Exploring TypeScript Type Manipulation: Mapped Types

One of the powerful features of TypeScript is its ability to manipulate types. This allows developers to create new types based on existing ones and perform various transformations on them. One of the ways to achieve this is through mapped types.

What are Mapped Types?

A mapped type is a way to transform an existing type by iterating over its properties and changing them according to a given transformation function. This allows for the creation of new types that share some similarities with the original type but may have modified or augmented properties.

How to Use Mapped Types

To use mapped types in TypeScript, you need to define a type that describes the transformation you want to apply to the properties. This type is then used to transform another type by looping over its properties and applying the transformation.

Here is an example:

type Point = { x: number; y: number };
// Creating a readonly version of Point
type ReadonlyPoint = Readonly;

// Creating a nullable version of Point
type NullablePoint = Partial;

// Creating a version of Point with all properties optional
type OptionalPoint = Partial;

In the above code, we define a type Point with x and y properties. We then create three new types using mapped types:

ReadonlyPoint: This type is created by applying the Readonly mapped type to Point. This transforms all properties of Point to be readonly.
NullablePoint: This type is created by applying the Partial mapped type to Point. This transforms all properties of Point to be nullable (i.e., they can be set to null or undefined).
OptionalPoint: This type is created by applying the Partial mapped type to Point. This transforms all properties of Point to be optional (i.e., they can be omitted when creating an instance of OptionalPoint).

Applying Transformations with Mapped Types

In addition to the built-in mapped types like Readonly and Partial, you can also define your own transformation functions to apply to types. These functions can be used with mapped types to create customized transformations.

Here is an example:

type Point = { x: number; y: number };
// A transformation function to add a 'z' property to a Point
type AddZProperty<T> = T & { z: number };
// Creating a Point with a 'z' property
type PointWithZ = AddZProperty<Point>;

In this example, we define a transformation function AddZProperty that takes a type T and adds a 'z' property of type number. We then create a new type PointWithZ by applying the AddZProperty transformation to the Point type.

Conclusion

Mapped types in TypeScript provide a powerful way to manipulate and transform types. They allow developers to create new types based on existing ones and apply various transformations to them. By using mapped types, developers can efficiently create modified or augmented versions of types without having to redefine them from scratch.

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Introduction

Welcome to YourSiteName – your ultimate destination for all things related to TypeScript and web development. In this article, we will explore the exciting world of TypeScript Type Manipulation and specifically focus on Mapped Types.

What are Mapped Types?

Mapped types are a powerful feature in TypeScript that allow you to transform existing types by applying certain operations to each property. They are a form of type transformation that enables you to create new types based on existing ones.

Benefits of Mapped Types

Mapped types provide several benefits, including:

Code reusability: You can reuse existing types and apply transformations to create new types, reducing the need for redundant code.
Consistency: Mapped types ensure consistency in your codebase by applying transformations uniformly across properties.
Type safety: Mapped types allow you to enforce specific typing rules by transforming the types of properties in a controlled manner.
Flexibility: Mapped types provide the flexibility to selectively transform properties, allowing you to create custom tailored types based on your requirements.

Commonly Used Mapped Types

Several commonly used mapped types in TypeScript include:

Partial: The Partial mapped type allows you to make all properties of a given type optional, effectively creating a type that allows partial values.
Required: The Required mapped type converts all optional properties of a given type into required properties, ensuring that all properties are present.
Readonly: The Readonly mapped type makes all properties of a given type read-only, preventing any modifications.
Pick: The Pick mapped type allows you to select a subset of properties from a given type and create a new type containing only those properties.
Record: The Record mapped type creates a new type where each key of a given set is mapped to a value of another given type.

Conclusion

In this article, we have explored the concept of Mapped Types in TypeScript and the various benefits they provide. Understanding and utilizing mapped types can greatly enhance your ability to create flexible, reusable, and type-safe code. Stay tuned for more articles on TypeScript and other web development topics on YourSiteName!

Understanding Mapped Types in TypeScript

Mapped types in TypeScript are a powerful feature that allow you to transform and manipulate existing types. They provide a way to create new types based on existing ones by applying a transformation to each property in the original type.

Basic Example

Let’s start with a basic example to understand how mapped types work. Suppose we have a type called Person with two properties: name and age.

type Person = {
name: string;
age: number;
};

Now, let’s say we want to create a new type called PartialPerson which is the same as Person, but with all properties optional. We can achieve this using a mapped type:

type PartialPerson = {
[K in keyof Person]?: Person[K];
};

In this example, we use the keyof keyword to get the keys of the Person type, and then iterate over each key using the in keyword. For each key, we append a question mark (?) to make the property optional, and assign the same type as the original property.

