Argument of type 'MonoTypeOperatorFunction<any>' is not assignable to parameter of type 'UnaryFunction<Observable<any>, Observable<any>>'

i am trying to migrate from rxjs 5 to 6 but i am having difficulties. when i try this

this._connectivity.isOnline().pipe(first()).subscribe((state) => {
  this.syncCheck(user.uid);
});

i am getting this error

Argument of type 'MonoTypeOperatorFunction<any>' is not assignable to parameter of type 'UnaryFunction<Observable<any>, Observable<any>>'.
  Types of parameters 'source' and 'source' are incompatible.
    Type 'import("/home/User/Desktop/projectname/node_modules/rxjs/Observable").Observable<any>' is not assignable to type 'import("/home/User/Desktop/projectname/node_modules/rxjs/internal/Observable").Observable<a...'.
      Property 'map' is missing in type 'Observable<any>'.

Solution

I found the same error with my code:

let source = of([1, 2, 3, 45, 56, 7, 7])
    .pipe(
        filter((x: number) => x % 2 == 0)
    );

TS2345: Argument of type 'MonoTypeOperatorFunction' is not assignable to parameter of type 'OperatorFunction'.

To fix this, remove type from filter function

 filter(x => x % 2 == 0)

Now you have error

The left-hand side of an arithmetic operation must be of type 'any', 'number', so make sure, that this annoying filter gets correct data type

filter(x => Number(x) % 2 == 0) // parse element to number

But now code stops work. Finally, to fix this, change of to from, at the beginning.

let source = from([1, 2, 3, 45, 56, 7, 7])
    .pipe(
        filter((x: number) => Number(x) % 2 === 0)
    )

let source = of(1, 2, 3, 45, 56, 7, 7)
.pipe(
        filter((x: number) => Number(x) % 2 === 0)
    )

So, cause of error was my initial data structure.

I think, that my example can help you with dealing similar problems.