是否可以在Flow中将Object强制转换为确切类型？

Question

I'm wondering how one might go about casting an Object to a user defined exact type in Flow. For example:

//@flow

type exactType = {|x : number|}

function test(a : Object) : exactType {
  if(typeof(a.x) === 'string'){
    return (a : exactType);
  }
  throw new Error('Incorrect type!')
}

Is it even possible to write such a function? Ie is it possible to convice the type checker that parameter a has only a single field named x with type number ?

Answer 1

There is no safe way to do this. However, you can always cast through any to circumvent the typechecker:

return ((a: any): exactType);

Ideally, you would refactor your code to avoid using Object , since it's an unsafe type. However, if that's not possible the best you can do is to carefully circumvent the typechecker like this.