A presentation on Functional Programming with Javascript by Deepankar Chopra, Senior Software Engineer at Ticketmaster

1. Functional Programming
With JavaScript
Deepankar Chopra
Senior Software Engineer
Ticketmaster

2. » What is Functional Programming?
» Mutations
» Side-Effects
» Pure Functions
» Higher Order Functions
» Composition
» Advantages & Disadvantages of FP
» How does FP differ from Object-Oriented programming
2Overview

3. What is Functional Programming?
Functional programming (often abbreviated FP) is the process of
building software by
» composing pure functions,
» avoiding
» shared state,
» mutable data
» side-effects.
» Managing state flows through composition
3

4. Mutations
Mutable is a type of variable that can be changed after it is created.
In JavaScript, only objects and arrays are mutable, not primitive
values.
(You can make a variable name point to a new value, but the
previous value is still held in memory.)
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5. Mutations 5
let a = { foo: 'bar' };
let b = a;
a.foo = 'test’;
console.log(a,b);
//{foo: "test"} {foo: "test"}
// avoiding mutation
let a = { foo: 'bar' };
let b = Object.assign({}, a);
a.foo = 'test’;
console.log(a,b);
//{foo: "test"} {foo: "bar"}

6. Side Effects
If a program / function modifies the state of something else
(outside its own scope), then it is producing a side effect.
This could effectively include something like "display a character on
the screen", or it could be changing the value stored in some
arbitrary RAM location, or anything similar.
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7. Side Effects include
» Modifying any external variable or object property (e.g., a global
variable, or a variable in the parent function scope chain)
» Logging to the console
» Writing to the screen
» Writing to a file
» Writing to the network
» Triggering any external process
» Calling any other functions with side-effects
7

8. Pure Functions
» Must return a value
» Can’t produce any side-effects
» Must return the same output for a given input
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9. Pure Functions Example 9
let a = 10;
function impure1(x) {
return x + a;
}
function impure2(x) {
a = 30;
return x + a;
}
console.log(impure1(10));//20
a+=10;
console.log(impure1(10));//30
console.log(impure2(10));//40
function pure(x, y) {
return x + y;
}
console.log(pure(10,10));

10. Composition
Function Composition is a mathematical concept, by which the
result of one function becomes the input of the next, and so on.
Composing functions together is like putting together a series of
pipes for our data to flow through.
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11. Composition Example
function addOne(x) {
return x + 1;
}
function timesTwo(x) {
return x * 2;
}
console.log(addOne(timesTwo(3))); //7
11

12. Higher Order Functions
A higher-order function is a function that does at least one of the
following:
» takes one or more functions as arguments
» returns a function as its result.
12

13. Higher Order Functions Example
function greaterThan(n) {
return function(m) { return m > n; };
}
let greaterThan10 = greaterThan(10);
let greaterThan11 = greaterThan(11);
console.log(greaterThan10(11));//true
console.log(greaterThan11(11));//false
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14. Advantages of FP
» Pure functions are easier to reason about
» Testing is easier, and pure functions lend themselves well to
techniques like property-based testing
» Debugging is easier
» Programs are more bulletproof
» Programs are written at a higher level, and are therefore easier
to comprehend
» Parallel/concurrent programming is easier
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15. Disadvantages of FP
» Not suitable in every situation
» Needs to be clubbed with Object oriented programming in some
cases.
» Makes heavy use of recursion
15

16. How does FP differ from OOP 16
Functional Programming
» Primary Manipulation Unit is
“Function”
» Stateless Programming Model
» Functions with No-Side Effects
» Order of execution is less
importance
Object Oriented Programming
» Primary Manipulation Unit is
Objects(Instances of Classes)
» Stateful Programming Model
» Methods with Side Effects
» Order of execution is more
important.

17. Any questions?
You can mail me at
deepankar.chopra@gmail.com
17THANKS!