AI solution for JavaScript Coding Question on BFE.dev
74. implement Observable Transformation Operators

function map(transformer) {
  return function (observable) {
    return new Observable(subscriber => {
      observable.subscribe({
        next(val) {
          try {
            subscriber.next(transformer(val));
          } catch (error) {
            subscriber.error(error);
          }
        },
        error(err) {
          subscriber.error(err);
        },
        complete() {
          subscriber.complete();
        }
      });
    });
  };
}

Explanation: The map function is being created by accepting a transformer function as an argument. This transformer function takes the value of the observable and returns a new transformed value. We return a new function inside the map function that we call on the observable to create a new instance of the Observable. We also need to create a new observer inside the new Observable instance that will be returned.

The new observer listens to the observable instance passed in as an argument to map function, and whenever the observable emits a new value, the next() callback is executed. Inside this callback, we call the transformer function with the emitted value from the observable that returns a new transformed value. We use this transformed value as argument to call the next() method on the subscriber object representing the new observer we created to wrap our transformed value in a new stream.

If the observable instance throws any error, the error() method is called on subscriber object we created, and if the observable instance completes, the complete() method is called.

Finally, we return the new Observable instance observable and that's how our map transformation operator is created.