Functional programming with JavaScript

as the name “functional programming” suggests, this lesson has a heavy focus on functions. In JavaScript you can describe a function in several ways:

• with the function keyword:

function func(params){
  ...doSomething...
  return result
}

• as a method (same behavior as the function keyword):

const methodHolder = {
  func(params){
    ...doSomething...
    return result
  }
}

• store it as an anonymous function in a variable

const func = function(params) {
  ...doSomething...
  return result
}

• and the best part: define it as an arrow function

const func = (params) => {
  ...doSomething...
  return result
}

Arrow functions have some benefits over the classical function/method declaration.

1. the this context gets set at the time of definition
2. it’s a little bit shorter to write (with only one parameter you can even omit the parentheses)

A short excursion on why this is weird in JS:

In the programming model of JavaScript it is totally fine to redefine the owner of a function when you call it, so whenever you use this, you have to make sure that the context is set correctly when you call the function.

A short example:

var person = {
  name: "Clara",
  greet() {
    return "Hello " + this.name;
  }
};
person.greet(); // -> 'Hello Clara'

var greet = person.greet;
greet(); // -> 'Hello '

greet.call({ name: "Peter" }); // -> 'Hello Peter'
greet.call({ noPerson: 500 }); // -> 'Hello undefined'

To solve this, like always, you have several possible ways to go.

1. Avoid this-context by using direct access to the owning object (BAD)

var person = {
  name: "Clara",
  greet() {
    return "Hello " + person.name;
  }
};
person.greet(); // -> 'Hello Clara'

var greet = person.greet;
greet(); // -> 'Hello Clara'

greet.call({ name: "Peter" }); // -> 'Hello Clara'
greet.call({ noPerson: 500 }); // -> 'Hello Clara'

2. Use a class and arrow-function for a fixed this-context:

class Person {
  constructor(name) {
    this.name = name;
    this.greet = () => "Hello " + this.name;
  }
}
var person = new Person("Lotte");
person.greet(); // -> 'Hello Lotte'
var greet = person.greet;
greet(); // -> 'Hello Lotte'

greet.call({ name: "Peter" }); // -> 'Hello Lotte'
greet.call({ noPerson: 500 }); // -> 'Hello Lotte'

3. Go functional and use an independent greeting function

const greet = name => "Hello " + name;
greet("Bert"); // -> 'Hello Bert'

The lesson-learned should be, only use this if you really have to, when possible use pure functions.

Now back to functional programming.

One fundamental building block is the usage of “pure” functions, this definition is close to the mathematical definition of functions f(x) -> y. A pure function returns a value purely defined by the parameters passed into it, and there should be no dependencies to variables defined outside of the function’s scope. The returned value is also always the same if the passed parameters are the same (no randomness or hold state in the function). Besides calculating the resulting value the function has no other effects. So never ever mutate the incoming values.

• pure or impure?

function f(x, y) {
  return x + y;
}

• pure or impure?

let count = 0;
function f(x, y) {
  count++;
  return x + y;
}

• pure or impure?

const f = (x, y) => x + y;

• pure or impure?

const y = 2;
const f = x => x + y;

• pure or impure?

const f = x => x + 2;

• pure or impure?

const f = (x, y) => {
  console.log(x, y);
  return x + y;
};

Another very important paradigm in functional programming is “declarative programming”. Instead of describing the control flow of the program you should care about the logic and behavior. For example use recursion instead of loops.

Task

• write a pure and declarative function calculating of the fibonacci sequence as a start use this imperative and impure implementation:

let a, b, temp;
const fibonacci = num => {
  a = 1;
  b = 0;
  while (num >= 0) {
    temp = a;
    a = a + b;
    b = temp;
    num--;
  }

  return b;
};

Here is a codesandbox with tests to work on this in the browser:

solution

Passing functions around

The parameters passed into a function can be data, like numbers, booleans, strings, objects… and also functions. So you can call a function with a function. Even the returned value can be a function.

Functions taking other functions as arguments or returning functions are called “higher-order-functions”

Getting back to the greeting function, this pattern allows you to define a function to return a function to generate a greeting message for a specific name.

const generateGreetingFunction = name => {
  return () => "Hello " + name;
};
const greetDavid = generateGreetingFunction("David");
greetDavid(); // -> 'Hello David'

or in a shorter way:

const generateGreetingFunctionShort = name => () =>
  "Hello " + name;

The most commonly used higher-order-functions in JS are array-functions like forEach, filter, map and reduce. Those functions take a function as input and call this function with every element of the array.

Examples:

const array = [1, 2, 3, 4, 5];

array.map(num => num * 2); // -> [2, 4, 6, 8, 10]

array.filter(num => num % 2 === 0); // [2, 4]

array.reduce(
  (accumulator, currentValue) => accumulator + currentValue,
  0
); // -> 15

But because normally you don’t develop just for the matter of programming, here is a reason why you should care about all this: It is a powerful tool to handle and transform data. So it is very useful if you are working in any field were you have some larger datasets. In this course we will use it to transform data into UI.

But first some exercises. As a training data set you can use the data from the Star Wars API provided in the code-sandbox.

Here is an example how to get the names from all residents of Tatooine:

const tatooineResidents = people
  .filter(person => {
    return planets.some(
      planet =>
        planet.id === person.homeworld &&
        planet.name === "Tatooine"
    );
  })
  .map(person => person.name);

Tasks:

• get familiar with the array functions
• get all starships with a hyperdrive rating over 2 which appear in the movie “A new Hope”
• are there any transports capable of carrying more than 8 passengers which are cheaper than 400000 credits
• how fast is the TIE bomber?
• how many characters are listed for the movie “The Phantom Menace”?
• list all the characters from the movie “Revenge of the Sith” matched to their species
• what is the longest opening crawl of all movies?

solution