What a pure function is

pureFunctions.js

const addNumbers(a,b) => return num + a + b; 

//Pure functions def: A function which returns the exact same result every time, when given the exact same arguments. 

//I.e. a function is NOT pure when the same arguments might give different outputs (i.e. imagine making API calls
//and getting 'num' from above through an API call, which in turn means the num value might be different, since
//the data fetched from the API might be different.