Transforming Property Types

Mapped types also allow you to transform the types of the properties. Let’s say we want to create a new type called PersonWithUpperCaseName which is the same as Person, but with the name property transformed to uppercase:

type PersonWithUpperCaseName = {
[K in keyof Person]: Person[K] extends string ? Uppercase<Person[K]> : Person[K];
};

In this example, we use a conditional type to check if the property type is a string (Person[K] extends string). If it is, we use the Uppercase utility type to transform the string to uppercase; otherwise, we keep the original type.

Combining Mapped Types

You can also combine multiple mapped types to create more complex transformations. Suppose we want to create a new type called PartialPersonWithUpperCaseName which is a combination of PartialPerson and PersonWithUpperCaseName:

type PartialPersonWithUpperCaseName = PartialPerson & PersonWithUpperCaseName;

In this example, we use the intersection type (&) to combine the two mapped types. Now, we have a type that is both partial and has the name property transformed to uppercase.

Conclusion

Mapped types in TypeScript provide a powerful tool for transforming and manipulating existing types. They allow you to create new types based on existing ones by applying a transformation to each property. By using the keyof keyword and conditional types, you can create complex transformations and combine multiple mapped types to achieve the desired result.

Using Mapped Types for Readonly Properties

In TypeScript, mapped types can be used to create new types based on existing ones. One of the powerful features of mapped types is the ability to create readonly properties on an existing type.

What are Readonly Properties?

Readonly properties are properties that cannot be modified once they are assigned a value. This can be useful in scenarios where you want to ensure that certain properties remain constant throughout the lifetime of an object or when you want to prevent accidental modifications.

Creating Readonly Properties with Mapped Types

To create readonly properties with mapped types, you can use the readonly modifier along with the -readonly mapped type operator. The syntax for creating readonly properties using mapped types is as follows:


type ReadonlyType<T> = {
readonly [K in keyof T]: T[K];
};

In the above code, we define a new type ReadonlyType that takes a generic type parameter T. We use the -readonly mapped type operator to specify that each property in T should be readonly. The resulting type ReadonlyType will have all the properties of T with the readonly modifier.

Here’s an example that demonstrates how to use mapped types to create readonly properties:


type Person = {
name: string;
age: number;
};
type ReadonlyPerson = ReadonlyType<Person>;
const person: ReadonlyPerson = {
name: "John Doe",
age: 25
};
person.name = "Jane Smith"; // Error: Cannot assign to 'name' because it is a read-only property.

In the above code, we define a type Person with properties name and age. We use the ReadonlyType mapped type to create a readonly version of Person called ReadonlyPerson. We then define a constant person of type ReadonlyPerson and assign values to its properties. When we try to modify the name property of person, TypeScript throws an error because it is a readonly property.

Conclusion

In this article, we explored how to use mapped types to create readonly properties in TypeScript. Readonly properties can be useful when you want to enforce immutability and prevent accidental modifications. By using the -readonly mapped type operator, you can easily create readonly versions of existing types.

Mapping Property Types with Conditional Types

In TypeScript, we can use conditional types to define mappings for property types based on certain conditions. Conditional types are a powerful feature that allow us to perform type transformations and create more flexible and reusable code.

Conditional Types

Conditional types in TypeScript allow us to define type transformations based on the properties of other types. They use the syntax T extends U ? X : Y, where T and U are types, X is the type transformation when the condition is true (T extends U), and Y is the type transformation when the condition is false.

Conditional types are especially useful when working with mapped types, as they allow us to customize the types of specific properties based on their key or value.

Mapping Property Types with Conditional Types

One common use case for conditional types is mapping the types of properties in an object. Let’s say we have an object with properties of different types, and we want to transform the type of each property based on its key or value.

“`typescript

type User = {

id: number;

age: number;

};

// Mapping property types based on the key

type MappedUser = {

[K in keyof User]: K extends “id” ? string : User[K];

};

// Mapping property types based on the value

type MappedUser2 = {

[K in keyof User]: User[K] extends string ? number : User[K];

};

// Usage

const user: MappedUser = {

id: “1”,

age: 25,

};

const user2: MappedUser2 = {

id: 1,

age: “25”,

};

“`

In the example above, we define a type User with properties id, name, and age. We then use conditional types to map the property types to string for the id property in MappedUser, and to number for the age property in MappedUser2.

