为什么流类型转换不能按预期对字符串文字起作用

Question

For the below example why type casting is not working in Flowtypes? What should be an ideal way of doing it?

type typeA = {
  name: 'ben' | 'ken',
};
type typeB = {
  name: string,
};
const objA: typeA = { name: 'ben' };
const objB: typeB = objA;

It gives error

Cannot assign `objA` to `objB` because in property `name`: Either  string [1] is incompatible with  string literal `ben` [2]. Or  string [1] is incompatible with  string literal `ken` [3].

However, for typescript, it is fine.

Answer 1

This is actually a TypeScript flaw IMO and Flow does it right. Let's see why:

type A = {
  name: 'ben' | 'ken';
}

type B = {
  name: string;
}

const a: A = { name: 'ben' }
const b: B = a;

b.name = 'jen';

console.log({ a });
// this logs { a: { name: 'jen' } } <- see how a.name has an invalid value!

In JS when you write b = a means that b is an "alias" for a and in fact they're the same object.

So if you make changes to b then those changes also reflect on a , so if you're permitted to "lean" the name type definition from a given list of strings to a general string, you can go and change a.name with "illegal" or better unwanted values!