Functional Programming

1.1 What is functional programming?

• a programming paradigm
• a code style
• a mindset
• a sexy, buzz-wordy trend

1.2 Why functional JavaScript?

• object-oriented JS gets tricky(prototypes? this?!?)
• safer, easier to debug/maintain
• established community

1.3 Do everything with functions

input → output

Non functional / imperative programming:

var name = “Anjana”;
var greeting = “Hi, I’m ”;
console.log(greeting + name);

> “Hi, I’m Anjana”

functional:

function greet(name) {
  return “Hi, I’m ” + name;
}
greet(“Anjana”);

> “Hi, I’m Anjana”

1.4 Avoid side effects

• use pure functions

Not pure:

var name = 'Foyez'
function greet() {
  console.log("Hi, I'm " + name)
}

Pure:

function greet(name) {
  return "Hi, I'm " + name
}

1.5 Use higher-order functions

• functions can be inputs/outputs

function makeAdjectifier(adjective) {
  return function (string) {
    return adjective + “ ” + string;
  };
}

var coolifier = makeAdjectifier(“cool”);
coolifier(“conference”);

> “cool conference”

1.6 Don't iterate

• use map, reduce, filter

1.7 Avoid mutability

• use immutable data

Mutation (bad!):

var rooms = [“H1”, “H2”, “H3”];
rooms[2] = “H4”;

// rooms => ["H1", "H2", "H4"]

No mutation (good!):

var rooms = [“H1”, “H2”, “H3”];
var newRooms = rooms.map(rm => rm === "H3" ? "H4" : rm);

// newRooms => ["H1", "H2", "H4"]
// rooms => ["H1", "H2", "H3"]

1.8 Persistent data structures for efficient immutability

• libraries: Mori, Immutable.js

1.9 FP libraries for JS

• Mori (http://swannodette.github.io/mori)
• Immutable.js (https://facebook.github.io/immutable-js/)
• Underscore (http://underscorejs.org)
• Lodash (https://lodash.com)
• Ramda (http://ramdajs.com)

1.10 Useful links

• Curry and Function Composition
• Composing Software: The Book
• An introduction to functional programming
• Functional Programming in Javascript: How and Why
• Functional JavaScript: Function Composition For Every Day Use
• An introduction to functional programming in JavaScript

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