Q345- What are pure functions?

A pure function is a function that:

1. Always returns the same output for the same input
2. Does not cause side effects

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🔹 Characteristics of Pure Functions

✅ 1️⃣ Deterministic Output

function add(a, b) {
  return a + b;
}

Same input → same output ✔

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✅ 2️⃣ No Side Effects

A pure function:

• Does NOT modify external variables
• Does NOT mutate input objects
• Does NOT perform I/O (API calls, DOM changes)

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❌ Impure Function Examples

❌ Modifies external state

let count = 0;
function increment() {
  count++;
}

❌ Mutates input

function addItem(arr) {
  arr.push(5);
}

❌ Depends on external data

function getTime() {
  return new Date();
}

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✅ Pure Version

function addItem(arr) {
  return [...arr, 5];
}

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🎯 Why Pure Functions Matter

✔ Easier to test
✔ Predictable behavior
✔ Better performance optimizations
✔ Foundation of functional programming
✔ Important in React & Redux

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🔹 Pure Functions in React

• Reducers must be pure
• React.memo relies on purity
• Prevents unexpected re-renders

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🎯 Short Interview Answer

A pure function always produces the same output for the same input and has no side effects. It does not modify external state or depend on external variables, making it predictable and easy to test.

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⭐ One-line summary

Pure functions are predictable, side-effect-free functions that always return consistent results.