This exercise starts with a `NonEmptyArray` that is currently `unknown`:

```tsx
type `NonEmptyArray`= unknown; 
```

## Challenge

Your challenge is to update ``NonEmptyArray` so that it accepts an argument and turns it into an array with a length of at least one.

As seen in the tests, there should be an error if an empty array is passed in.

Hint: The TypeScript docs on the <a href="https://www.typescriptlang.org/docs/handbook/2/objects.html#tuple-types" target="_blank" rel="noopener">Tuple type</a> may come in handy.

Using TypeScript’s built-in types allows you to add constraints when working with arrays.

The first step to solving this challenge is to update ``NonEmptyArray`to accept`T`. Since we are working with arrays, we can replace` unknown`with an array of`T`:

```tsx
type NonEmptyArray<T> = Array<T>; 
```

This will still accept arrays with a length of zero, so there's more work to be done.

In order to avoid empty arrays, we need to specify a tuple.

For example, we could update `NonEmptyArray` like this:

```tsx
type NonEmptyArray<T> = [T, T, T]; 
```

However, the problem now is that we can't have an array with more than 3 items.

The solution to this challenge is to use the rest parameter `...` along with `Array<T>`:

```tsx
type NonEmptyArray<T> = [T, ...Array<T>]; 
```

The first `T` enforces that there is at least one item in the array, while supporting however many more items may be there thanks to the rest parameter.

This technique is used in the Zod library, and is useful in a variety of situations in application code as well.

Enforce a Minimum Array Length in a Type Helper

0:01 In order to solve this, we need to take this and add a T to it. That's the first step. We need to add a type argument. The second step here is if we add, for instance, Array T here, then this is going to make the first steps of this happy.

0:16 What we're saying here is we're saying, "We have a NonEmptyArray and this is just now going to represent an array," except it's erroring here, because it accepts an array of zero length, which is not good.

0:29 The way to do this is we could specify this with a tuple. We can actually add T, T, T, whatever we wanted to here. This would force us to represent an array with three members in.

0:42 If we had const example is NonEmptyArray with a T here. In fact, let me say number. Then, it would make us go one, two, three.

0:52 It would actually error if we try to add a fourth one here, because source has four elements, but target only allows three.

1:00 The way to do this then is, the way to say at least one is, if you think about this in JavaScript, in fact function arguments even, you can use a rest parameter here.

1:10 You can say, T Array or Array T. Let's use this syntax. What this is doing is it's saying, "OK, you have to have one element in this array, but then the rest of the array could be as long as you want or as short as you want."

1:28 That means that you have to pass in at least one, but if you pass in one, it will be fine. If you want to make this like, you have to pass in at least two, then we would say T, T. Pass in at least three, like this for instance. You get the idea.

1:41 This is a really nice little bit of syntax to process these different types of arrays. This is used in the library zod, for instance, and this is one that you may find is really helpful for application code as well. This TypeHelper has a lot of different use cases.

Constraining Type Helpers to Non-Empty Arrays

0:00 In this exercise, we're going to be creating a type helper called NonEmptyArray. This NonEmptyArray is going to accept an argument here, a T probably. It's going to turn it into an array of at least one length.

0:14 We can see here that we're expecting an error if we pass in an empty array into this function, makeEnum. It's fine if you accept one or three or a hundred different numbers of that rank. That's your challenge. You'll need a little bit of knowledge about tuples as well. Good luck.

The Type Transformations workshop is your guide to TypeScript features and patterns that allow you to manipulate types to produce exactly what you want. As you begin to see how the pieces fit together, you’ll find yourself writing less code that is easier to maintain.

There are over 50 exercises in this workshop, each with a challenge designed to help you learn to wield the power of transforming strings into objects, objects into unions, and everything in between.



Learn to manipulate types for cleaner, maintainable code with over 50 hands-on exercises, from string manipulation to advanced unions and mapped types.

Type Transformations Workshop

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Understanding how to transform types to other types is crucial because many problems in TypeScript come down to how to get one type to become another.

Type Transformations Workshop Welcome

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Typeof Keyword, and Type Level

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Use a Utility Type to Extract Function Parameters

Extract Function Parameters Into A Type

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It’s quite useful to be able to extract the awaited result of a Promise.

Use Utility Types To Extract a Promise's Result

Extract The Awaited Result of a Promise

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Create Unions from Objects Using Two Operators

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Creating types from types is one of the most powerful features of TypeScript

Creating Types from Types

The 'typeof' operator transforms a runtime construct into a type.

The 'typeof' Operator

The 'keyof' operator can be used to extract the keys of an object type.

The 'keyof' Operator

Built-in TypeScript utilities that assist with transforming types.

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Typing Promises and Async Requests

Inference Basics

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Learn three different types of union in this example: enums, unions, and discriminated unions.

Union Terminology Examples

Understand The Terminology Around Unions

Often we’ll need just a part of a union type. 

TypeScript has a utility type that can directly grab specific members of a discriminated union.

Extract From A Union Using a Utility Type

Extracting Members of a Discriminated Union

Sometimes, you need a union with just a small part omitted.

TypeScript has a useful utility type that can extract all but a specified part of a union!

Use a Utility Type to Remove a Single Member of a Union

Excluding Parts of a Discriminated Union

Discover the power of union types in TypeScript and how they handle different possibilities over time, creating a multiverse in your code.

The Power of Union Types in TypeScript

TypeScript gives you a way to a look up a type’s properties individually.

