FP in JS 1

fp_js_1.js

// This function is pure because it's determinisic.
// It has no side effects.
// Nothing outside the function can influence the output.
// addTen(5) will _always_ be 15
const addTen = input => {
  return input + 10;
};

// Fetch the number from the database
const numberFromDb = DB.getNumber();

// This function is not pure because it's not deterministic.
// It has a side effect (the numberFromDb value).
// We cannot know for sure that the outcome will be the same
// every time we call it, and that's why it's not pure
const addWithNumberFromDb = input => {
  return input + numberFromDb;
};