Transforming streams with combinators

When we work with streams, we often want to refine the values in a stream with some logic. In Bacon.js, combinators enable refinement.

Combinators let us build complex structures with simple functions. Combinators solve the same problem as the composition pattern in object-oriented programming.

For example, this is how we can use the map combinator:

doubledValue = Bacon.once(1).map(number => number * 2)

Commonly used combinators in Bacon.js

• map – transforms values in the stream
• filter – lets through values in the stream
• flatMap – spawns a stream with a new source
• combine – combines the latest values of two streams using a two-arg function
• merge – merges two streams into one stream that delivers events from both

There are many other combinators in Bacon.js, but the above ones are the fundamental combinators. Once you understand them, you can rather easily understand all the other combinators that you encounter.

Exercise: the map combinator

Create a stream that sequentially emits the numbers 1, 2 and 3. Then transform that stream with the map combinator such that the resulting stream will emit the values 1, 8 and 27.

Exercise: the filter combinator

Again, create a stream that sequentially emits the numbers 1, 2 and 3. With the help of the filter combinator, let through only values that are even.

Next: the flatMap combinator

The flatMap combinator captures a fundamental programming concept. Let’s take a closer look at it.