Why should you prefer pure functions in JavaScript?
Pure functions (same input, same output, no side effects) are easier to test, debug, and reason about. They do not depend on or modify external state, which makes them predictable and safe to use in any order.
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More FAQs in Functional Programming Best Practices in JavaScript
Prefer pure functions, use immutability (spread, new copies), use map/filter/reduce instead of loops, compose small functions, avoid side effects in callbacks, always pass an initial value to reduce, use const by default, and use pipe/compose for pipelines.
Use spread to create new arrays/objects: [...arr, 4] instead of arr.push(4); { ...obj, x: 2 } instead of obj.x = 2. For complex state, use libraries like Immer (with produce) or Immutable.js.
For readability and immutability, yes. But for very large arrays or performance-critical code, a single loop may be faster than chaining multiple methods (each creates a new array). Optimize only if profiling shows a problem.
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