--- title: How sort works in JavaScript slug: how-sort-works-in-javascript published_at: 2020-12-28 18:35:02 +0000 updated_at: 2026-03-04 20:14:37 +0000 summary: description: Learn why sorting a list of numbers in javascript yields unexpected results and how to use a compare function. Also, we'll show how the sort function on the Array.prototype might be implemented by re-implementing it our own `Array.prototype.mySort` that accepts our own compare function. 00:00 sort basics 02:00 Implement mySort with the merge sort algorithm ### Code https://gist.github.com/cjavdev/9fbfcce4fb16f0a70e43c8507b4962ea tags: [javascript, learn to code, beginner, sort, mergesort, array.sort, Array.prototype.sort, Js, Learn to code] views: 3878 author: CJ Avilla url: https://www.cjav.dev/videos/how-sort-works-in-javascript youtube_url: https://www.youtube.com/watch?v=2DATa8qEzWc youtube_id: 2DATa8qEzWc embed_url: https://www.youtube.com/embed/2DATa8qEzWc thumbnail_url: https://i.ytimg.com/vi/2DATa8qEzWc/hqdefault.jpg type: video --- # How sort works in JavaScript *Published: December 28, 2020* *Views: 3878* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=2DATa8qEzWc) [![How sort works in JavaScript](https://i.ytimg.com/vi/2DATa8qEzWc/hqdefault.jpg)](https://www.youtube.com/watch?v=2DATa8qEzWc) ## Description Learn why sorting a list of numbers in javascript yields unexpected results and how to use a compare function. Also, we'll show how the sort function on the Array.prototype might be implemented by re-implementing it our own `Array.prototype.mySort` that accepts our own compare function. 00:00 sort basics 02:00 Implement mySort with the merge sort algorithm ### Code https://gist.github.com/cjavdev/9fbfcce4fb16f0a70e43c8507b4962ea ## Transcript let's talk about the default sort for arrays so if you uh have played around in javascript you'll notice that you can create an array like this right and it's a list of numbers 1 2 3 9 34 and 11 and we can create another array called sorted array and that will be the result or the return value from calling array.sort this method on the array class and then we will just console log the sorted array now when we run this you'll see something really interesting okay so if we run node array.js the result is a sorted array but it's not sorted as we expect so here we're getting 1 and then 11 and then 2 3 34.99 and so what actually happens is by default the objects in the array don't always have to be the same type so some of these could be strings some of them could be objects some of them could be numbers some of them could be boolean and so in order to create this generic sort function they needed some like common denominator some lowest common denominator in order to compare each of the items in the list and so what um what we do by default in javascript is convert all of these numbers into strings and then we compare the unicode point value so right now we're converting one into a string and then comparing that with the rest of the objects and using some sorting algorithm right and then because 11 when you convert it to a string and compare that to two 11 comes before too because when we're comparing the unicode values one is before two so it's more like an alphabetical sort uh with the string values than it is a numeric sort and so the way we can get around this is that array.sort accepts an optional function argument this is a compare function sometimes called a comparator and so we can pass it a function that expects two arguments a and b and a and b represent each pair of items in the list that we're going to be comparing so the underlying algorithm to sort the items we don't actually have to care about or know about we just have to write a function here and pass in a function that will return some value that is either negative zero or positive and the the comparator uh so this comparator function will be used by the underlying algorithm when figuring out like which order to put things in so in the case of numerix the shortcut is to return a minus b and this will give us a list of sorted numbers that is in order so if that's all you needed you wanted to come and check out how to sort a list of numbers cool you can go use this and this is really kind of like all you have to do is pass in this compare function you might want to pull it out and say like sort or like number compare as a function that is uh equal to this thing um and then you can just like create a little helper and then pass that in when you need it number compare and then when you run it it'll compare the numbers and sort them as integers now if you wanted to get a little deeper what i'm going to do is re-implement sort so i'm going to create a new method called my sort to look at how this might be done under the hood okay so if you want to check that out stick with me and we'll dive right in so here what i want to do is um so we have our number compare let's also show what the default compare probably looks like so default compare is going to be a function that takes a and b and it returns instead of returning a minus b so in this case a minus b right like this is going to be if the first number is 1 and the second number is 2 then it's going to be 1 minus 2 and it'll return negative 1 and that says put a before b so 1 should appear before 2. similarly if we get 34 and then 11 right a is going to be 34 and 11 is going to be b so 34 minus 11 is 23 and 23 is greater than zero so we're going to swap these so that's kind of kind of how it works so default compare works like this so it actually returns a.2 string something like 8.2 string.locale compare b.2 string and so this is kind of like what the default is so if we pass in default compare that's the same as what we saw in the very beginning 1 and then 11 and then 2 3 34.99 that's that's useful if you are alphabetically sorting something so that's the default compare all right so what i'm going to do now is implement my sort and i'm going to implement this as a merge sort so merge sort is an algorithm i like a lot so the first thing we need to do is add a new method to the array prototype so we can say like array.proto type dot my sort is a new function and this is going to take in by default it doesn't take anything but we can accept this optional argument i'm just going to say fn i'm going to call it fn to accept the function as an argument the compare function the comparator function and then here rather than calling sort i'm going to call my sort and my sort by default we'll just we'll pass nothing in to begin with and we'll yeah actually let's just let's just implement this without any comparator function just to talk about how my sort works all right so the first thing that you do with a merge function it's usually written as two separate functions so we'll say merge is our other function and that's going to take the left and the right and eventually some comparator but what we can say is like if we want to like iterate over left and we want to iterate over right and compare the first uh the first item in each array and if the left is less then we're going to put that at the front if the right is less we'll put that at the front so we want to like say something like let's let um we'll create instance variables for left and right and a result this is going to be like the resulting array that we're going to return okay so we're implementing the merge here first and we want to say like while left.length is greater than zero and right dot length is greater than zero we want to check uh so let's d destructure left into l and um the rest of left and so we're sort of like splitting it up and we can do the same