JavaScript Sorting Arrays
Sorting an Array
The sort() method sorts an array alphabetically:
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.sort(); // Sorts the elements of fruits
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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();
// First sort the elements of fruits
fruits.reverse();
// Then reverse the order of the elements
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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});
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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});
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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>
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Sorting an Array in Random Order
Example
const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){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)
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
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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
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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:
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:
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:
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;
});
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