Pure Function

A function must pass two tests to be considered “pure”. 
1. Same inputs always return same outputs
  const add = (x, y) => x + y;

  add(2, 4); // 6

2. No side-effects
  A few examples of side-effects are: 
    1. Mutating your input
    2. console.log
    3. HTTP calls (like AJAX/fetch)
    4. Changing the filesystem
    5. Querying the DOM
    
// code with side effects 
const impureAssoc = (key, value, object) => {
  object[key] = value;
};

const person = { name: 'Bobo' };
const result = impureAssoc('shoeSize', 400, person);

console.log(person);  //  { name: 'Bobo', shoeSize: 400 }


// code with no side side effects 
const pureAssoc = (key, value, object) => ({
  ...object,
  [key]: value
});

const person = { name: 'Bobo' };
const result = impureAssoc('shoeSize', 400, person);

console.log(person); //  { name: 'Bobo', shoeSize: 400 }