Functional Programming in JavaScript

Pure Functions Same input always produces same output. No side effects. Easy to test and reason about.

Immutability Never mutate data. Use spread operator, Object.assign, or structuredClone for copies. Arrays: use map, filter, reduce instead of push, splice.

Composition Build complex functions from simple ones. `const getUserDisplayName = compose(capitalize, getFirstName)`. Use pipe for left-to-right reading.

Higher-Order Functions Functions that take or return functions. map, filter, reduce are the big three. Custom HOFs encapsulate behavior patterns.

Practical Tips - Keep functions small and focused - Favor expressions over statements - Use destructuring for clarity - Pipeline operations on collections