如何修复以下javascript代码以使其与flowtype一起使用？

Question

Given the following snippet I am getting the error below... how should I can fix it?

This type is incompatible with the expected return type of number enum.

type Size = 1 | 2 | 3;
const GetSize = (): Size => {
    const result: number = parseInt(Math.random() * 20);
    if (result > 3) {
        return 3;
    }
    return result;
};

You can try it online ...

Edit:

The issue is related to the conversion from integer to enum.

Answer 1

Math.random() returns a "floating point" number between [0, 1). So it can return 0.5 or 0.31231. The types are indeed incompatible. Even if it returned either 0 or 1 with equal probability, you'd still be outside the constraints of your Size type.

One way to fix this would be to do something like return (int)result + 1 . But I don't know if Flow is smart enough to understand that. A proper way would be to have if/else s for all cases you anticipate, and explicitly return 1, 2 or 3 based on those. Like you're doing for results larger than 3.

Answer 2

Math.random() returns a random number between 0 and 1, it's return type is number .

Your defined return type is stricter than that, it's 1 | 2 | 3 1 | 2 | 3 1 | 2 | 3 . Math.random() doesn't comply with with the return type you defined, so it can't be returned. That's the error.

Answer 3

It is not possible to cast it because in flow number is not a subtype of 1 | 2 | 3 1 | 2 | 3