thing with actually let's only do that if so if left zero so if the first item in the left array is less than the first item in the right array then we want to put the first element from the left array onto our result so we're going to push l else we want to push r right so we're going to structure the right array and push r okay so here what we're doing is we're saying like if there's anything left in either the left array or the right array so this this merge function takes two arrays that are already sorted and then starts popping off the first element from each array and if either of those two arrays have something in them then we're going to compare the first elements in each and if there's something in the left then we want to we're using destructuring here to do kind of like pull off the first and the rest and save the rest uh and then similar for the right we're going to pull off the right and save the right and we're pushing on those first elements that are the least or like the smallest and then we can concatenate when we return the result it's possible that like we have a left array that is completely smaller than the right array or vice versa so after we've gone through and checked and compared those items we want to concatenate our result with left and right uh okay so let's so this is this is our merge function let's test that before we get too far so console.log merge of one two five with uh four six seven and let's see what we get let's cancel this out okay so our merge is working one two four five six seven so merge works that's great um okay so now we're gonna take advantage of merge so we wanna take our our array and we want to split that in half and we're going to sort the left and then pass that into merge and then we're going to sort the right and pass that into merge and sort of recursively call my sort so let's see so we want to get we want to get the halfway mark so the halfway mark is going to be like the ceiling of the half of this length and then the left is going to be this dot slice 0 up to half and the right is going to be this dot slice half okay so this should give us sort of the left and right sides of the array and what we want to return is um what do we want to return we want to return merge so we're going to merge two arrays and the things that we want to merge is left.mysort so we're going to recursively call mysort and then right.mysort okay so this is the this is the two this is the two arrays that we're going to um that we're going to merge together um okay and then we need one other thing we need our base case so since we're recursively calling my sort here on these like objects inside we need some base case where by if we get to this case we break and and return so um i like the base case of an array with zero or one elements and returning just just returning that same array because um an array with zero elements is sorted in an array with just one element is also sorted so we can say like if this dot length is less than or equal to one then we'll return return this okay so if we run this again what are we going to get okay we're not actually exercising anything um all right i'm not sure if this is going to work the first time but we'll give it a shot okay so we it actually did so 1 2 3 11 34 99 so now we have a function here called my sort which is a method that we implemented right so we took in my sort or we we added my sort to the array prototype which means that on instances of arrays we can call this method and because we're using this method style of uh invoking the function where we have some object on the left and then a dot and then we have like the parens on the right this inside of the function this will refer to the object that was on the left so here this is bound to the instance of the array that we defined at the top let's just move that down for clarity so arr is this array that we're working with so this this dot length if this dot length is less than or equal to one we return this um if otherwise we're gonna split it in half we're gonna merge the left then we're gonna or we're gonna sort the left and then we're going to sort the right and we'll pass those two left and right into merge which will merge the two together and this is kind of the merge sort algorithm so now let's extend our our sort function to accept a function itself so our my sort right so like this is the function that we defined right now or previously it didn't accept any arguments but we're going to add a single argument called fn and fn is going to be our comparator so now mysore can take in a function and so we need to define that function so if we define it as or if we just pass in this default compare right so default compare we want this to sort it in like uh alphabetical order so default compare is going to be passed into our function here we need to pass that down uh when we're calling my sort recursively and we also need to pass it down to merge so we need to take our merge function and make this accept a function itself and rather than just straight up comparing left the first item in the left array and the in the first item in the right array what we can do is we can pass the the first item in the left array and then the first item in the right array as arguments to our function and then if that is less than zero then we can um we can do the same action so rather than just straight up comparing is the the first item in the left array less than the the first item in the right array we want to use this function so we're going to call the function and pass in two elements and then whatever the function defines as the the direction that should be sorted that's what will what will be used so now our function is being called here and it's being called function style so we don't have access to the underlying array because when we call something function style this gets bound to the window or to the global the global scope right um and so we're passing in default compare we expect to get back sort of alphabetical order and we do so that's cool um and then if we passed in you know our number compare we should get uh you know numeric sort good now if we if we pass in nothing maybe we should expect to get the alphabetic uh sort but it turns out that fn is not a function here so in order to make our mysore work the same way as the normal sort with some default some reasonable default then i might come up here and say like if fn is undefined then set fn equal to default compare okay and then we'll run this same function and we get so by default now it is still the um you know alphabetically sorted um and then if we change our my sort to now take in the number compare then it will use that function when it's sorting so if we wanted to we could say like number compare reverse or something so maybe we want our mysore to take in like a and b and then we could take our number compare and say return number compare of a and b and then multiply that by minus one so we're going to get number compare is going to give us uh some number and then by my um multiplying by negative one we're going to flip the sign and we should get a our array in reverse order so this is how we would like get a you know um a list of numbers in descending order so okay that's uh that's all i wanted to show i'll drop in some links to gists with all of the code for uh for this episode uh hopefully that was helpful uh if you found this useful and you'd like to see more videos like that i would appreciate the the the old subscribe alright friends see ya --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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