Functional Programming in JavaScript:

- Why?

• Less bugs because it will be easier to reason about.
• Less time because more re-use of code.

In all functional programmin languagess, functions are values

Divide your code in smaller simpler functions and compose them together using higher-order functions.

A higher order funtion is a function that takes another function as an argument. Examples:

let orders = [
  1340,
  600,
  345,
  1230,
  3456,
  12345
]

Array.filter: Iterates over an array and returns a new array of those items for which the provided funtion returns true Provided functions should return true/false ideally based on some codtion applied on array item.

// Syntax: filter(function(item, index, object) {
//   ...code...
// })

// Get orders that are multiple of 10
orders.filter((x) => x % 10 === 0)

Array.map: Iterates over an array and returns a new array on which provided funtion has performed some operation Provided functions should ideally perfrom some desired operation on the array item and return the new value.

// Syntax: map(function(item, index, object) {
//   ...code...
// })

// Get all orders in float values
orders_float = orders.map((x) => x.toFixed(2))

Array.reduce: Iterates over an array and returns a single value that is obtained by applying a function against each value and an accumulator

// Syntax: reduce(function(accumulator, item, index, object) {
//   ...code...
// }, startValueOfAccumulator)

// Get sum of all values
orders.reduce((acc, x) => acc + x, 0)

Advanced example use of Reduce:

// Group animals by species
let animals = [
  {name: 'A', species: 'dog'},
  {name: 'B', species: 'dog'},
  {name: 'C', species: 'cat'},
  {name: 'D', species: 'rat'},
  {name: 'E', species: 'bat'},
  {name: 'F', species: 'cat'},
]

let res = animals.reduce((species_group, animal) => {
  species_group[animal.species] = species_group[animal.species] || []
  species_group[animal.species].push(animal.name)
  return species_group
}, [])

console.log(res)

Source:

• Mattias Petter Johansson - Functional programming in JavaScript - Youtube Playlist
• Link: https://www.youtube.com/playlist?list=PL0zVEGEvSaeEd9hlmCXrk5yUyqUag-n84