Question

Consider how we would have written the previous example (finding the person with the earliest year of birth) without higher-order functions. The code is not that much worse.

var min = ancestry[0];
for (var i = 1; i < ancestry.length; i++) {
  var cur = ancestry[i];
  if (cur.born < min.born)
    min = cur;
}
console.log(min);
// → {name: "Pauwels van Haverbeke", born: 1535, …}

There are a few more variables, and the program is two lines longer but still quite easy to understand.

Higher-order functions start to shine when you need to compose functions. As an example, let’s write code that finds the average age for men and for women in the data set.

function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}
function age(p) { return p.died - p.born; }
function male(p) { return p.sex == "m"; }
function female(p) { return p.sex == "f"; }

console.log(average(ancestry.filter(male).map(age)));
// → 61.67
console.log(average(ancestry.filter(female).map(age)));
// → 54.56

(It’s a bit silly that we have to define plus as a function, but operators in JavaScript, unlike functions, are not values, so you can’t pass them as arguments.)

Instead of tangling the logic into a big loop, it is neatly composed into the concepts we are interested in—determining sex, computing age, and averaging numbers. We can apply these one by one to get the result we are looking for.

This is fabulous for writing clear code. Unfortunately, this clarity comes at a cost.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.