Error when merging typescript interface function declaration(Merging Interfaces)

Question

interface Dog {
    speak(words: string): string;
}

interface Dog {
    speak(num: number): number;
}

const dog: Dog = {
    speak(wordsOrNum): string | number {
        return wordsOrNum;
    }
}

Interfaces are implemented as in the example code above, and declarations are merged.

However, when I try to implement a function in an object, an error occurs. Is this part possible?

Try to use wordsOrNum: unknown in the implementation

Parzh
– Parzh

2022-02-23 12:37:59 +00:00
Commented Feb 23, 2022 at 12:37 — Parzh
– Parzh, Commented Feb 23, 2022 at 12:37

captain-yossarian from Ukraine · Accepted Answer · 2022-02-23 12:42:25Z

1

In order to make it work you should assure TypeScript that input value is equal to return value:

interface Dog {
    speak(words: string): string;
}

interface Dog {
    speak(num: number): number;
}

const dog: Dog = {
    speak<Input extends string | number>(wordsOrNum: Input): Input {
        return wordsOrNum;
    }
}

Playground

According to your type definition speak(wordsOrNum): string | number wordsOrNum might be a number and return type might be a string or vice versa

answered Feb 23, 2022 at 12:42

captain-yossarian from Ukraine

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2 Comments

zero86 Over a year ago

You are using generics. It seems like a good way. thank you.

zero86 Over a year ago

const dog: Dog = { speak<T extends Dog>(wordsOrNum:T): T { return wordsOrNum; } }

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