Use Indexed Access Types to Extract Object Properties

Extract Object Properties into Individual Types

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Simple Syntax Used to Access Parts of a Discriminated Union

Extract the Discriminator from a Discriminated Union

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The Annotation Used to Infer an Object's Values as Literal Types

Resolve an Object’s Values as Literal Types

TypeScript gives you a couple of different ways to pass a union to extract multiple members of another union.

Extract Specific Members From A Union with Indexed Access

Create a Union From an Object's Values

You’ll often need to get all of an object’s values. 

There is an operator in TypeScript that gives us an elegant solution to this problem.

Use Two Operators With Indexed Access to Get All of an Object's Values

Get All of an Object’s Values

We can also create unions out of arrays. 

There are ways in TypeScript to make a union from the entire array, or just some specified elements.

Methods Used to Create Unions out of Array Values

Create Unions out of Array Values

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Union Types

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Discriminated Unions

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'as const' Operator

Indexed access types let you access a member of a type: an object value, array member, or class method.

Indexed Access Types

Unions and Indexing

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Template Literal with Strings

Only Allow Specified String Patterns

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Extracting String Pattern Matches with Template Literals

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Passing Unions Into Template Literals

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Using S From ts-toolbelt to Split a String Into a Tuple

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Template Literals in Mattermost

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Template Literal Types

Template Literals

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Introducing Type Helpers

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Let's create a Maybe type that can handle null and undefined

The Unconstrained Maybe Type Helper

Creating a Maybe Type Helper

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Use Constraints to Limit Type Parameters

Ensure Type Safety in a Type Helper

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Add Support for Multiple Types in a Type Helper

Create a Reusable Type Helper

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Set a Default Type Value in a Type Helper

Optional Type Parameters in Type Helpers

Functions can be used as constraints for Type Helpers, but not all functions have the same shape for their arguments.

Support Function Type Constraints with Variable Arguments

Functions as Constraints for Type Helpers

Leverage an essential TypeScript concept to constrain types to anything but null or undefined.

Exclude null and undefined from the Maybe Type

Constraining Types for Anything but null or undefined

Explainer: Type Helpers in Redux

Type Helpers

Conditional types are powerful, and are where TypeScript becomes less about "annotations you add to your JS" and more like its own meta programming language.

Compare and Return Values with Extends and the Ternary Operator

Add Conditional Logic to a Type Helper

We need to handle cases where something undesirable is passed in to a conditional. We’ll do this with a union, nested ternaries, and a special type.

Prevent Unwanted Type Scenarios from Happening

Refine Conditional Logic in a Type Helper

Daniel Rosenwasser discusses how new features are added to TypeScript, using the example of conditional types inspired by Knockout.js.

How TypeScript Added Conditional Types

If you find yourself in some complex conditional logic, harness the power of infer.

Infer Elements Inside a Conditional with Infer

Introducing infer for Conditional Logic

infer can extract out all the members of the type arguments without needing to dive deep into the element itself to understand its structure.

Use infer with Generics to Extract Types from Arguments

Extract Type Arguments to Another Type Helper

In this exercise, you’ll use what you know about infer and template literals to get only a person’s last name from their full name.

Pattern Matching on Template Literals with Infer

Extract Parts of a String with a Template Literal

The benefits of using template literal types for expressing string requirements over regexes, as explained by TypeScript's program manager.

Template Literal Types Were Nearly Regexes

You may want to extract from a library function's return, but don't want to declare a type annotation for it. This is solved with infer and a conditional type.

Optionally Infer the Return Type of a Function

Extract the Result of an Async Function

Long ternary statements become difficult to read. Luckily, there’s an alternative way to write longer conditionals with some help from a built-in type.

Two Methods for Extracting the Result of Multiple Possible Functions

Extract the Result From Several Possible Function Shapes

There’s a gotcha when working with union and conditional types known as the Distributive Conditional Type. In this exercise, we'll find workarounds.

Using Generic Context to Avoid Distributive Conditional Types

Distributivity in Conditional Types

Conditional Types and Infer

TypeScript provides a powerful type useful for turning union types into objects.

Use Mapped Types to Create an Object from a Union

Map Over a Union to Create an Object

Mapped types aren’t just for unions. They can also be used on objects.

Map Over the Keys of an Object

Mapped Types with Objects

It can be useful to transform an object’s keys while you’re mapping them. You can do this in a mapped type without changing your original object!

Remapping Object Keys in a Mapped Type

Transforming Object Keys in Mapped Types

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How Excalidraw uses Mapped Types to Save Lines of Code

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Selective Remapping with Conditional Types and Template Literals

Conditionally Extract Properties from Object

TypeScript allows you to create new objects by mapping over discriminated unions to pull out only the data you need.

Two Techniques for Mapping a Discriminated Union to an Object

Map a Discriminated Union to an Object

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Create a Union of Tuples by Reindexing a Mapped Type

Map an Object to a Union of Tuples

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Map an Object to a Union of Template Literals

Transform an Object into a Union of Template Literals

Use the key remapping technique to transform a discriminated union into a union

Iteratively Map and Remap to Transform Types

Transform a Discriminated Union into a Union

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Mapped Types

Combine techniques from previous exercises to extract dynamic parameters from a path.

Extract from String with Mapped Types, Template Literals, and infer.

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Create a Discriminated Union through Intermediary Transformations

Transform an Object into a Discriminated Union

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Create an Object using Mapped Types, Conditional Types, and TypeScript Keywords

Transform a Discriminated Union with Unique Values to an Object

Partials allow you to construct types where some higher-level properties are optional. DeepPartial goes further by allowing all properties to be optional.

Transcript

Constraining Type Helpers to Non-Empty Arrays

Challenge

Transcript