[00:00] I've got a fun exercise for you here. We have a type of sandwich, and we're expecting the sandwich type to be equal to a bunch of different things. Rye sandwich with cheese, rye sandwich with ham, rye sandwich with salami, brown sandwich with cheese, brown sandwich with ham, brown sandwich with salami. Basically, all the possible combinations of rye, brown, white, and cheese, ham, salami,

[00:18] which ends up being these various possibilities. Your job is to try to work out the most efficient way to type this sandwich type while essentially using as few lines of code as possible. You can do this in a single line of code using template literal types. Good luck.


In this exercise, we want to build a `Sandwich` type.

This `Sandwich` is expected to encompass all possible combinations of three types of bread (`"rye"`, `"brown"`, `"white"`) and three types of filling (`"cheese"`, `"ham"`, `"salami"`):

```tsx
type BreadType = "rye" | "brown" | "white";

type Filling = "cheese" | "ham" | "salami";

type Sandwich = unknown;
```

As seen in the test, there are several possible combinations of bread and filling:

```tsx
type tests = [
  Expect<
    Equal<
      Sandwich,
      | "rye sandwich with cheese"
      | "rye sandwich with ham"
      | "rye sandwich with salami"
      | "brown sandwich with cheese"
      | "brown sandwich with ham"
      | "brown sandwich with salami"
      | "white sandwich with cheese"
      | "white sandwich with ham"
      | "white sandwich with salami"
    >
  >,
];
```

## Challenge

Your challenge is to use a template literal type to define the `Sandwich` type. This will require combining template literal types with union types, and can be done in only one line of code.


By combining template literal types with union types, you can create a type that represents a string that matches one of several patterns.

[00:00] Okay, the key lesson with this is that when you pass a union to a template literal type, it basically like expands. So let's say we have, okay, we need bread type and we need filling. So we can start to see that we need to pass bread type into this sandwich.

[00:16] And so we can add something to the template literal type, we can say sandwich like that. And we get rye brown and white sandwiches. But what happens if we also pass in the filling? So let's say, and we say filling inside here, you might think that we'd end up with sort

[00:33] of, I don't know, some sort of strange combination, or it wouldn't sort of expand out as you like might imagine. But what we get is we get rye sandwich cheese, rye sandwich ham, rye sandwich salami, we get all the possible like setups for this. And so what you get if you add another bread type, then it's going to go even further.

[00:52] And then you get so many more like 20 more here. Rye sandwich cheese rye, rye sandwich cheese brown, rye sandwich cheese white, rye sandwich ham rye, rye sandwich ham brown. Isn't this crazy? And so to fulfill this, we just need to say with filling. Look at that. And so you get this amazing, amazing just spreading of all of the things that you add

[01:12] in to a template literal. You can go further with this as well. You can say like, let's say this is 20 bucks. And so this will actually put in into the template literal. So it will sort of be stringified there. Let's say it's 20 bucks and let's maybe add a dollar sign or maybe it's 40 bucks here.

[01:31] Now you've got 20 on some and you've got 40 on others. I mean, this is just such powerful stuff. And like being able to just build out unions very quickly from this. It's a power that you need to use sometimes, but quite rarely.

[01:48] And mostly I'm showing it to you because it's pretty fun being able to build this stuff. And when you see in the future, oh yes, I've got a use case for this. You'll remember the bread type and the filling and how they go together.


When you pass a union type to a template literal, it expands.

Following the pattern of the tests, we can see that the desired results are named:

```tsx
bread "sandwich with" filling
```

That means we should pass the `BreadType` and `Filling` unions to the `Sandwich` template literal with the string `"sandwich with"` in between:

```tsx
type BreadType = "rye" | "brown" | "white";
type Filling = "cheese" | "ham" | "salami";
type Sandwich = `${BreadType} sandwich with ${Filling}`;
```

TypeScript generates all the feasible pairings, leading to the type `Sandwich` being the same as:

```tsx
| "rye sandwich with cheese"
| "rye sandwich with ham"
| "rye sandwich with salami"
| "brown sandwich with cheese"
| "brown sandwich with ham"
| "brown sandwich with salami"
| "white sandwich with cheese"
| "white sandwich with ham"
| "white sandwich with salami"
```


Using template literals like this enables you to quickly construct unions using other unions' combinations.

While this can be useful, it's a capability that should be used cautiously.

For each additional union you add, the number of combinations increases exponentially.

For example, if you add another type of bread along with a couple possible prices, it can get out of hand quickly:

```tsx
type Sandwich = `${BreadType} with ${Filling}: ${20 | 40}`;
```

While this challenge was more fun, there are actually some legit use cases for this functionality.

By passing a union to a template literal type, you can generate a type that represents all possible combinations of the union.

Passing Unions to Template Literal Types

Represent All Possible Type Combinations with Template Literals

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Unexpected Token Error in the Browser

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Compile TypeScript to JavaScript with the TypeScript CLI

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Compiling TypeScript Files to a Directory

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Create a new project with Vite and TypeScript to improve upon our current setup.

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Use Vite for a Frontend App with TypeScript

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Comparing Vite with tsc

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Quickly Create Scripts with tsx

TypeScript in The Build Process

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Learn about the two different syntaxes for defining arrays in TypeScript.

Two Syntaxes for Arrays

Add an Array to an Object

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Representing an Array of Objects

Arrays of Objects in TypeScript

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Rest Parameters in TypeScript

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Tuples for Precise Array Structures in TypeScript

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Function Types

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The `void` Type

Typing an Event Listener

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TSDoc Comments

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The Organize Imports Hotkey in VS Code

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Handling Null Values in TypeScript

Understanding union types and assignability is essential to working effectively with TypeScript.

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Represent All Possible Type Combinations with Template Literals

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