如何在Flowtype中将返回值定义为truthy？

Question

With a function like this the return value will always be truthy, not necessary true , and not necessary any . Is there any way to define that the return value for this function will be truthy ?

function cleanTruthy (value: any) {
  if (!value) return true
  return value
}

Answer 1

It seems this is possible by overloading a method in a separate module declaration. The following works (I'm not sure if it's possible without module declarations):

/* @flow */

declare function cleanTruthy(V: false | 0 | null | typeof undefined | ""): true;
declare function cleanTruthy<V: any>(v: V): V;


const truthy: 1 = cleanTruthy(1);
const falsy: true = cleanTruthy(false);

Here is a working version .

However, type polymorphism is not usually the best way to go, and flow will have to recompile every time you change a module file.

A better approach might be to create a function like this, that returns a solid boolean based on the logic of truthiness:

function isTruthy(v: any): boolean {
  return !!v;
}

if(isTruthy(v)) {
  // do something
}