown): unknown;
function returnWhatIPassInExceptFor1(t: unknown): unknown {
  ...
```

With these changes the string tests are erroring because they are typed as `unknown`.

To fix this, we can add a generic `<T>` to the overload.

We want it to say when `t` is not `1`, make it the same as type `T`, and return that `T`:

```typescript
function returnWhatIPassInExceptFor1(t: 1): 2;
function returnWhatIPassInExceptFor1<T>(t: T): T;
function returnWhatIPassInExceptFor1(t: unknown): unknown {
  ...
```

With this change in place, we can hover over one of the string tests and see that we have inference:

```typescript
// Hovering over
const a = returnWhatIPassInExceptFor1("a");

// Popup shows:
function returnWhatIPassInExceptFor1<"a">(t: "a": "a") (+1 overload)
```

However if we hover over the test with `1`, notice that there is no type argument being inferred:

```typescript
// Hovering over
const result = returnWhatIPassInExceptFor1(1);

// Popup shows:
function returnWhatIPassInExceptFor1(t: 1): 2 (+1 overload)
```

## Mapping Different Type Arguments with Generics

We can use generics on different function overload signatures to map different type argument setups.

For example, let's add a function overload with a slot that has `T1` and `T2`. Then if the generics were ignored we would say `hello` and then get `goodbye`:

```typescript
function returnWhatIPassInExceptFor1<T1, T2>(t: 'hello'): 'goodbye'; 
```

Now if we called `returnWhatIPassInExceptFor1` and pass in arguments of `wow` and `cool`, it would force us to be in that generic slot or that function overload:

```typescript
// Hovering over
const goodbye = returnWhatIPassInExceptFor1<"wow", "cool">()

// Popup shows:
returnWhatIPassInExceptFor1<T1, T2>(t: "hello"): "goodbye"
```

## Recapping the Solution

Again, here's what the solution looks like all together:

```typescript
function returnWhatIPassInExceptFor1(t: 1): 2;
function returnWhatIPassInExceptFor1<T>(t: T): T;
function returnWhatIPassInExceptFor1(t: unknown): unknown {
  if (t === 1) {
    return 2;
  }
  return t;
}
```

We can have different generic signatures on different function overloads.

This means `a`, `b` and `c` are being inferred correctly, but the `1` isn't.

You may be asking what we should type it as, but for this example I actually like `unknown` and `unknown`. With this setup anything can be passed in, and it's not being constrained.

The `<T>(t: T): T;` represents it pretty well, because we don't know what's going to be passed in. Really it doesn't matter much because the function itself is so simple.

The big takeaway from this solution is that you can add generics to a specific function as needed.

Typing Different Function Use Cases

0:00 The solution here, we know it's probably going to involve a function overload. If we add in a function overload here, let's handle this case first, because that's pretty simple to write. When we get a 1, we're going to return a 2. Now these are erroring because, of course, the function implementation signature is not exposed to the outside world. We need to write something in here. 

0:23 If we add this, then a is going to be unknown, b is going to be unknown, and C is going to be unknown. What we can do is we can add a generic here. We can add a t. This is just like all the other generics that we've used. We can have T extends string, whatever, whatever, whatever. Then we can do exactly the same stuff as we had before.

0:44 In this case, when t is not equal to 1, then we're going to just treat it as T and infer it. If you hover over this, then we get the inference here. You notice if we hover over this, there's no type argument being inferred here. What this means is we can use generics on different function overload signatures to map different type argument setups.

1:10 Let's say we had T1, T2 here. Imagine these were just totally ignored. This means you need to say hello, and then you get goodbye. I've got that exercise on the brain.

1:23 Now if you were to say const goodbye equals returnWhatIPassInExceptFor1, and you were to manually pass in two type arguments, let's say wow and then cool like this, then it would force you to be in that generic slot or that function overload, which is nutty.

1:47 You can have different generic signatures on different function overloads. What this means then is that a is being inferred correctly, b is being inferred correctly, and c is being inferred correctly, but this one isn't. The question is now what do we type this as.

2:03 I like unknown and unknown being here, because we could pass in anything. This isn't being constrained by anything. It really is unknown as the signature. Then you could do unknown or 2. We do need to handle the case here. We have if t equals 1, return 2.

2:32 This represents it pretty well, because we really don't know what's going to be passed in there. It doesn't matter because the function itself is so simple. That's the lesson here, is that you can add generics onto a certain function overload if you want to.

Generics in Function Overloads

0:00 This exercise is a little bit tricky. This function says returnWhatIPassInExceptFor1. What I want this function to do is basically return what I pass in, so this should be typed as a, this should be typed as b, this should be typed as c, except for when you pass in 1 here. 

0:21 You basically want to have...You're probably going to need a generic here to pass in through all the stuff when you just pass in whatever you want to, but when you return in 1, the result should be of type 2. 

0:34 You'll need some function overloads here. You'll also need a generic. That, I think, is all the clues I'm going to give you. Good luck.

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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.

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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.

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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

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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?

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

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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

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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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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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Generics in Function Overloads

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