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Practical Functional Programming in JavaScript - Data Last

richytong profile image Richard Tong Updated on ・2 min read

Welcome back to my series on practical functional programming in JavaScript. Last time, I dove into why functional programming was worth it. This time, I'll elaborate on a core functional programming concept that causes a lot of confusion for newcomers to functional programs: data last.

For the most comfortable read, you should have knowledge of Array.prototype.map and decent programming fundamentals.

Note: I use methods from my functional programming library rubico in a couple places to illustrate my points. I link documentation where applicable.

What is data last?

Data last is a programming convention wherein the data of a procedure is provided as the last parameter. This is in contrast to data first, where the data is the first parameter - you are probably more used to seeing this one.

This is data first. Quite literally, the Array (our data) is first.

[1, 2, 3, 4, 5].map(number => number * 2) // > [2, 4, 6, 8, 10]

This is data last. The Array (our data) is now last.

map(number => number * 2)([1, 2, 3, 4, 5]) // > [2, 4, 6, 8, 10]

map in this case is a partially applied function from rubico.

Documentation for map

Why does this matter?

Consider the program

const double = x => x * 2

const square = x => x * x

const doubleSquare = n => {
  const doubled = double(n)
  const squared = square(doubled)
  return squared

doubleSquare(3) // > 36

doubleSquare here is rather hand-holdy and imperative. However, since data is last for both double and square, we can rewrite doubleSquare using the functional approach in terms of just the two functions.

const double = x => x * 2

const square = x => x * x

const doubleSquare = pipe([

doubleSquare(3) // > 36

Documentation for pipe

Look ma, no variables! Data last allows us to write larger programs as compositions of smaller ones. This is a powerful concept for code reuse, and core to the functional programming paradigm. This idea is extensible at any scale; from small scripts to production workloads, anything you can represent with a function falls under this model.

I'll leave you today with a couple excerpts from the Unix philosophy:

Write programs that do one thing and do it well.
Write programs to work together.

We just discovered a powerful way for programs to work together via a simple convention: data last. Next time, we'll examine how we can consistently write programs that do one thing and do it well. Be on the lookout for Side Effects and Purity.

Posted on Jun 24 by:


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While this is a nice brief introduction, I think you should demonstrate this with some sort of partial application. That's where the ideas really shine.


Would you like to help me with this? I'm adding flatMap as the 24th function github.com/a-synchronous/rubico/is...


Perhaps it would have been better for me to go into partial application a bit more rather than just link the docs for map. Thank you for the feedback, i'll find a way to work it in