#147 What are Pure Functions?

A Pure Function is a function that always:

1. Returns the same output for the same input.
2. Does not cause side effects (like modifying global variables, DOM, API calls, etc.).

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🔹 Characteristics of Pure Functions

• Deterministic → same input → same output.
• No Side Effects → doesn’t modify external state (variables, objects, DOM).
• Immutability → works with copies instead of changing original data.

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🔹 Example of Pure Function ✅

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

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

👉 No matter how many times you call add(2,3), it will always return 5.

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🔹 Example of Impure Function ❌

let count = 0;

function increaseCount(num) {
  count += num;  // modifies external variable (side effect)
  return count;
}

console.log(increaseCount(2)); // 2
console.log(increaseCount(2)); // 4 (different output for same input ❌)

👉 Output is different for the same input because it depends on external state (count).

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🔹 Pure Functions in React

In React, components should be pure functions:

• They take props as input.
• Return JSX as output.
• Do not modify props or external state.

✅ Pure React Component Example:

function Greeting({ name }) {
  return <h1>Hello, {name}!</h1>;
}

❌ Impure React Component Example:

let greetingCount = 0;

function Greeting({ name }) {
  greetingCount++;  // modifies external variable ❌
  return <h1>Hello, {name}! Count: {greetingCount}</h1>;
}

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🔹 Why Pure Functions are Important?

• Predictable → easy to test & debug.
• Reusable → can be reused without worrying about external state.
• Helps React → React’s reconciliation (diffing) works better with pure functions.
• Performance → works well with memoization (React.memo, useMemo).

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✅ In short for interview:

“A pure function always returns the same output for the same input and does not cause side effects. In React, functional components are designed to be pure — they depend only on props and state, making them predictable and easy to test.”