How To Sort Array Elements in Javascript and Ruby


Sorting an array is one of the most common things we have to do as software engineers. It can also sometimes be the fastest way to calculate a data point–for example, the highest possible product of any three numbers in an array.

The most standard method to use in both Javascript and Ruby is the .sort method. However, as we will see next, the name of the method is where the similarities between the two languages end.


In Javascript, the .sort method mutates the original array. So, if you need to keep the original array for whatever reason, you better only sort a copy of the original array.

Let’s look at the following code:

const arr = [1,4,-10,-200,-3,30,42,5,-1500,3];

–> which outputs: [ -10, -1500, -200, -3, 1, 3, 30, 4, 42, 5]

Several things to point out here:

  1. Even if we replaced line 2 with const newArray = arr.sort(), we’d get the same output (arr is still mutated).
  2. Notice that we were able to sort the array even though we declared it with const and not let. That’s because although const prevents us from pointing arr to a new place in memory (i.e. using the = symbol), it still allows us to rearrange the internal elements. Similarly, we could still use .push(), .pop(), etc, on an array declared with const.
  3. Notice that 30 came before 4, and 42 came before 5. What gives? And what’s going on with the negative numbers–shouldn’t they be: -1500, -200, -10, and -3? When you don’t add parameters to the .sort() method, it treats each digit kind of like the way a dictionary treats each letter in a word. So, just like “gone” comes after “go” but before “hi”, so too does “42” come after “4” but before “5”. The same thing is happening to the negative numbers.

If we want to correctly sort by numerical value, we need to insert a function as a parameter in the .sort() method.

  • To sort in ascending order, we’d use: arr.sort((a,b) => a-b);
  • To sort in descending order, we’d use: arr.sort((a,b) => b-a);

The ascending order sort would output: [ -1500, -200, -10, -3, 1, 3, 4, 5, 30, 42]

The descending order sort would output the exact reverse.


Let’s now look at how to do the same thing in Ruby. Here, the default .sort method does not mutate the original array.

Let’s look at the following code:

arr = [1,4,-10,-200,-3,30,42,5,-1500,3]
print arr

–> which outputs: [1,4,-10,-200,-3,30,42,5,-1500,3]

If we want to mutate the original array, we need to use .sort! instead of .sort:

arr = [1,4,-10,-200,-3,30,42,5,-1500,3]
print arr

–> which outputs: [-1500, -200, -10, -3, 1, 3, 4, 5, 30, 42]

Several things to point out here:

  1. Notice that, unlike in Javascript, the default .sort method in Ruby sorted by numerical value instead of by digit order. The default method does the exact same thing as this more defined sort: arr.sort { |a, b| a <=> b }
  2. If we wanted to sort in descending order, we can use: arr.sort { |a, b| b <=> a }
  3. To do more complex sorts, check out the .sort_by method. This guide by Andrew Allen is good.
Vahid Dejwakh
Vahid Dejwakh
Software Engineer, Co-Creator of the Fjord Framework

Vahid writes about interesting ideas at the intersection of software, system design, data, philosophy, psychology, policy, and business. He enjoys coffee and has a palate for spicy and diverse foods.

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