What we get

So, now, does FP get you these five characteristics?

• In FP, the goal is writing separate independent functions, which are joined together to produce the final results.
• Programs written in functional style usually tend to be cleaner, shorter, and easier to understand.
• Functions can be tested on its own, and FP code has advantages for that.
• You can reuse functions in other programs, because they stand on their own, not depending on the rest of the system. Most functional programs share common functions, several of which we'll be considering in this book.
• Functional code is free from side effects, which means you can understand the objective of a function by studying it, without having to consider the rest of the program.

Finally, once you get used to FP ways, code becomes more understandable and easier to extend. So, it seems that all five characteristics can be ensured with FP!

For a well balanced look at reasons for FP, I'd suggest reading Why Functional Programming Matters, by John Hughes; it's available online at www.cs.kent.ac.uk/people/staff/dat/miranda/whyfp90.pdf. It's not geared towards JS, but the arguments are easily understandable, anyway.