The Interface in TypeScript will not function properly when used on a variable (Object) that has been declared with the value returned from a function

I am currently in the process of developing an application using Ionic v3. Strangely, I am encountering issues with my interface when trying to assign a variable value returned by a function.

Here is an example that works without any problems:

export interface Quote {
    title: string;
}


export class HomePage {

   quote: Quote;

   constructor(public navCtrl: NavController) {
       var foo = { title: 'Lorem ipsum dorem.' };
       this.quote = foo; // Successfully logs as {title: "Lorem ipsum dorem."}
   }
}

However, when attempting to retrieve the value from a function, things start to malfunction. (Even with the same interface)

quote: Quote;

constructor(public navCtrl: NavController) {
   // Retrieving and setting quote
   this.quote = this.getQuote();
   console.log(this.quote); // Outputs: {title: "Lorem ipsum dorem."}
}

// Retrieve random quote
getQuote(category: string = undefined) : object {
    const quotes = [
        {
            title: 'Lorem ipsum dorem.'
        },
        {
            title: 'Lorem ipsum dorem.'
        },
        {
            title: 'Lorem ipsum dorem.'
        }
    ];

    if (category == undefined) {
        return quotes[Math.floor(Math.random() * quotes.length)];
    }
}

When attempting to package the app into an APK file, the following error message is displayed:

line: 21
  Type 'object' is not assignable to type 'Quote'. Property 'title' is missing in type '{}'.
  L20:  // Retrieving and setting quote
  L21:  this.quote = this.getQuote();
  L22:  console.log(this.quote);

It seems like setting the value of this.quote within the function could be a possible solution. However, I am still curious as to why this approach is not functioning properly. Any help or insights would be greatly appreciated.

Answer №1

The issue lies within the data types being used. You have defined the result as an object:

getQuote(category: string = undefined) : object {..}

However, it should actually be of type Quote:

getQuote(category: string = undefined) : Quote {

Answer №2

Entity in TypeScript refers to a broad group that includes all entities. By casting your entity as Entity, the program loses visibility of its attributes, making it unable to recognize if your entity aligns with the interface.

To address this issue, simply update your function to

fetchData(type: string = undefined) : Data

Answer №3

To ensure code integrity, make sure to properly specify the return value of getQuote as Quote and the array type as an array of quote in TypeScript. This will allow TypeScript to enforce the correct data types. Here's an example implementation:

getQuote(category: string = undefined) : Quote {
    const quotes: Quote[] = [
        {
            title: 'Lorem ipsum dorem.'
        },
        {
            title: 'Lorem ipsum dorem.'
        },
        {
            title: 'Lorem ipsum dorem.'
        }
    ];

    if (category == undefined) {
        return quotes[Math.floor(Math.random() * quotes.length)];
    }
}

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