Pure vs Impure Functions in JavaScript

Understanding the difference between pure and impure functions is crucial for writing predictable, testable code.

✅ Pure Functions

A pure function is a function that:

• Returns the same output for the same input.
• Has no side effects (does not modify external state).

Example:

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

console.log(add(2, 3)); // Always returns 5

✅ No mutation, no reliance on external state.

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❌ Impure Functions

An impure function:

• May produce different outputs for the same inputs.
• Causes side effects (e.g., modifies global variables, logs to console, etc.).

Example:

let counter = 0;

function increment() {
  counter += 1;
  return counter;
}

console.log(increment()); // Changes global state

❌ Depends on external state and produces side effects.

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🔍 Comparison Table

Feature	Pure Function	Impure Function
Output	Same for same input	May vary
Side Effects	None	Yes
Testable	Easy	Hard
Predictability	High	Low

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✅ Why Prefer Pure Functions?

• Easier to test and debug.
• Enhances reusability and composability.
• Leads to more predictable code — a key concept in functional programming and Redux.