Xtykutl

The following code results in an error:

type NextItem = string | { [string]: string };



const next: NextItem = 'foo';

const isString = typeof next === 'string';

const toName = isString ? next : Object.keys(next)[0];

                                 ^ string [1] is not an object.

but getting rid of the isString variable fixes it:

type NextItem = string | { [string]: string };



const next: NextItem = 'foo';

const toName = typeof next === 'string' ? next : Object.keys(next)[0];

I understand why, but I'm hoping that someone might offer a better solution. I need to reuse the isString variable and would like to keep my code both DRY and SIMPLE (easy to read). So no "clever" (hacky) solutions please.

Flow's refinements are generally based on direct checks at the time of use, so it will only ever see your isString variable as a boolean with no special meaning.

That leaves you with a few separate options:

• Tweak the control flow of your code so there are two clear branches, assuming there are many more checks for isString, you can always make a clear branch to handle both cases.


• 
  

  Inline the typeof next === 'string' check.

  
  
  

  const toName = typeof next === 'string' ? next : Object.keys(next)[0];
  
  

  
  


• 
  

  Use a predicate function to centralize the isString logic

  
  
  

  function isString(arg: mixed): boolean %checks {
  
     return typeof arg === "string"; 
  
  }
  
  
  
  const toName = isString(next) ? next : Object.keys(next)[0];