How do I sort an array of numbers in Javascript?

Let’s say you have a Javascript array of numbers like this:

let numbers = [85, 83, 29, 70, 4, 0, 17, 8, 55];

You want to sort this array. You might think you can do this using the array’s sort method like this:


If you do that, though, the results aren’t what you might expect.

[0, 17, 29, 4, 55, 70, 8, 83, 85]

Default Behavior of sort()

Everything looks OK until you get to 4 and realize the numbers are sorted alphabetically instead of numerically. The sort methods default behavior is an alphabetical sort, but, fortunately, it can be customized to sort by any criteria you want.

Customizing the Array Sort

To customize the sort order, you pass it a function to compare two items in the array. sort will pass to your function two items in the array (conventionally the parameters a and b in your function). sort looks for one of three different outcomes from your function:

  • If your function returns a value less than 0, a comes first.
  • If your function returns a value greater than 0, b comes first.
  • If your function returns 0, the order of these two items remains unchanged.

In order to sort numbers, you’ll need to write a function that returns a negative number if a is less than b, returns a positive number if b is less than a, and returns 0 if the numbers are the same.

This can easily be accomplished by subtracting the numbers. Subtracting the two numbers will yield 0 if the numbers are the same. Which number needs to be subtracted from which to yield a negative number if a is smaller? I believe that’s going to be a - b, but I like to confirm that by testing it with some actual numbers.

Testing Our Solution

Let’s say a is 1 and b is 2. We’d want our sort function to return a negative number in this case. Insert the sample values into my proposed formula of a - b and you’ll have 1 - 2 which yields -1. Looks good so far! Now, let’s see if it yields a positive number when a is larger.

This time, we’ll make a equal 2 and b equal 1. Plug that into the formula and we have 2 - 1 which yields 1 — a positive number. That’s exactly what we want.

This means all our function needs to do is return the result of a - b. We can do our sort by passing an anonymous function into the sort function like this:

numbers.sort((a, b) => a - b);

but if we’re going to be sorting multiple arrays in this way, we probably want to declare a function and pass it into the sort instead like this:

const numberSorter = (a, b) => a - b;

Now we can pass numberSorter to any array’s sort method to sort it numerically. Here’s the array after the sort with our custom sorting function:

[0, 4, 8, 17, 29, 55, 70, 83, 85]

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Sorting Other Data

Even though the default sort doesn’t work in every case, by writing custom sorter functions, it becomes very flexible. You can write virtually any type of sort you want by setting up the sorter function to produce the three possible outcomes we discussed above. Use this example to get your data organized in whatever order you need!

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