JavaScript Interview Question #177

What is a pure function?

Advanced JavaScript, Practical Problems & Senior Interview Scenarios Senior Advanced

Quick Interview Answer

A pure function produces the same output for the same inputs and does not create observable side effects outside the function. It does not depend on hidden mutable state such as a changing global variable.

Detailed Explanation

A pure function produces the same output for the same inputs and does not create observable side effects outside the function. It does not depend on hidden mutable state such as a changing global variable.

Pure functions are easier to test, reason about, cache, and reuse. Real applications also require side effects such as network requests and DOM updates, so a useful design is to keep core calculations pure and isolate side effects at clear boundaries.

Code Example

function calculateTotal(price, quantity) {
    return price * quantity;
}

console.log(calculateTotal(50, 3)); // 150

// No global state is read or changed.