A Functional Programmer’s Introduction to JavaScript from Eric Elliot's Composing Software

intro-functional-programming.js

// EXPRESSIONS AND VALUES

// A variable declared with the const keyword can’t be reassigned. This restriction is a good thing.
const hola = 'Hola';

// TYPES
const arreglo = [1, 2, 3];

const a = 'a';
const b = 'b';

const oA = { a };
const oB = { b };

// It iterates over the properties in oA and assigns them to the new object, 
// then does the same for oB, overriding any keys that already exist on the new object.
const c = { ...oA, ...oB }; // { a: 'a', b: 'b' }

// If object spread operator does not work

const d = Object.assign({}, oA, oB); // { a: 'a', b: 'b'}

const e = Object.assign({}, oB, oA); // { b: 'b', a: 'a'}

// Mutating an existing object rather than creating a new object is usually a BUG.

// DESTRUCTURING
const [t, u] = ['a', 'b'];
t; // 'a'
u; // 'b'

const bleep = {
	blop: 'blop'
}

// Equivalent to const blop = blep.blop;
const { blop } = bleep;

// COMPARISONS AND TERNARIES
4 + 4 === 8; // true

14 - 7 === 7 ? 'Sip!' : 'Nop!'; // Sip!
14 - 7 === 53 ? 'Sip!' : 'Nop!'; // Nop!

// FUNCTIONS AND SIGNATURES

const double = x => x * 2;
double(5); // 10

const orZero = (n = 0) => n;
orZero(); // 0
orZero(2); // 2

// NAMED ARGUMENTS

const createBand = ({
	name = 'Anonymous',
  	avatarThumbnail = '/avatars/anonymous.png'
}) => ({
	name,
  	avatarThumbnail
});

const exilio = createBand({
	name: 'Exilio',
    avatarThumbnail: '/avatars/world-emoji.png'
});

/* { name: 'Exilio', avatarThumbnail: '/avatars/world-emoji.png' } */

// REST AND SPREAD

const unaCola = (cabeza, ...cola) => cola;
unaCola(1, 2, 3); // [2, 3]