Deterministic Output
A pure function always returns the same output for the same input.
It does not rely on or change external state.
JavaScript Tutorial
JavaScript Pure Functions
Pure functions are predictable: same input, same output, no side effects.
They are the foundation of reliable and testable code.
Why We Need It
When functions are pure, debugging becomes easier because behavior is consistent.
They are also safer to reuse and refactor.
Syntax
function fn(input) { return output; }Basic Example
1. Pure function
function add(a, b) {
return a + b;
}
console.log(add(2, 3));No external state, same output for same inputs.
Real World Example
2. Impure function
let total = 0;
function addToTotal(amount) {
total += amount;
return total;
}This function changes external state, so it is impure.
Multiple Use Cases
No Side Effects
Pure functions do not modify global variables, DOM, or external data.
This makes them easy to test and reason about.
Why It Matters
Pure functions are predictable and safe to reuse.
They are ideal for calculations, transformations, and utilities.
More Examples
3. Pure transformation
function toUpper(words) {
return words.map((w) => w.toUpperCase());
}
console.log(toUpper(["a", "b"]));Input array is not modified, output is new.
4. Avoid side effects
function formatPrice(amount) {
return "$" + amount.toFixed(2);
}
console.log(formatPrice(99));Pure utility functions are easy to reuse.
Comparison
Without
let total = 0;
function add(amount) {
total += amount;
return total;
}With
function add(total, amount) {
return total + amount;
}Common Mistakes and Fixes
Mutating inputs
Return new values instead of modifying parameters.
Using global state
Pass all required data via parameters.
Hidden side effects
Avoid logging or IO in functions meant to be pure.
Interview Questions
What is a pure function?
It returns the same result for the same input and has no side effects.
Why are pure functions easier to test?
They do not depend on external state.
Give an example of a side effect.
Modifying a global variable or logging to the console.
Practice Problem
Practice: Write a pure function that converts Celsius to Fahrenheit.
// TODO: function toF(c)
One Possible Solution
function toF(c) {
return (c * 9) / 5 + 32;
}
console.log(toF(0));Frequently Asked Questions
What makes a function pure?
Same inputs produce same output and no side effects.
Are pure functions always better?
Not always, but they are easier to test and reuse.
Is console.log a side effect?
Yes, it interacts with external state.
Try It Yourself
Try calling the function multiple times with the same input.