Part of the Fundamentals of Functional Programming document.

Prev: FOFP 1.4 A different approach with map

In our previous example, we defined a helper function square and used it like this:

function square(x) {
  return x * x;
}

nums.map(square)
// [1, 4, 9, 16, 25]

Helper functions

Let's go one step further and write something that will produce helper functions for us. We'll move away from squaring numbers, but stay on the simple theme of increasing integers.

Consider this:

function times(n) {
  return function(x) {
    return x * n;
  }
}

What's going on? We have a function, which is returning a function. Yes, it's that higher-order nature again. Here we're defining a function that takes a multiple n. The scope defined by that function's block (the outermost {...}) closes over the value passed for n, creating a so-called "closure", with the value forged into the inner function that's returned.

The inner function also expects a value x that will be multiplied by that value of n.

Let's have a look at how we might use that:

var double = times(2);
var triple = times(3);

So take triple. What is it that we have, as a result of calling times(3)? Well, we have a function expecting an argument:

triple(6)
// 18

So really, with times, we have a function, that takes a value, and produces a function, that takes a value, that produces a value. If you're familiar with type signatures at all, for example from Haskell, you'd represent this like so:

f :: a -> (a -> a)

or simply:

f :: a -> a -> a

Partial application

You could even use times like this:

times(3)(4)
// 12

In some ways, our simple times function embodies some of the essence of partial application [4.12.1.4]. Calling times with a single argument:

times(3)

is a partial application, and results in a function which is waiting for the second argument:

<function-produced-by-times(3)>(4)

Using helpers

Now we have everything we need to use map to process our list, with a helper function:

nums.map(times(2))
// [2, 4, 8, 8, 10]

Neat!

Next: FOFP 2.1 A look at filter