Functional programming in JavaScript focuses on treating computation as the evaluation of mathematical functions and avoiding changing state and mutable data. Some key aspects of functional programming include first class functions, reducing side effects, pure functions, and concepts from functional languages like lambda calculus, monads, functors, and higher order functions. While JavaScript is not a purely functional language, many functional programming principles and patterns like immutable data, first class functions, and avoiding shared state can be applied to write more predictable and testable code.

1. Functional Programming
in JavaScript
Len Smith <@ignu>

2. (and
dog
pics)

3. Functional
Programming

4. • Lamda Calculus
• Monads
• Monoids
• Functors
• Combinators
• Reducers
• Transducers
• Lenses
• Higher Order Functions
• Applicatives
• Pattern Matching

5. Scott Wlaschin

6. Scott Wlaschin

7. –Wikipedia
“…treats computation as the evaluation of
mathematical functions and avoids
changing-state and mutable data”
Functional
Programming

9. • First Class Functions
• Reduce Side E
ff
ects
Essentially

10. Natural to
Write
Side Effects

11. not a pure functional language

13. “Lisp in C’s Clothing”

14. You are all functional
programmers

17. Props State
DOM
render()

18. Data
DOM

19. First Class Functions

20. First Class Functions
• Functions as variables
• Accept functions as arguments
• Return Functions as the value from other functions

21. Functions as variables

22. Functions as parameters
Anonymous Function

23. Returning Functions
closure

24. Pure Functions

26. Ramda

27. Impure is handy
debounce()
once()
now()

28. Reduce the
unpredictable
surface area

29. Pure Functions
• Testable
• Portable
• Readable
• Parallelizable
• Memoizable

30. Imperative Memoized

32. Memoization

33. Memoization

36. For Loops
“What not How”

37. This is why modern languages give us
Map/Filter/Reduce

39. Power to
not
name everything

41. getName vs. prop(“name”)

43. Build our own lodash/ramda

44. Identity Function

45. Identity Function

46. Currying

47. Data First.. Why?
_.map(data, function)
Could use partial application…
getNames(people)

48. lodash-fp
ramda

51. composing functions

52. Less Nouns, More Verbs

53. Put it all together

54. https://tinyurl.com/seattlejsfp

56. Some
Opinionated
Tangents
(don't @ me)

57. Immutability
“Just don’t mutate things”
Object.freeze(x)

59. Types

60. “Doing TDD I’m
building a domain
speci
fi
c compiler”
🙃

61. ()=> ()=> ()=> ()=>
Verify all these contracts
n: number n: number n: number n: number

63. Thanks!
Len Smith
@ignu