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JavaScript Sorting Arrays

Sorting an Array

The sort() method sorts an array alphabetically:

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.sort();

Reversing an Array

The reverse() method reverses the elements in an array.

You can use it to sort an array in descending order:

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.sort();
fruits.reverse();

Numeric Sort

By default, the sort() function sorts values as strings.

This works well for strings ("Apple" comes before "Banana").

However, if numbers are sorted as strings, "25" is bigger than "100", because "2" is bigger than "1".

Because of this, the sort() method will produce incorrect result when sorting numbers.

You can fix this by providing a compare function:

Example

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a - b});

Use the same trick to sort an array descending:

Example

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b - a});

The Compare Function

The purpose of the compare function is to define an alternative sort order.

The compare function should return a negative, zero, or positive value, depending on the arguments:

function(a, b){return a - b}

When the sort() function compares two values, it sends the values to the compare function, and sorts the values according to the returned (negative, zero, positive) value.

If the result is negative, a is sorted before b.

If the result is positive, b is sorted before a.

If the result is 0, no changes are done with the sort order of the two values.

Example:

The compare function compares all the values in the array, two values at a time (a, b).

When comparing 40 and 100, the sort() method calls the compare function(40, 100).

The function calculates 40 - 100 (a - b), and since the result is negative (-60),  the sort function will sort 40 as a value lower than 100.

You can use this code snippet to experiment with numerically and alphabetically sorting:

<button onclick="myFunction1()">Sort Alphabetically</button>
<button onclick="myFunction2()">Sort Numerically</button>

<p id="demo"></p>

<script>
const points = [40, 100, 1, 5, 25, 10];
document.getElementById("demo").innerHTML = points;

function myFunction1() {
  points.sort();
  document.getElementById("demo").innerHTML = points;
}

function myFunction2() {
  points.sort(function(a, b){return a - b});
  document.getElementById("demo").innerHTML = points;
}
</script>

Sorting an Array in Random Order

Example

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(){return 0.5 - Math.random()});

The Fisher Yates Method

The above example, array.sort(), is not accurate. It will favor some numbers over the others.

The most popular correct method, is called the Fisher Yates shuffle, and was introduced in data science as early as 1938!

In JavaScript the method can be translated to this:

Example

const points = [40, 100, 1, 5, 25, 10];

for (let i = points.length -1; i > 0; i--) {
  let j = Math.floor(Math.random() * (i+1));
  let k = points[i];
  points[i] = points[j];
  points[j] = k;
}

Find the Highest (or Lowest) Array Value

There are no built-in functions for finding the max or min value in an array.

However, after you have sorted an array, you can use the index to obtain the highest and lowest values.

Sorting ascending:

Example

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a - b});
// now points[0] contains the lowest value
// and points[points.length-1] contains the highest value

Sorting descending:

Example

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b - a});
// now points[0] contains the highest value
// and points[points.length-1] contains the lowest value

Sorting a whole array is a very inefficient method if you only want to find the highest (or lowest) value.

Using Math.max() on an Array

You can use Math.max.apply to find the highest number in an array:

Example

function myArrayMax(arr) {
  return Math.max.apply(null, arr);
}

Math.max.apply(null, [1, 2, 3]) is equivalent to Math.max(1, 2, 3).

Using Math.min() on an Array

You can use Math.min.apply to find the lowest number in an array:

Example

function myArrayMin(arr) {
  return Math.min.apply(null, arr);
}

Math.min.apply(null, [1, 2, 3]) is equivalent to Math.min(1, 2, 3).

My Min / Max JavaScript Methods

The fastest solution is to use a "home made" method.

This function loops through an array comparing each value with the highest value found:

Example (Find Max)

function myArrayMax(arr) {
  let len = arr.length;
  let max = -Infinity;
  while (len--) {
    if (arr[len] > max) {
      max = arr[len];
    }
  }
  return max;
}

This function loops through an array comparing each value with the lowest value found:

Example (Find Min)

function myArrayMin(arr) {
  let len = arr.length;
  let min = Infinity;
  while (len--) {
    if (arr[len] < min) {
      min = arr[len];
    }
  }
  return min;
}

Sorting Object Arrays

JavaScript arrays often contain objects:

Example

const cars = [
  {type:"Volvo", year:2016},
  {type:"Saab", year:2001},
  {type:"BMW", year:2010}
];

Even if objects have properties of different data types, the sort() method can be used to sort the array.

The solution is to write a compare function to compare the property values:

Example

cars.sort(function(a, b){return a.year - b.year});

Comparing string properties is a little more complex:

Example

cars.sort(function(a, b){
  let x = a.type.toLowerCase();
  let y = b.type.toLowerCase();
  if (x < y) {return -1;}
  if (x > y) {return 1;}
  return 0;
});

Stable Array sort()

revised the Array sort() method.

Before 2019, the specification allowed unstable sorting algorithms such as QuickSort.

After ES2019, browsers must use a stable sorting algorithm:

When sorting elements on a value, the elements must keep their relative position to other elements with the same value.

Example

const myArr = [
  {name:"X00",price:100 },
  {name:"X01",price:100 },
  {name:"X02",price:100 },
  {name:"X03",price:100 },
  {name:"X04",price:110 },
  {name:"X05",price:110 },
  {name:"X06",price:110 },
  {name:"X07",price:110 }
];

In the example above, when sorting on price, the result is not allowed to come out with the names in an other relative position like this:

X01 100
X03 100
X00 100
X03 100
X05 110
X04 110
X06 110
X07 110

Complete Array Reference

For a complete Array reference, go to our:

.

The reference contains descriptions and examples of all Array properties and methods.

Test Yourself With Exercises

Exercise:

Use the correct Array method to sort the fruits array alphabetically.

const fruits = ["Banana", "Orange", "Apple", "Kiwi"];
;

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