14. What is a pure function?

Pure Function in JavaScript

A pure function is a function that:

1. Always returns the same output for the same input.
2. Has no side effects (does not modify external variables, change state, or interact with I/O like databases, files, or the DOM).

Example of a Pure Function

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

console.log(add(2, 3)); // Output: 5
console.log(add(2, 3)); // Output: 5 (Always the same for same input)

✅ Same input, same output
✅ No side effects

Example of an Impure Function

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let total = 0;

function addToTotal(num) {
  total += num;
  return total;
}

console.log(addToTotal(5)); // Output: 5
console.log(addToTotal(5)); // Output: 10 (Different output for same input!)

❌ Modifies an external variable (total) → Side effect
❌ Does not always return the same result for the same input

Why Use Pure Functions?

✅ Predictability – Easy to test and debug.
✅ Referential Transparency – Can be replaced with their output without changing behavior.
✅ Avoids Side Effects – Safer for functional programming.

Real-World Example of a Pure Function

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function calculateDiscount(price, discount) {
  return price - price * (discount / 100);
}

console.log(calculateDiscount(100, 10)); // Output: 90

✅ No external state changes
✅ Always predictable

Key Takeaways

• Pure functions return the same output for the same input.
• They do not modify external state or cause side effects.
• Ideal for functional programming and unit testing.

Would you like a real-world use case for pure functions? 🚀