imperative-state.js

let state = {
  foreground: '#999999',
  background: '#FFFFFF';
};

const imperativeMakeBackgroundBlack = () => {
  state.background = '#000000';
};
// directly changes the state object outside of its internal scope

const declarativeMakeBackgroundBlack = state => ({...state, background: '#000000'});
// takes current state as its input and returns new state with changed value
// without changing the original state

let turtles = ['Galápagos tortoise', 'Greek Tortoise'];

const imperativeAddTurtle = turtle => turtles.push(turtle);
// changes the turtles external array and returns the length of the new array

const declarativeAddTurtle = turtles => turtle => [...turtles, turtle];
// takes 'array of turtles' and the 'new turtle' as its input.
// It returns new array of turtles without changing the original array