Consider Using Mapped Object Type Instead: Improved Efficiency

When it comes to specifying dynamic keys, using union types can create potential inefficiencies. One common solution to this problem is called a discriminated union, which encloses the union in a struct and includes an enum member that indicates the member type currently stored in the union. However, there is another option to consider: using a mapped object type instead.

What is a Mapped Object Type?

A mapped object type is a type that maps values from an existing object to a new object. This is particularly useful when dealing with dynamic keys, as it allows for flexible and efficient key-value pairings. In TypeScript, mapped object types are implemented using index signatures, which specify the property name and its corresponding value type.

Advantages of Using a Mapped Object Type

There are several advantages to using a mapped object type over union types. For one, it allows for dynamic key creation and flexible key-value pairings without sacrificing efficiency. Additionally, mapped object types enable easy iteration and transformation of keys and values, making it ideal for dynamic data structures.

Example of Using a Mapped Object Type

Let’s say we have an object that contains user information, but the keys are dynamic and change depending on the user. Instead of using a union type, we can create a mapped object type to handle this scenario efficiently:

type User = { 
  [key: string]: string | number; 
}; 

This code allows for flexible key-value pairings and dynamic key creation without sacrificing efficiency. Later on, if we need to access or transform the keys or values, it can be done easily through iteration or other methods.

While union types in a discriminated union can offer a solution to dynamic key-value pairing, there is another option worth considering: mapped object types. These types provide similar functionality while also allowing for flexible key creation and easy key-value iteration and transformation.

Understanding Mapped Object Types

In TypeScript, objects can be defined and mapped according to their property types. This is known as a mapped object type. By accessing the properties of another type and altering them, we can produce a new type that retains the original properties with slight modifications. In this way, mapped object types bring greater efficiency and organization to our code.

Pros and Cons of Using Mapped Object Types

Pros

• Improved efficiency in code: Mapped object types provide improved efficiency by allowing for the creation of new types without duplicating code.
• Greater accuracy in type checking: Mapped object types allow for more precision in type checking, resulting in fewer potential errors in the code.
• More flexibility in specifying dynamic keys: With mapped object types, developers have more flexibility in specifying dynamic keys, which can be useful in certain applications.

Cons

• Can be more complex to understand and implement: Mapped object types can be difficult to understand and implement, especially for developers who are new to TypeScript.
• May not always be necessary for simpler projects: Mapped object types may not always be necessary for simpler projects or those with lower complexity.

How to Use Mapped Object Types

Step 1: Define the Key Type

The first step in using mapped object types is to define the key type. This is done using the syntax key in type. For example, if you want to create a mapped object type that has string keys, you would define the key type as key in string.

Step 2: Define the Value Type

The second step is to define the value type. This is done using the same syntax as the key type: value in type. For example, if you want the values in your mapped object type to be of type number, you would define the value type as value in number.

Step 3: Implement the Mapped Object Type

Once you have defined both the key type and the value type, you can implement the mapped object type. This is done by enclosing the key-value pair in curly braces, and separating each pair with a semicolon. Here’s an example:

interface MyMappedObjectType {
   [key in string]: value in number;
}

In this example, we have defined an interface called MyMappedObjectType that has string keys and number values. You can use this interface as a type for any object that has string keys and number values.

Examples of Mapped Object Types in Action

Mapped object types have proven to be a valuable tool in TypeScript projects, providing increased accuracy and efficiency in code. Here are some real-life examples of how mapped object types have been successfully implemented:

Example 1: Omitting Properties

In a project that requires a user to log in, certain user data may need to be omitted from the response to ensure secure data transmission. By using the Omit mapped object type, certain properties can be removed from the user object before it is sent back to the client.

For example:

interface User {
  name: string,
  email: string,
  password: string,
  age: number
}

const user: User = {
  name: "John",
  email: "[email protected]",
  password: "password123",
  age: 25
}

type PublicUser = Omit
const publicUser: PublicUser = {
  name: "John",
  email: "[email protected]",
  age: 25
}
,>

As shown in the example, the password property is removed from the User interface using Omit and a new type, PublicUser, is created with the password property omitted.

Example 2: Readonly Properties

In a project that requires some properties to be immutable, the Readonly mapped object type can be used to make certain properties read-only.

For example:

interface Config {
  apiUrl: string,
  maxRequests: number
}

const config: Config = {
  apiUrl: "https://example.com/api",
  maxRequests: 10
}

config.apiUrl = "https://example2.com/api" // Allowed

type ReadonlyConfig = Readonly
const readonlyConfig: ReadonlyConfig = {
Api",
  maxRequests: 10
}
 error

As shown in the example, the Readonly mapped object type is used to make the configuration properties read-only, preventing the apiUrl property from being updated.

Example 3: Merging Interfaces

In a large project, interfaces may need to be merged to maintain organization and clarity. Merging interfaces using the & operator makes it easy to add new properties while maintaining distinct interface types.

For example:

interface User {
  name: string,
  email: string
}

interface Admin {
  adminLevel: number
}

const user: User = {
  name: "John",
  email: "[email protected]"
}

const admin: Admin = {
  adminLevel: 2
}

interface User {
  name: string,
  email: string
}

interface Admin {
  adminLevel: number
}

interface AdminUser extends User, Admin {}

const adminUser: AdminUser = {
  name: "John",
  email: "[email protected]",
  adminLevel: 2
}

As shown in the example, the two interfaces, User and Admin, are merged using the & operator to create a new interface, AdminUser, with all properties of both interfaces.

Conclusion

Using a mapped object type in TypeScript can be beneficial for maintaining code clarity and avoiding errors caused by implicit relationship between variables. TypeScript provides built-in mapped types such as Omit, Partial, Readonly, Exclude, Extract, NonNullable, and ReturnType that serve as utilities for developers. By considering the benefits and drawbacks of using mapped object types, developers can decide whether implementing them in their own projects is necessary.

References

• TypeScript Documentation on Mapped Types
• LogRocket Article on Mapped Object Types
• Codeburst Article on Mapped Object Types