Custom Array Sort Algorithms in JavaScript

JavaScript's Default Array Sort

JavaScript's Array.sort defaults to a String sort. This catches many people off guard when attempting to sort an Array of type Number.

// ❌ Default search is a String search
const numbers = [10, 1, 3, 15]
numbers.sort() // [ 1, 10, 15, 3 ]

In the above example, each Number is converted to a String and then sorted using a String sort.

At first, this can seem like a WTF JavaScript moment, but this happens because an Array can contain mixed elements and JavaScript doesn't know how it should sort. So sort defaults to a String sort.

const array = [1, 2, 3, 'Joel', 4, { userId: 123 }]

When we want something other than a String sort, we have to be explicit.

Custom Sort Compare Function

Creating a custom sort compare function is pretty easy. The function takes two elements, then we return -1 if the first is lower and 1 if it's higher. 0 for the same.

const compareFunction = (a, b) => {
    // Pseudo Code
    if (a is less than b) return -1
    if (a is more than b) return 1
    return 0
}

Then pass that function to the sort method.

myArray.sort(compareFunction)

This flexibility will allow us to be creative with our sorting algorithms.

Number Sort

To sort a Number Array we could create a custom compareNumbers function and pass that into Array.sort.

const compareNumbers = (a, b) => a - b

const numbers = [10, 1, 3, 15]
numbers.sort(compareNumbers) // [ 1, 3, 10, 15 ]

Custom Object Sort

Let's say we had some data that looks like this:

const customers = [
    { id: 1, orders: ['a-1000', 'x-2000', 'c-8000'] },
    { id: 2, orders: ['a-1010'] },
    { id: 3, orders: ['a-1040', 'c-8050'] },
]

Our requirement is to sort by the number (length) of orders. So the order should be 2, 3, 1.

We can do that with a custom compareOrderLength function that will sort by customer.orders.length.

const compareOrderLength = (a, b) => a.orders.length - b.orders.length

customers.sort(compareOrderLength)
/**
 * [
 *   { id: 2, orders: [ 'a-1010' ] }, 
 *   { id: 3, orders: [ 'a-1040', 'c-8050' ] }, 
 *   { id: 1, orders: [ 'a-1000', 'x-2000', 'c-8000' ] }
 * ]
 */

Complex Custom Sorting

I recently had a use case where an API was returning data that looked like this.

// API Response
["1", "10", "2", "BLA", "BLA2", "3"]

The Array contained all String items, but the business wanted the items to display like "1, 2, 3, 10, BLA, BLA2".

That meant, I had to detect when the String was a Number and Sort the "numbers" first and the text after.

As complex as that sounds, the sort algorithm wasn't too bad.

const isNumeric = (num) => !isNaN(num)

const customCompare = (a, b) => {
    if (isNumeric(a) && !isNumeric(b)) return -1
    if (!isNumeric(a) && isNumeric(b)) return 1
    if (isNumeric(a) && isNumeric(b)) return a - b
    return a < b ? -1 : 1
}

// [ '1', '2', '3', '10', 'BLA', 'BLA2' ]

End

So just remember the default Array sort is a String sort. To sort by anything else, you must create a compare function and pass that into sort.

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Photo by Kelly Sikkema on Unsplash