Type 'Observable<boolean | "">' is not assignable to type 'Observable<boolean>' TS2322

Question

I'm selecting from NgRx store @ngrx/store": "7.3.0".

// typeof state.id == string

export const selectIsSomethingPresent = createSelector(selector, state => {
  // standard null check
  return state.id && state.id !== "12345";
});

and I'm assigning it to an Observable

isSomethingPresent$ !: Observable<boolean>;

someMethod() {
  this.isSomethingPresent$ = this.store.select(selectIsSomethingPresent);
}

I'm getting this error:

Type 'Observable<boolean | "">' is not assignable to type 'Observable<boolean>'.

Why does typescript act like that?

fyi return ("12345" && true) throws same error.

Asaf Aviv · Accepted Answer · 2019-09-03 15:11:20Z

2

Because state.id type is a string, an empty string && true will return an empty string and not a boolean so you need to convert state.id to boolean

return Boolean(state.id) && state.id !== "12345";

edited Sep 3, 2019 at 15:11

answered Sep 3, 2019 at 15:03

Asaf Aviv

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3 Comments

Kacper Cieluch Over a year ago

Them why return "12345" && true throws the same same error?

Andrew Allen Over a year ago

@KacperCieluch see stackoverflow.com/questions/12693787/…. Note you can use !!state.id instead of Boolean(state.id)

Asaf Aviv Over a year ago

@AndrewAllen IMO it is more readable to use Boolean() then !!

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