Pure Functions

🔹 Definition

A Pure Function is a function that:

1. Given the same input, always returns the same output.
2. Has no side effects (doesn’t modify variables, objects, or states outside of its scope).

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🔹 Characteristics of Pure Functions

• Deterministic → Same input → Same output.
• No side effects → Doesn’t modify external variables, objects, databases, or DOM.
• Self-contained → Only depends on its arguments, not external state.

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🔹 Examples

✅ Pure Function Example

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

console.log(add(2, 3)); // 5
console.log(add(2, 3)); // 5 (always the same result)

👉 Same input (2,3) will always give 5.

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❌ Impure Function Example (side effect)

let total = 0;

function addToTotal(a) {
  total += a;  // modifies external variable
  return total;
}

console.log(addToTotal(5)); // 5
console.log(addToTotal(5)); // 10 (different result for same input)

👉 Depends on external state (total), so it’s not pure.

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❌ Impure Function Example (randomness)

function getRandomNumber() {
  return Math.random();
}

console.log(getRandomNumber()); // e.g. 0.123
console.log(getRandomNumber()); // e.g. 0.984

👉 Same input (none), but different outputs each time → impure.

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🔹 Real-World Example

Pure Function (Safe for financial calculations)

function calculateTax(amount, taxRate) {
  return amount * taxRate;
}

console.log(calculateTax(1000, 0.2)); // 200
console.log(calculateTax(1000, 0.2)); // 200 (consistent)

Impure Function (Not safe)

let taxRate = 0.2;

function calculateTaxImpure(amount) {
  taxRate = taxRate + 0.01;  // modifies global state
  return amount * taxRate;
}

console.log(calculateTaxImpure(1000)); // 210
console.log(calculateTaxImpure(1000)); // 220 (different!)

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🔹 Why Pure Functions Matter?

• ✅ Easier to test (no dependencies on external state).
• ✅ Easier to debug (predictable behavior).
• ✅ Useful in functional programming & frameworks like React (pure components).
• ✅ Help in immutability and avoiding bugs.

👉 Would you like me to also explain how React components follow the idea of pure functions (like render() being pure) with examples?