What is the reason for encountering a TypeScript error when using a union type?

Question

What is the reason for encountering a TypeScript error when using a union type?

interface Bird {
  age:string,
  eat:()=>any
}

interface Fish {
  age:string,
  swim:()=>any
}
type Pet = Fish | Bird;

everything looks good so far, defining a Pet type

const pet:Pet={
  age:"sd",
  eat:()=>{}
}

when trying to return a Pet type, an error occurs.

function test():Pet{
  return {
    age:"sd",
    eat:()=>{}
  }
}

test().eat(); //results in a TypeScript error

I'm puzzled as to why they are both the same TypeScript type, yet test().eat() throws an error.

typescript

Answer 1

Answer №1

Looking at your code:

interface Bird {
  age:string,
  eat:()=>any
}

interface Fish {
  age:string,
  swim:()=>any
}
type Pet = Fish | Bird;

Therefore, objects of type Pet:

Will have the attribute age since it's present in both Fish and Bird, and
Will have either swim() or eat() because lacking both would mean it's neither a Fish nor a Bird, although we are uncertain which one (note that it could have both).

The issue arises from your factory function test():Pet being defined to return a Pet that might not include the eat() method for a Fish. Consequently, calling test().eat() results in a type error. While it is true that test() actually returns a Bird, this information is not conveyed through the return type.

To address this problem, consider changing test():Pet to one of the following:

test():Bird, or
Omitting the return type for test() and letting type inference determine it.

Answer 2

Looking at your code:

interface Bird {
  age:string,
  eat:()=>any
}

interface Fish {
  age:string,
  swim:()=>any
}
type Pet = Fish | Bird;

Therefore, objects of type Pet:

Will have the attribute age since it's present in both Fish and Bird, and
Will have either swim() or eat() because lacking both would mean it's neither a Fish nor a Bird, although we are uncertain which one (note that it could have both).

The issue arises from your factory function test():Pet being defined to return a Pet that might not include the eat() method for a Fish. Consequently, calling test().eat() results in a type error. While it is true that test() actually returns a Bird, this information is not conveyed through the return type.

To address this problem, consider changing test():Pet to one of the following:

test():Bird, or
Omitting the return type for test() and letting type inference determine it.

What is the reason for encountering a TypeScript error when using a union type?

Answer №1

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