JavaScript: The Definitive Guide, 6th Edition

Functional Programming

JavaScript is not a functional programming language like Lisp or Haskell, but the fact that JavaScript can manipulate functions as objects means that we can use functional programming techniques in JavaScript. The ECMAScript 5 array methods such as map() and reduce() lend themselves particularly well to a functional programming style. The sections that follow demonstrate techniques for functional programming in JavaScript. They are intended as a mind-expanding exploration of the power of JavaScript’s functions, not as a prescription for good programming style.^[14]

Processing Arrays with Functions

Suppose we have an array of numbers and we want to compute the mean and standard deviation of those values. We might do that in nonfunctional style like this:

var data = [1,1,3,5,5];  // This is our array of numbers

// The mean is the sum of the elements divided by the number of elements
var total = 0;
for(var i = 0; i < data.length; i++) total += data[i];
var mean = total/data.length;                  // The mean of our data is 3

// To compute the standard deviation, we first sum the squares of 
// the deviation of each element from the mean.
total = 0;
for(var i = 0; i < data.length; i++) {
    var deviation = data[i] - mean;
    total += deviation * deviation;
}
var stddev = Math.sqrt(total/(data.length-1));  // The standard deviation is 2

We can perform these same computations in concise functional style using the array methods map() and reduce() like this (see ECMAScript 5 Array Methods to ...