sorting-arrays-in-javascript.md

Sorting arrays in JavaScripts

Today I learnt about sort function that allows to conveniently sort arrays in JavaScript. The simplest way to use it is to simply call it on an array as such:

var fruit = ['cherries', 'apples', 'bananas'];
fruit.sort(); // ['apples', 'bananas', 'cherries']

The default sort order is defined by Unicode code points. This brings unexpected results when we try to sort an array of numbers as such:

var scores = [1, 10, 2, 21]; 
scores.sort(); // [1, 10, 2, 21];

In the example above, 10 comes before 2 which is most likely not what you would expect. However, since 10 comes before 2 in Unicode code point order, it will be treated as smaller than 2.

compareFunction

The compareFunction is where really all the meat lays. It allows us to sort arrays made up of any type of objects by defining compareFunction where we either specify whether two objects are equal, or which one of them is bigger.

compareFunction accepts two parameters a and b, and would be called as such:

let scores = [1, 10, 2, 21]; 
scores.sort((a, b) => {
	a = parseInt(a);
	b = parseInt(b);

	if (a === b) return 0;
	if (a > b) return 1;
	else return -1;
}); // [1, 2, 10, 21]

To reiterate, we pass a so called compareFunction to sort with two parameters, a, and b. If a and b are equal according to our criteria, we return 0. If a is bigger than b, we return 1. Otherwise, we return -1.

You can probably see by now how compareFunction can be used to sort literally anything. For example, below is the function I wrote today that sorts an array of dates in ascending order. Note that it uses Moment.js for convenience but that does not change the idea. Cool stuff if you ask me :)

filteredDates.sort(function(a, b) {
  let aDate = moment(a.fullTime, "YYYY MM DD, HH:mm:ss")
  let bDate = moment(b.fullTime, "YYYY MM DD, HH:mm:ss")
  let dateA = new Date(aDate)
  let dateB = new Date(bDate)
  return dateA - dateB;
});

Resources

• MDN Array.prototype.sort()