What this code class is about What functional programming is How it applies to JS Look at functional JS libraries Introduction (What?) Functional programming is a way & style of writing code Write logic using re-usable bits of code Programming with functions Why? Improve re-usability, by abstracting: broader abstractions that are not domain specific Improve readability, by leaving out: pointfree Ease refactoring, by centralizing side effects Ease testing, with pure functions How? FP does not depend on specific language features, but some make it easier Use mathematical terms, don't need to remember them but might stumble upon them The FP tools Functions as values Major one is ability to use functions as values, which is possible in JS Higher order functions Functions that return functions Partial application presetting some arguments Currying converting a function that takes multiple arguments into a sequence of functions Composition Combine multiple functions into one functions Compose & pipe What makes a function pure? What is a function? Relationship between a set of inputs and outputs Given the same arguments, it will return the same value Pure functions are cacheable No side effects, doesn't affect anything outside Side effects are not bad, they are needed, but hard to test Immutability less for your brain to track prevents unintentional side effects preserves state Category theory A broad concept, there are categories with rules We contain something in a 'type' that we expect to behave a certain way For example in JS we have arrays that have a map method to transform it's content Exercises Exercise 1 Beginning of https://composed.adamgiese.com/ Exercise 2 Create lambda wrapper for 'api scripts' Exercise 3 Maybe monad Conclusion The scale of FP 0% <> 100% 1-liners (lodash) - Ramda - Sanctuary - Folktale Dealing with arrays Array.map, Array.filter, Array.reduce Array.flat, Array.some, Array.every, Array.slice, Array.concat The extremes 0: quick and dirty, good for one-off scripts 100: not useful, not output/effect/side-effect Where to be on this scale depends on the project and the people you work with Frontend tends to be more left because you don't want libraries JS is adding more FP style support https://github.com/tc39/proposal-pipeline-operator https://github.com/tc39/proposal-partial-application