Instructor: Rename this double to multiply, and then let's invoke this with a 3. We need to take the number and return a function. Even though we're not using the number yet, we're still working, because we're invoking this with a number, and then returning this, and it's getting the source from pipe.
Let's pass this number down to a double subscriber and I want to rename this to multiply subscriber, and I'll just pass in that number as well. Now, inside of the constructor here, constructor will take that subscriber or destination, whatever we want to call it, and then a number.
I call super with the subscriber and then assign a number. Now, I can access this number down in my next call and just pass it right here. Now, this number, if I look at console log, this.number should come through this 3 each time, so three times that number will now be 3, 6, 9, 12, 15.
If I just go ahead and duplicate this line and pass in a 4, we'll see now we have a multiply operator that can take any argument. You'll see the 3s, and the 4s, the 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, and the multiples of 3s, and the multiples of 4s.
I'm going to go ahead and extract this to a separate file and the file that's next to it called multiply.js. I will export my function. I need to import subscriber. Import subscriber from RxJS. Now, I can just import from my multiply file. Just multiply.
I'll clear out these unused imports, and you can see we now have a custom operator importing from another file which I can use anywhere in my project.