All 10 Functional Programming questions
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JavaScript › Functional Programming

What is a pure function?

Beginner

Answer

A pure function always returns the same output for the same input and produces no observable side effects (no mutating external state, no I/O), which makes it predictable, testable, and easy to reason about in isolation.
// Pure
function add(a, b) { return a + b; }

// Impure — depends on and mutates external state
let total = 0;
function addToTotal(n) { total += n; }
Real-world example Preferring pure calculation functions in a reducer or utility layer for predictable, easily-testable behavior.

Common follow-ups: Can a pure function still throw an error?

Error Handling
All 10 Functional Programming questions