By using conditional types, we can create flexible and reusable code that adapts to different scenarios and transformations. This allows us to write type-safe and robust applications in TypeScript.

Conclusion

Conditional types in TypeScript provide powerful tools for mapping property types based on conditions. By using conditional types in conjunction with mapped types, we can create flexible and reusable code that adapts to different transformations. This allows us to write type-safe and robust applications in TypeScript.

Creating Partial Types with Mapped Types

In TypeScript, mapped types provide a powerful and flexible way to transform and manipulate existing types. One useful application of mapped types is creating partial types.

What are Partial Types?

Partial types are types that allow for some properties of an object to be optional. This means that we can create objects where certain properties can be left undefined or omitted.

By default, all properties of an object in TypeScript are required. However, using mapped types, we can create partial types that specify which properties are optional.

Using Mapped Types to Create Partial Types

In TypeScript, the Partial utility type is a pre-defined mapped type that allows us to create partial types easily. The Partial type takes an object type as its argument and returns a new type where all properties are optional.

For example, suppose we have the following object type:

type Person = {
name: string;
age: number;
address: string;
}

We can create a partial type from the Person type using the Partial utility type as follows:

type PartialPerson = Partial<Person>;

The PartialPerson type will now allow the properties name, age, and address to be optional.

Using Partial Types

Partial types can be used to create more flexible and reusable code. They are particularly useful when dealing with optional properties in function arguments and object initialization.

For example, consider the following function:

function printPerson(person: Person) {
console.log(person.name, person.age, person.address);
}

If we want to call this function with an object that only has a subset of the Person type’s properties, we would run into a TypeScript error.

However, by using a partial type, we can modify the function signature as follows:

function printPerson(person: Partial<Person>) {
console.log(person.name, person.age, person.address);
}

Now, we can call the printPerson function with an object that only has some of the properties of the Person type, and TypeScript won’t raise any errors.

Conclusion

Mapped types in TypeScript allow us to create partial types, which are types that allow for some properties to be optional. By using the Partial utility type, we can easily create partial types from existing object types. Partial types provide flexibility and reusability in code, especially when dealing with optional properties in function arguments and object initialization.

Modifying Property Types with Mapped Types

Introduction

In TypeScript, mapped types provide a way to create new types by transforming the properties of an existing type. They allow you to modify or add properties, change the type of existing properties, or create new properties based on the existing ones.

Modifying Property Types

One of the most common use cases for mapped types is to modify the types of existing properties in a type. This can be useful when you need to change the type of a property to meet specific requirements.

To modify property types, you can use key remapping in mapped types. Key remapping allows you to iterate over each property in a type and modify its type. You can specify the new type using the `as` keyword.

type Person = {
name: string;
age: number;
};
type ModifiedPerson = {
[K in keyof Person]: Person[K] | null;
};
const person: ModifiedPerson = {
name: "John",
age: null,
};

In the example above, we have a `Person` type with `name` and `age` properties. We use a mapped type to create a new type called `ModifiedPerson`, where the type of each property in `Person` is modified to include `null` as a possible value. This allows us to assign `null` to the `age` property in the `person` object.

Adding and Removing Properties

In addition to modifying property types, mapped types can be used to add or remove properties from a type.

To add a new property, you can use the `&` operator to include the new property along with the existing properties in the mapped type. To remove a property, you can use the `Omit` utility type or exclude the property using the `Exclude` utility type.

type Person = {
name: string;
age: number;
};
type ExtendedPerson = Person & {
address: string;
};
type RemovedProperty = Omit;

const person: ExtendedPerson = {
name: "John",
age: 30,
address: "123 Main St",
};
const removedProperty: RemovedProperty = {
name: "John",
};

In the example above, we first define a `Person` type with `name` and `age` properties. We then use a mapped type to add an `address` property to the `Person` type, creating a new type called `ExtendedPerson`. We can now assign the `person` object, which includes the `address` property.

In the second example, we use the `Omit` utility type to remove the `age` property from the `Person` type, creating a new type called `RemovedProperty`. We can now assign the `removedProperty` object, which only contains the `name` property.

Conclusion

Mapped types in TypeScript allow you to modify property types, add or remove properties, or create new properties based on an existing type. They provide a powerful way to manipulate types and tailor them to your specific needs.

Applying Mapped Types to Union Types

In TypeScript, mapped types can also be applied to union types. This allows us to transform each member of a union type using the same mapping function, resulting in a new type with transformed members. Let’s explore how this works with some examples.

