Is it a bad practice to pass instance as function parameter? [closed]

Question

So I am learning how to use the factory function in javascript/typescript. I created a to-do list project where I had to pass the instance as a parameter to the function. I wanted to ask is it a bad practice to use this approach

eg

const Animal = ()=>{
    const legs = (noOfLegs:number)=>{
        console.log(noOfLegs)
    }
    return {
        legs
    }
}

const lion = Animal();

//not sure how to typeheck the instance passed to the function 
const randomFunc = (lion:ReturnType<typeof Animal>)=>{
    lion.legs(4)
}

secondly, if there is no issue with the passing instance as a parameter to the function then how type-check the parameter?

Answer 1

While you have found an approach that happens to work, it would typically be much simpler to define a class. In TypeScript, a class does two things. First, it defines a JavaScript class with a constructor and so on. Secondly, it defines an interface that we can use for typing. Then:

class Animal {
  legs(noOfLegs: number) {
    console.log(noOfLegs);
  }
}

const lion = new Animal();

const randomFunc = (lion: Animal)=>{
    lion.legs(4)
}

Alternatively, we could keep your existing function but just define an interface in addition. For example:

interface Animal {
  legs(noOfLegs: number): void;
}

const Animal = (): Animal => {
    const legs = (noOfLegs:number)=>{
        console.log(noOfLegs)
    }
    return {
        legs
    }
}

const lion = Animal();

const randomFunc = (lion: Animal)=>{
    lion.legs(4)
}

This works because values and types are in different namespaces, though in practice you might want to consider different names, for example an Animal interface but a makeAnimal function.