CODEWITHSUNDEEP
typescript
Eliav Louski
Eliav Louski

Reputation: 5224

Typescript why given function parameter that extends type of this parameter not allowed?

it must be an easy one but I can't figure out why typescript throws an error.

taking this question and tweaking at a little bit:

type Fruit = "Orange" | "Apple" | "Banana";
const myFruit: Fruit = "Apple";
const myString: string = myFruit;   //accepted!

// now with functions
type fruitFunc = (key: Fruit) => void;
type stringFunc = (key: string) => void;
const myFruitFunc: fruitFunc = (key: Fruit) => {};
const myStringFunc: stringFunc = myFruitFunc;   //Error!
// Type 'fruitFunc' is not assignable to type 'stringFunc'.
//   Types of parameters 'key' and 'key' are incompatible.
//     Type 'string' is not assignable to type 'Fruit'.ts(2322)

Why is that happening? I'm not trying to assign a string to fruit, but the opposite...

Upvotes: 1

Views: 294

Answers (1)

Orelsanpls
Orelsanpls

Reputation: 23515

Typescript tells you that string is not compatible with "Orange" | "Apple" | "Banana" which is true.

I know you tell yourself that "Orange" | "Apple" | "Banana" are all strings, so typescript should allow the function assignation but it's not how typescript works.

Typing is here to ensure that functions gets what they asked for. For example, the function myFruitFunc is waiting for Fruit as parameter, if you send a Plant to it, the function won't be prepared and will have unexpected behavior.

Putting the myFruitFunc that is waiting for Fruit inside a function pointer that would wait for a string, makes the use of the function unsafe.

That's why typescript is raising you an error.

All considered, if you still wants to store myFruitFunc into myStringFunc you can cast your function, telling typescript

it's okay, I know what I'm doing.

const myStringFunc: stringFunc = myFruitFunc as stringFunc;

Upvotes: 1

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