// Pure functions
function square(x) {
    return x * x;
}
function squareAll(items) {
    return items.map(square);
}

Pure functions 's return value depends solely on the values of its arguments. No observable "side-effects" such as network or database calls. Pure functions do not modify the values passed to them. In the second example, the function does not modify the items array, instead creates a new array with map().

// Impure functions
function square(x) {
    updateXInDatabase(x);
    return x * x;
}
function squareAll(items) {
    for (let i = 0; i < items.length; i++) {
        items[i] = square(items[i]);
    }
}