Functional programming in JavaScript

FUNCTIONAL
PROGRAMMING IN
JAVASCRIPT
Or, “How lambdas can change your life”

Some basic questions
 What is functional programming and how does
it differ from imperative programming?
 What is JavaScript?
 What is functional programming in JavaScript,
and how can it help me?

Two Models of Computation
 Lambda calculus: Emil Post, 1936
 Turing Machine: Alan Turing, 1936-1937
 Proved equivalent in computational power by
Turing in 1937

Turing Machine
 Infinite tape
 Head moves over tape, can read symbol,
change symbol, go left, or go right
 Modifying state in a particular order, according
to instructions
 “Mechanized symbol evaluation based on
assignment and time ordered evaluation”

Imperative Languages
 Assignment sequences
 Same name can be associated with several
values
 Fixed evaluation orders
 New values can be associated with the same
name through command repetition

A classic imperative loop
function fib(n) {
var a = 0, b = 1, f = 1;
for (var i = 2; i <= n; i++) {
f = a + b;
a = b;
b = f;
}
return f;
};

Order is important
var x = 1, y = 2, buffer;
buffer = x;
x = y;
y = buffer;
// x = 2, y = 1
var x = 1, y = 2, buffer;
x = y;
buffer = x;
y = buffer;
// x = 2, y = 2

Lambda Calculus
 Roots in mathematical logic and proof theory
 Turing machines = Kleene’s recursive function
theory = lambda calculus = generalised von
Neumann machines
 Attempt to describe computation in terms of
computable functions

Functional Languages
 Based on structured function calls: a(b(c(3)))
 So what? You can nest functional calls in C!
 But in pure functional languages, this is the
only way to transform values: a name is only
ever associated with one value
 Functions as first-class citizens
 No assignment = no side effects
 Can be executed in different orders but give
the same result

Recursion instead of looping
function fib(n) {
if (n <= 2) {
return 1;
}
return fib(n-1) + fib(n-2);
}

Functions as first-class citizens
var arr = [42, function() { return 42; }];
var obj = {key: 42, func: function() { return 42; }}
var x = function() { return 42; };
var y = function() { return function() { return 42} };
42 + (function() { return 42; })()
//=> 84
function weirdAdd(n, f) { return n + f() }
weirdAdd(42, function() { return 42 });
//=> 84

JavaScript
 Developed by Brendan Eich at Mozilla
 Scheme in the browser
 “Lisp in C’s clothing”
 Supports many programming paradigms:
 Classical object-oriented
 Prototype-based
 Imperative
 Functional

Functional JavaScript
 Functions as first-class citizens
 Anonymous functions
 Native methods:
 .map()
 .reduce()
 .forEach()
 Event-based programming (DOM API,
XmlHttpRequest)
 Closures

.map()
var numbers = [1, 4, 9];
var roots = numbers.map(Math.sqrt);
console.log(roots);
var doubledRoots = roots.map(
function(n) {
return n * 2;
}
);
console.log(doubledRoots);

.map() imperative alternative
var numbers = [1, 4, 9];
var roots = [];
for (var i=0; i<numbers.length; i++) {
roots.push(Math.sqrt(numbers[i]));
}
console.log(roots);
var doubledRoots = [];
for (var i=0; i<roots.length; i++) {
doubledRoots.push(roots[i] * 2);
}
console.log(doubledRoots);

.reduce()
[0, 1, 2, 3, 4].reduce(
function(previousValue, currentValue, index, array) {
return previousValue + currentValue;
}
);

Event callbacks
document.addEventListener('click', function() {
console.log('You clicked somewhere!');
});
var clicked = function() {
console.log('You clicked somewhere!’)
};
document.addEventListener('click', clicked);

Closures
function makeFunc() {
var name = "Mozilla";
function displayName() {
console.log(name);
}
return displayName;
}
var myFunc = makeFunc();
myFunc();

Self-executing functions
var counter = (function(){
var n = 0;
return function() {
console.log('Times run: ‘+(++n));
}
})();

99 Bottles of Beer: imperative
var lyrics = [];
for (var bottles = 99; bottles > 0; bottles--) {
lyrics.push(bottles + ' bottles of beer on the wall');
lyrics.push(bottles + ' bottles of beer');
lyrics.push('Take on down, pass it around');
if (bottles > 1) {
lyrics.push((bottles - 1) + ' bottles of beer on the wall.');
} else {
lyrics.push('No more bottles of beer on the wall!');
}
}

99 bottles of beer: functional
function lyricSegment(n) {
return _.chain([])
.push(n + ' bottles of beer on the wall')
.push(n + ' bottles of beer')
.push('Take one down, pass it around')
.tap(function(lyrics) {
if (n > 1) {
lyrics.push((n - 1) + ' bottles of beer on the wall');
} else {
lyrics.push('No more bottles of beer on the wall');
}
})
.value();
}

99 bottles of beer: functional
function song(start, end, lyricGen) {
return _.reduce(_.range(start, end, -1),
function(acc, n) {
return acc.concat(lyricGen(n));
}, []);
}

10 green bottles!
function greenBottles(n) {
return _.chain([])
.push(n + ' green bottles sitting on the wall')
.push(n + ' green bottles hanging on the wall')
.push('And if 1 green bottle should accidentally fall')
.tap(function(lyrics) {
if (n > 1) {
lyrics.push('There'll be '+ (n - 1) + ' green bottles hanging on the wall');
} else {
lyrics.push('There'll be no more bottles hanging on the wall');
}
})
.value();
}

Conclusions
 JavaScript brought functional programming
into the mainstream
 Its functional features are indispensable for
event-driven programming and for general
elegance
 Just a glimpse of what “pure” functional
languages provide (immutability, purity,
algebraic data types)
 But enough to make JavaScript a powerful and
flexible language
 Explore underscore.js!

Functional programming in JavaScript

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