Differentiating Typescript will discard attributes that do not adhere to an Interface

Question

Differentiating Typescript will discard attributes that do not adhere to an Interface

Currently, I am working with an API controller that requires a body parameter as shown below:

insertUser(@Body() user: IUser) {}

The problem I'm facing is that I can submit an object that includes additional properties not specified in the IUser interface.

What would be the most straightforward and efficient approach to ensure that the user strictly adheres to the defined type, or alternatively, how can I clone it while maintaining only the properties outlined in the IUser interface?

javascript typescript

Answer 1

Answer №1

My approach is to focus on creating a framework-agnostic solution. The goal is to address how TypeScript can restrict passing values of types with additional fields, emphasizing the need for exclusive behavior within the type itself.

It's important to note that enforcing such behavior solely at the type level may not always be practical. While compile-time checks can ensure type consistency, runtime validation becomes necessary when dealing with data from APIs.

To implement exclusive type behavior at the type level, consider the following:

type IUser = {
  name: string;
  lastname: string;
}

type OnlyExistingFields<Base, Extender> = {
  [K in keyof Extender]: K extends keyof Base ? Extender[K] : never
}

type IMoreUser = IUser & {
  age: number;
}

const user1: IUser = {
  name: 'John',
  lastname: 'Doe'
}

const user2: IMoreUser = {
  name: 'Jane',
  lastname: 'Doe',
  age: 30
}

declare function insertUser<U extends OnlyExistingFields<IUser, U>>(user: U): void

insertUser(user1) // 👍 correct type
insertUser(user2) // 🔴 error due to extra field

The above implementation ensures that only arguments of a specific type (or its exact subset) are accepted, maintaining type exclusivity.

While the type-level check is effective, it's crucial to acknowledge that runtime enforcement presents challenges. A runtime converter can help eliminate unwanted fields:

// Converter function to extract only IUser fields from an object
function toIUser<T extends IUser>(input: T): IUser {
    return {
      name: input.name,
      lastname: input.lastname
    }
}
const convertedUser = toIUser(user2); // convertedUser now has only IUser fields

Answer 2

My approach is to focus on creating a framework-agnostic solution. The goal is to address how TypeScript can restrict passing values of types with additional fields, emphasizing the need for exclusive behavior within the type itself.

It's important to note that enforcing such behavior solely at the type level may not always be practical. While compile-time checks can ensure type consistency, runtime validation becomes necessary when dealing with data from APIs.

To implement exclusive type behavior at the type level, consider the following:

type IUser = {
  name: string;
  lastname: string;
}

type OnlyExistingFields<Base, Extender> = {
  [K in keyof Extender]: K extends keyof Base ? Extender[K] : never
}

type IMoreUser = IUser & {
  age: number;
}

const user1: IUser = {
  name: 'John',
  lastname: 'Doe'
}

const user2: IMoreUser = {
  name: 'Jane',
  lastname: 'Doe',
  age: 30
}

declare function insertUser<U extends OnlyExistingFields<IUser, U>>(user: U): void

insertUser(user1) // 👍 correct type
insertUser(user2) // 🔴 error due to extra field

The above implementation ensures that only arguments of a specific type (or its exact subset) are accepted, maintaining type exclusivity.

While the type-level check is effective, it's crucial to acknowledge that runtime enforcement presents challenges. A runtime converter can help eliminate unwanted fields:

// Converter function to extract only IUser fields from an object
function toIUser<T extends IUser>(input: T): IUser {
    return {
      name: input.name,
      lastname: input.lastname
    }
}
const convertedUser = toIUser(user2); // convertedUser now has only IUser fields

Differentiating Typescript will discard attributes that do not adhere to an Interface

Answer №1

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