HOF.md

Higher Order Functions

There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies.

Abstraction

Abstractions hide details and give us the ability to talk about problems at a higher (or more abstract) level. Abstraction help to cover up and hide important details which we don't like to share with anyone.

Higher Order Functions

Functions that operate on other functions, either by taking them as arguments or by returning them, are called higher-order functions.

function greaterThan(n) {
  return m => m > n;
}
let greaterThan10 = greaterThan(10);
console.log(greaterThan10(11));
// → true

Another Example...

function noisy(f) {
  return (...args) => {
    console.log("calling with", args);
    let result = f(...args);
    console.log("called with", args, ", returned", result);
    return result;
  };
}
noisy(Math.min)(3, 2, 1);
// → calling with [3, 2, 1]
// → called with [3, 2, 1] , returned 1

1. For Each Loop

There is a built-in array method, forEach, that provides something like a for/of loop as a higher-order function.

["A", "B"].forEach(l => console.log(l));
// → A
// → B

2. Filter

Filter function filters out the elements that don't pass the TEST function passed in it.

It returns a new array that has filtered values.

Suppose we have some SCRIPT data, then:

SCRIPT.filter(s => s.direction == "ttb")

3. Map

Map function is used for mapping each value of the passed iterator object to a particular another value based on the transform method passed.

SCRIPT.map(s=> s.name);

4. Reduce

The higher order operation that computes single vaue from available data, called Reduce operation (or Fold).

It builds a value by repeatedly taking a single element from the array and combining it with the current value.

Parameters to reduce are: Array, Combining function and a Start value.

console.log([1, 2, 3, 4].reduce((a, b) => a + b));
// → 10

function characterCount(script) {
  return script.ranges.reduce((count, [from, to]) => {
    return count + (to - from);
  }, 0);
}

console.log(SCRIPTS.reduce((a, b) => {
  return characterCount(a) < characterCount(b) ? b : a;
}));
// → {name: "Han", …}

Composabiity

function average(array) {
  return array.reduce((a, b) => a + b) / array.length;
}

console.log(Math.round(average(
  SCRIPTS.filter(s => s.living).map(s => s.year))));
// → 1165
console.log(Math.round(average(
  SCRIPTS.filter(s => !s.living).map(s => s.year))));
// → 204