Demo: single

single with a condition asserts there is only one match.

single( v=> (condition))

  1.For more than one match it emits *SequenceError*
  2.For no match, it emits *NotFoundError*
  3.For EMPTY input, it emits EmptyError

single without a condition asserts there is only one value.

//Emits either of the 2 data streams
//based on Math.random() outcome
let input = iif(()=>(Math.random()>0.5),
               of(2,5,25,5,15,5), //outputs 25
               of(3,5,25,5,25,5)  //outputs "E"
            ).pipe(concatMap(v=>of(v).pipe(delay(100*v))));

//asserts there is only one match
let process = single(v => (v%25 == 0));

//Catch the error and convert it into the given observable
let catchError = catchError(e => of("E"));

let output = input.pipe(process,catchError)
             .subscribe(v=>console.log(v))

Demo: single RunReset

Demo: single