Example 1: Mapping a Union of String literals to a Union of Number literals

Suppose we have a union type of string literals representing different colors:

type Color = 'red' | 'blue' | 'green';

We can define a mapped type called ColorToNumber that transforms each member of the Color union to a number literal:

type ColorToNumber = {
[K in Color]: number;
};

Now, the type ColorToNumber represents a new union type of number literals corresponding to each member of the original Color union. For example:

type ColorAsNumber = ColorToNumber['red' | 'blue' | 'green'];  // Equivalent to: number | number | number
// ColorAsNumber is now equivalent to: number

By applying the ColorToNumber mapped type to the Color union, we get a type that represents the same set of colors, but with each color transformed into a number.

Example 2: Mapping a Union of Objects to a Union of Their Property Types

Let’s consider a union type of objects with different properties:

type Shape = { type: 'circle'; radius: number; } | { type: 'square'; sideLength: number; } | { type: 'triangle'; base: number; height: number; };

We can define a mapped type called ShapeProperties that transforms each member of the Shape union to its corresponding property types:

type ShapeProperties = {
[K in keyof Shape]: Shape[K];
};

The keyof keyword is used to iterate through each property of the Shape union. In this example, the ShapeProperties type represents a new union type consisting of the property types of each member of the Shape union. For example:

type ShapeProps = ShapeProperties['type' | 'radius' | 'sideLength' | 'base' | 'height'];  // Equivalent to: 'circle' | number | number | number | number
// ShapeProps is now equivalent to: 'circle' | number

By applying the ShapeProperties mapped type to the Shape union, we get a new type that represents the same set of objects, but with each property type extracted and placed in a union.

Conclusion

Applying mapped types to union types in TypeScript allows us to transform each member of a union type using the same mapping function. This can be useful in various scenarios where we need to transform or extract specific information from a union of types. By leveraging the power of mapped types, we can create more flexible and reusable code.

Using Mapped Types to Extract Properties

Mapped types in TypeScript allow you to create new types based on the properties of an existing type. One common use case for mapped types is extracting a subset of properties from an object or interface.

Extracting properties

To extract properties from an object or interface, you can use the Pick mapped type. The Pick type allows you to select only the specified properties from the original type.

Here’s an example:

interface Person {
name: string;
age: number;
address: string;
}
type PersonNameAndAge = Pick;

const person: PersonNameAndAge = {
name: 'John',
age: 25,
};
console.log(person); // Output: { name: 'John', age: 25 }

In the example above, we have an interface Person with three properties: name, age, and address. We use the Pick type to create a new type PersonNameAndAge that only includes the name and age properties from the Person interface. We then create an object of type PersonNameAndAge and assign it to the person variable.

Limitations of extracting properties

It’s important to note that when you extract properties using the Pick type, you can only include properties that exist in the original type. If you try to include a property that doesn’t exist, TypeScript will throw an error.

Here’s an example:

interface Person {
name: string;
age: number;
address: string;
}
type PersonNameAndAge = Pick;  // Error: Property 'email' does not exist on type 'Person'

const person: PersonNameAndAge = {
name: 'John',
age: 25,
};

In the example above, we’re trying to extract the name, age, and email properties from the Person interface. However, since the email property doesn’t exist in the Person interface, TypeScript throws an error.

Conclusion

Using the Pick mapped type in TypeScript allows you to extract a subset of properties from an object or interface. This can be useful when you only need certain properties from an existing type, and want to create a new type that only includes those properties. Just remember to make sure the properties you’re trying to extract actually exist in the original type to avoid TypeScript errors.

Combining Mapped Types with Keyof Operator

In TypeScript, mapped types and the keyof operator can be combined to create powerful type manipulations. The keyof operator is used to retrieve the keys of an object type, and mapped types allow for the transformation or manipulation of these keys and their corresponding values.

Understanding the Keyof Operator

The keyof operator in TypeScript allows us to retrieve the keys of an object type. It produces a union of literal types containing all the keys of the object type.

For example, consider the following object type:

type Person = {
name: string;
age: number;
address: string;
};
type PersonKeys = keyof Person; // "name" | "age" | "address"

Here, the keyof operator retrieves the keys of the Person type and creates a union type containing the string literals “name”, “age”, and “address”. This can be useful when we want to perform actions on all the keys of an object type, such as creating a new type with modified keys.

Combining Mapped Types with Keyof Operator

By combining the keyof operator with mapped types, we can create new types with modified keys or values.

