What is the difference between declarative and imperative code in JavaScript?
Imperative describes how to do it (loops, mutations). Declarative describes what to do (map, filter, reduce). FP is declarative: arr.filter(n => n > 1).map(n => n * 2) instead of a for loop with push.
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More FAQs in Functional Programming in JavaScript: The Basics
A style based on pure functions (no side effects), immutability (return new copies, do not mutate), function composition (chain small functions), and declarative style (describe what, not how). JS supports it because functions are first-class.
A function that given the same input always returns the same output and has no side effects (does not modify external state, does not read mutable external state). Pure functions are easy to test and reason about.
Use spread to create new arrays/objects instead of mutating: [...arr, 4] instead of arr.push(4); { ...obj, x: 2 } instead of obj.x = 2. Use libraries like Immer or Immutable.js for complex state.
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