This document provides a brief introduction to functional programming concepts including pure functions, higher order functions, avoiding side effects, and using declarative rather than imperative style. It emphasizes composing software with pure functions, treating functions as first-class objects, and using map and reduce functions over iteration to transform and combine collections of data in a non-mutating way.

1. Functional Programming
A brief, yet practical introduction

2. What is functional programming?
● Immutable data, first class functions, currying, mapping, reducing,
pipelining, recursing…
Software composed of pure functions
“
”

3. Pure Functions
★ A pure function is a function that has no side effects
The function doesn’t rely on data outside the current function, and it
doesn’t change data that exists outside of the current function
★ Given the same inputs, it will always return the same output
“
”

4. DO everything with functions

5. Avoid side effects
var potato = 0;
function increment() {
return potato += 1;
}
increment(potato) {
return potato + 1;
}
non-functional
functional

6. Declarative not imperative
function sumOfSquares(nums) {
var i, sum = 0, squares = [];
for (i = 0; i < nums.length; i++) {
squares.push(nums[i]*nums[i]);
}
for (i = 0; i < squares.length; i++) {
sum += squares[i];
}
return sum;
}
console.log(sumOfSquares([1, 2, 3, 4, 5]));
Imperative
★ Not how, but what to do

7. Declarative not imperative
function sumOfSquares2(nums) {
return nums
.map(function(num) { return num * num; })
.reduce(function(start, num) { return start + num; }, 0)
;
}
console.log(sumOfSquares([1, 2, 3, 4, 5]));
Declarative

8. Higher Order functions
★ A function that takes a function as an input or returns a function as output,
or both
function speakTruth(name) {
return function (string) {
return name + “ ” + string;
};
}
var truthifier = speakTruth(“Ed”);
truthifier(“is bad at foos”);
=> “Ed is bad at foos”

9. Don’t iterate
★ Use map and reduce
A Map takes a function and a collection of items. It
makes a new, empty collection, runs the function on each
item in the original collection, and inserts each return
value into the new collection. It then returns the new
collection.
Map
Reduce takes a function and a collection of
items. It returns a value that is created by
combining the items.
Reduce

10. Map, an eggjampool
var names = [“Mary”, “Isla”, “Sam”];
var code_names = [“Mr. Pink”, “Mr. Orange”, “Mr. Blonde”];
for(var i in code_names) {
var randomIndex = Math.floor(Math.random() * code_names.length);
var randomValue = code_names[randomIndex];
names[i] = randomValue;
}
console.log(names);
=> ["Mr. Blonde", "Mr. Pink", "Mr. Pink"]
Iterative Approach

11. Map, an eggjampool cont.
var names = [“Mary”, “Isla”, “Sam”];
var code_names = [“Mr. Pink”, “Mr. Orange”, “Mr. Blonde”];
names = names.map(function(item) {
var randomIndex = Math.floor(Math.random() * code_names.length);
var randomValue = code_names[randomIndex];
return randomValue;
});
console.log(names);
=> ["Mr. Orange", "Mr. Orange", "Mr. Blonde"]
Using Map

12. Pure Functions, an example
★ A pure function is a function that has no side effects
var a = 0;
function increment() {
return a += 1;
}
increment(a) {
return a + 1;
}
non-functional
functional

13. The end