Pure function vs impure function

pure-function-vs-impure-function.js

const pureMultiplyItemsBy = (arrayOfNumbers, multiplier) => {
  return arrayOfNumbers.map(_ => _ * multiplier);
}

const impureMultiplyItemsBy = (arrayOfNumbers, multiplier) => {
  for (let i = 0; i < arrayOfNumbers.length; i++) {
    arrayOfNumbers[i] *= multiplier;
  }
  return arrayOfNumbers;
}

let x = [1,2,3];
pureMultiplyItemsBy(x, 2);
console.log(x); // [1,2,3] - no side effects

let y = [1,2,3];
impureMultiplyItemsBy(y, 2);
console.log(y); // [2,4,6] - side effect