Flip function

flip.js

// our example array to work with
const myArr = [{
    name: 'bob',
    id: 5,
}, {
    name: 'scott',
    id: 4,
}, {
   name: 'matt',
   id: 6,   
}, {
    name: 'chris',
    id: 1
},
];

// the Problem: sorting an array ascending is usually a - b, descending is b - a
const mySortedArray = myArr.concat().sort((a, b) => a - b));

// for an object we might want to sort by a specific property
const anotherSortedArray = myArr.concat().sort((start, end) => start.id - end.id);
 
// lets extract our sorting function, and make it so we can pass in any property to sort by
const ascendingProp = prop => (start, end) => start[prop] - end[prop];

const sortedArray = myArr.concat().sort(ascendingProp('id'));

// unfortunately if we want this to be descending we run into some trouble.

// we can create a funciton, which take a function, 
// then takes two parameters and flips them in execution
const flip = func => (first, second) => func(second, first);

const descendById = flip(ascendingProp('id'));

const descendingArray = myArr.concat().sort(descendById);

console.log(sortedArray); // result: [{ id:1, name: 'chris'}... etc]
console.log('descending', descendingArray); // result: [{ id: 6, name: 'matt'}... etc]