Problems inferring type with typescript

Question

I have a TypeScript function which removes a "path" in a Firebase database. It return a Promise which then resolves to a boolean flag indicating whether content was actually there in the first place (aka, no need to delete if it doesn't exist).

The function looks like this:

export function remove(path: string): Promise<boolean> {
  return exists(path)
    .then(existance => {
      return existance
        ? getRef(path).remove()
          .then(() => Promise.resolve(true))
        : Promise.resolve(false);
    })
    .catch((err: any) => {
      console.log(`There was an error trying to remove the path "${path}": `, err);
      return Promise.reject(err);
    });
}

This, however, leads to the error messages below (note the difference in message is based on where in the expression i'm hovering):

If anyone's interested in the exists() function -- which, btw, has no problems with TypeScript -- it is:

export function exists(path: string): Promise<boolean> {
  return getRecord(path)
    .then(snap => snap.val()
      ? Promise.resolve(true)
      : Promise.resolve(false)
    )
    .catch(Promise.reject);
}

Anyway, i'm at a loss as to why I'm getting these errors and would love any insight anyone may have.

p.s. i'm using TypeScript 2.2.2

---

UPDATE: as has been pointed out in the comments ... these errors appear to only occur when using the popular Bluebird promise library

Answer 1

There is currently are less than intuitive ways to get Bluebird promises to override the global Promise that TypeScript provides. The issue is discussed here:

• DefinitelyTyped Issue
• This one is probably more informative actually

So in short, my problems were more the impact of using Bluebird Promises in some cases but native Promises in others. To force the use of Bluebird a working solution is to:

/src/[xxxx].js

import * as Bluebird from './bluebird';

/src/bluebird.js

declare module 'bluebird-global' {
  import * as Bluebird from 'bluebird';
  global { export interface Promise<T> extends Bluebird<T> {} }
}

In this way you should always get Bluebird promises and the error I ran into will be avoided.

---

Update ... even better solution. Include the @types/bluebird-global module and you're done!

Note: if in your tsconfig.js file you're using the "types" parameter (most do not) then add bluebird-global to the section:

types: [
  "bluebird-global"
]