The limitations of Typescript types influence the program's behavior

Question

The limitations of Typescript types influence the program's behavior

As a newcomer to the Typescript environment, I am currently developing a test application to familiarize myself with it. However, I have encountered an issue regarding type restrictions that seems to be not working as expected.

In my class, I have defined an array as a member field like this:

listings: Array<ICryptListingItem> = [];

The interface ICryptListingItem is structured as follows:

export interface ICryptListingItem {
  name: string;
  something: number;
}

I am puzzled as to why the compiler allows me to do this:

this.listings = listings.data.map((listing) => {
      return {
        name: listing.name
      }
    });

The objects returned from the listings.data.map method do not fully implement the interface set for the array's type. What concept am I failing to grasp here?

Appreciate any insight you can provide. Thanks in advance.

javascript typescript

Answer 1

Answer №1

When working with TypeScript, it automatically handles certain processes for you. In the code example provided, there seems to be some missing information that may affect the outcome. Here is an example:

export interface ICryptListingItem {
    name: string;
    something: number;
}

class MyThing {
    listings: Array<ICryptListingItem> = [];

    doSomething() {
        const listings = {
            data: [
                { name: "the keeper" },
                { name: "the seeker" }
            ]
        };

        // This part will generate an error
        this.listings = listings.data.map((listing) => {
            return {
                name: listing.name
            }
        });
    }
}

The issue could be related to the type of either listings or listings.data, which might be inferred as any. Consequently, the result of the map function call would also be of type any, making it a permissible type to assign to this.listings.

Answer 2

When working with TypeScript, it automatically handles certain processes for you. In the code example provided, there seems to be some missing information that may affect the outcome. Here is an example:

export interface ICryptListingItem {
    name: string;
    something: number;
}

class MyThing {
    listings: Array<ICryptListingItem> = [];

    doSomething() {
        const listings = {
            data: [
                { name: "the keeper" },
                { name: "the seeker" }
            ]
        };

        // This part will generate an error
        this.listings = listings.data.map((listing) => {
            return {
                name: listing.name
            }
        });
    }
}

The issue could be related to the type of either listings or listings.data, which might be inferred as any. Consequently, the result of the map function call would also be of type any, making it a permissible type to assign to this.listings.

The limitations of Typescript types influence the program's behavior

Answer №1

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