For example, consider the following mapped type that adds the “optional” modifier to all the keys of an existing object type:

type MakeKeysOptional<T> = {
[K in keyof T]?: T[K];
};
type OptionalPerson = MakeKeysOptional<Person>;
// { name?: string; age?: number; address?: string; }

In this example, we defined a mapped type MakeKeysOptional that takes an object type T and iterates over its keys using the keyof operator. For each key K in T, the mapped type adds the optional modifier (?) to the key and sets its corresponding value as T[K]. The result is a new object type OptionalPerson with all the keys of Person made optional.

We can also combine multiple mapped types with the keyof operator to customize both keys and values:

type MakeKeysUppercase<T> = {
[K in keyof T as Uppercase<K>]: T[K];
};
type UppercasePerson = MakeKeysUppercase<Person>;
// { NAME: string; AGE: number; ADDRESS: string; }

In this example, we defined a mapped type MakeKeysUppercase that changes the casing of the keys to uppercase using the Uppercase utility type. The mapped type iterates over the keys of T using the keyof operator and sets the uppercase key as the new key in the resulting type UppercasePerson with the original values from T[K].

Conclusion

The combination of mapped types and the keyof operator in TypeScript allows us to perform powerful type manipulations by customizing keys and values of object types. By understanding these concepts, we can create more flexible and expressive type definitions in our code.

Advanced Examples of Mapped Types in Action

1. Filtering Out Keys

Mapped types can be used to filter out certain keys from an existing type. For example, let’s say we have a type representing a user object:

type User = {
id: number;
name: string;
age: number;
email: string;
};

Now, let’s say we want to create a new type that excludes the “id” and “email” keys from the User type. We can achieve this using a mapped type:

type ExcludeKeys<T, K> = Pick
type UserWithoutIdAndEmail = ExcludeKeys<User, 'id' | 'email'>;

The UserWithoutIdAndEmail type will now only have the “name” and “age” keys.

2. Creating Optional Properties

Mapped types can also be used to create new types with optional properties. For example, let’s say we have a type representing a product object:

type Product = {
id: number;
name: string;
price: number;
};

Now, let’s say we want to create a new type that has all the properties of the Product type, but with optional properties. We can achieve this using a mapped type:

type OptionalProperties<T> = {
[P in keyof T]?: T[P];
};
type ProductWithOptionalProps = OptionalProperties<Product>;

The ProductWithOptionalProps type will now have the same properties as the Product type, but all of them will be optional.

3. Transforming Property Types

Mapped types can also be used to transform the types of certain properties in an existing type. For example, let’s say we have a type representing a user object:

type User = {

id: number;

age: number;

email: string;

};

Now, let’s say we want to create a new type where the “age” property is transformed to a string type. We can achieve this using a mapped type:

type TransformPropertyType<T, K extends keyof T, U> = {
[P in keyof T]: P extends K ? U : T[P];
};
type UserWithTransformedAge = TransformPropertyType<User, 'age', string>;

The UserWithTransformedAge type will now have the same properties as the User type, but the “age” property will be of type string instead of number.

4. Readonly Properties

Mapped types can be used to create new types with readonly properties. For example, let’s say we have a type representing a user object:

type User = {

id: number;

age: number;

email: string;

};

Now, let’s say we want to create a new type where all the properties are readonly. We can achieve this using a mapped type:

type ReadonlyProperties<T> = {
readonly [P in keyof T]: T[P];
};
type ReadonlyUser = ReadonlyProperties<User>;

The ReadonlyUser type will now have the same properties as the User type, but all of them will be readonly.

FAQ:

What are mapped types in TypeScript?

Mapped types in TypeScript are a powerful feature that allow you to transform existing types by applying a certain operation to each property.

How can I create a mapped type in TypeScript?

You can create a mapped type in TypeScript by using the `keyof` operator to iterate over the keys of an existing type and applying a transformation to each key.

Can I modify the type of each property using mapped types?

Yes, you can modify the type of each property using mapped types in TypeScript. For example, you can make all properties optional or readonly, or even change the type of each property to a different one.

Is it possible to create new properties using mapped types?

Yes, it is possible to create new properties using mapped types in TypeScript. You can add new properties to the existing type or remove existing ones.

Are mapped types only applicable to object types?

No, mapped types are not only applicable to object types. They can also be applied to union types, intersection types, and even primitive types like string or number.

Can I use conditional expressions in mapped types?

Yes, you can use conditional expressions in mapped types in TypeScript. Conditional types allow you to create complex mappings based on the type of each property.

Are there any limitations to using mapped types in TypeScript?

While mapped types are very powerful, there are some limitations to keep in mind. For example, they cannot be used to transform the type of a property based on its value, only on its key.