Join egghead, unlock knowledge.

Want more egghead?

This lesson is for members. Join us? Get access to all 3,000+ tutorials + a community with expert developers around the world.

Unlock This Lesson
Become a member
to unlock all features

Level Up!

Access all courses & lessons on egghead today and lock-in your price for life.


    Updating Data with Scan

    John LindquistJohn Lindquist

    You often need to update the data flowing through the stream with custom logic based on what you need to output. This lesson covers how to use scan for collecting and updating the outputs as your stream progresses.



    Become a Member to view code

    You must be a Member to view code

    Access all courses and lessons, track your progress, gain confidence and expertise.

    Become a Member
    and unlock code for this lesson




    to be able to start and then stop our stream, and then start so that it continues it, two, three instead of going back to zero and one.

    You'd think, "OK, let's create a variable. Call it count. Set it to zero." Then in my subscribe block, I can just say console.log, count++. I'll hit save.

    Then now, when I start, we'll get zero, one, two. I'll stop and then start again. Everything works as expected. Then again, this is wrong, undo all of these. The proper way to gather and collect data in RxJS is to use the scan operator.

    Scan works just like reduce in JavaScript arrays. We take a function and initializer. Our initializer is just going to be a count of zero or an object with a count of zero. Then, that comes into this function as an accumulator.

    When this function is called, I get this accumulator as this first argument. We'll just go ahead and return a new object with a property called "count," which is accumulator.count plus one. For something more advanced, you'll probably use object assign, but we just have one property for now.

    I'll go ahead and save. I'll hit start. We go one, two, three. I'll hit stop, and then hit start again, and re-continue five, six, seven. This now reads as clicking a start button, which switches to a timer that can stop when you click stop.

    Then, each time a new interval is pushed or you get a new tick, we start with this value which is this accumulator here.

    Then we run this function, which is just going return an object with a count on it. It's going to take that previous accumulator find account property, add one. Then is going to be pushed into my subscribe and logged out...